# How To Calculate Muffler Size and Exhaust Pipe Diameter

If you’re a math wiz and/or an engineer, you’re probably going to like this article and the resources we’ve linked to. However, if you find yourself getting stuck (or bored) with the info below, here are the key take-aways:

1. The factory exhaust pipe diameter is usually a good choice for most vehicles.

2. The muffler manufacturers are doing all the math for us – no need to reinvent the wheel. If they say it will work for your vehicle, it will probably work for your vehicle.

3. We’ve got an easy-to-read exhaust system size table that is good for quick calculations.

### Breaking Down The Problem

While we’re not going to go through and list out all the formulas and calculations you need to figure this exactly, we will break down the problem, explain how you would go about figuring things out scientifically, and then leave you with some good quick-and-dirty exhaust system math as well as some interesting links.

The science goes like this…

1) Mass of air that the engine breathes in + mass of fuel = mass of exhaust gases

Conservation of mass, right?

2) To calculate the volume of air the engine takes in, we multiply the displacement of the engine by the engine RPM and then divide by two (it takes two full revolutions for the engine to exhaust it’s entire air volume). We then convert that to volume to mass.

3) To make the calculations easy, you want to assume that combustion is perfect, i.e. there aren’t any byproducts, any unburned fuel, etc. It’s easier to assume perfect combustion and then “back in” to the actual numbers using an estimate after the fact.

4) Since you’re assuming perfect combustion, it’s easy to figure out how much fuel mass is added to the exhaust.

5) Once you know the mass of the exhaust gas, you just figure out how much volume that mass would occupy. Of course, you have to adjust for expansion due to the high exhaust gas temperature.

That’s it! Of course, when you sit down to figure it, you’ll find that getting a good scientific estimate takes a lot of work (which is why we don’t bother with it here).

### Quick and Dirty Exhaust System Math

Easy Way To Estimate: Your intake system needs to flow 1.5 CFM per engine horsepower, and your exhaust system needs to flow 2.2 CFM per engine horsepower.

Good Way To Estimate: Take engine RPM x engine displacement, then divide by two. This is the intake volume. Use this same volume of air for the exhaust system, but then correct for thermal expansion (you need to know exhaust temps to figure things out).

Exhaust Pipe Size Estimate: A good section of straight pipe will flow about 115 CFM per square inch of area. Here’s a quick table that shows how many CFM each common pipe size will flow, as well as the estimated max horsepower for each pipe size:

 Pipe Diameter (inches) Pipe Area (in2) Total CFM (est.) Max HP Per Pipe Max HP For A Dual Pipe System 1 1/2 1.48 171 78 155 1 5/8 1.77 203 92 185 1 3/4 2.07 239 108 217 2 2.76 318 144 289 2 1/4 3.55 408 185 371 2 1/2 4.43 509 232 463 2 3/4 5.41 622 283 566 3 6.49 747 339 679 3 1/4 7.67 882 401 802 3 1/2 8.95 1029 468 935

NOTE: These numbers are just estimates. All pipes are assumed to be 16 gauge steel.

The table above is probably over-estimating pipe size, but you can see that a 400 hp vehicle with a dual exhaust system only needs 2 1/4 – 2 1/2 inch pipes. Anything larger is overkill.

Great forum discussion that really discusses the details of the scientific calculations:Β http://www.eng-tips.com/viewthread.cfm?qid=104735

A good general article about designing the perfect exhaust system: http://www.popularhotrodding.com/enginemasters/articles/hardcore/0505em_exh/index.html

1. onyeh
Aug 16 2011

it was a very good article. i red and i was impressed. it solved my problem to make more efficiently exhaust for racing need. do you have any article about header? size of pipe diameter and how long it can be to give an optimum performance. i’m atuner from indonesia. an in my country, something like this was very rare. i have searched on the web, but found no article that make me delightfull to solve header problem. if you have some, it will make me very gratefull. thanxs before.

• Kurt
Feb 25 2012

@ onyeh,
search for the haynes website and consider purchasing performance tuning for 4 strokes and performance tuning for forced induction. They are EXTREMELY useful and have a great amount of information for real motoring enthusiasts and professional tuners.

2. briankmizell
Aug 31 2011

This is in regards to this:

“Good Way To Estimate: Take engine RPM x engine displacement, then divide by two.”

My question is… for the engine displacement do I used cubic inches or CC?

thank for an informatic article,

Brian Mizell

3. Jason
Sep 19 2011

Brian – It can be either. Just remember that whatever units you use (cubic inches or cubic centimeters) must be converted to a unit of mass. My suggestion? Stick with metric. Conversions are much easier.

4. Nov 21 2011

Hello

Great table but it may not be as accurate as you may think as i have a very quiet 3″ system on my 3.0L turbo Nissan skyline with 315kw@all 4 wheel, the problem is it’s 315kw on 15psi boost and 335kw on 19PSI boost, so did a google search for pipe size vs horse power

Kind regards

Geoffrey

• Jan 10 2012

Just a quick update

I removed the rear muffler and increased power to 355kw@ all four wheel.

New rear muffler fitted dropped the power to 345kw@ all four

made up a 3″ mandrel bent pipe to replace the rear muffler 350kw@ all four

Now it’s just too noisy

Destroyed my clutch before Christmas so have not played since

best so far is a 11.66 quarter at 201km with street tyres

Hoping to up the power to 400kw@ all four

kind regards

Geoffrey

5. Buz
Jan 8 2012

Very informative! I do not understand “overestimating pipe size”. Do you mean I can make more power with each size pipe in the chart? I have a non mandrel 1.75″ system on a Daihatsu Rocky that originally made 94hp with 1.5″ from the cat back. Am I properly sized, it needs all the HP and tq it can get.

• Jason
Jan 8 2012

Buz – The size of your exhaust system pipes should be matched to the power of your engine. So, if you have an engine that generates less than 100hp, the proper single exhaust tube size is 1 3/4″ . Smaller (like 1 5/8″) is probably just fine (might actually help with low end torque to got to 1 5/8″), but if you go much bigger you’ll actually rob power from your engine.

Also, when I wrote that our chart was over-estimating size, it means that the diameters listed aren’t exact. Instead, they’re rounded to the common exhaust pipe sizes so you can buy pipe…if we just used math to arrive at the diameter, we’d end up with some funky pipe sizes that would be impossible to buy. Hopefully that makes sense (the article is a bit cryptic).

• Mark Brewer
Feb 23 2016

Jason, Thank you very much for posting this, it is very simple to understand and quite helpful.

6. Michael Walton
Jan 10 2012

Hi there Jason, Great reading, Im running a 572ci big block chev on a twin 2.5 inch system with an x pipe and flowmaster mufflers(620hp)primary pipe size is 2 inch going into 3 inch collectors i have been thinking about going to a twin 3 inch system but am wondering if i would see any noticible difference in the car as I seldom go beyond 4500rpm on the street(Max rpm 6000)the system is ceramic coated so cost a bit what are your thoughts?

• Jason
Jan 10 2012

Michael – If I’ve done the easy math right, your big block could pump out nearly 1,100 CF of exhaust per minute at 4500 RPM. If you’ve got a section of pipe that’s only 2″ in diameter, than you’ve got a bottleneck that’s probably restricting flow on your vehicle.

However, dual 3″ is a lot..I’m thinking dual 2.5″ or dual 2 3/4″ is probably enough. Based on all my readings and conversations with people who would know first hand, it’s better to be a little on the small side (2.5″) than too big (3″).

• Michael Walton
Jan 10 2012

Jason, Thanks for the reply the only 2 inch pipes in the system are the primary header pipes(this is quite a big size primary pipe size as I understand) I think i will stick with the dual 2.5 system as its only a street driven vehicle (I do drive it hard often though)People comment on how awesome the car sounds it currently has Flowmaster delta 40 series mufflers on it but im thinking of trying out the super 44s as i understand they have a deeper tone than the delta 40s(Can you comment on this?)

• Jason
Jan 10 2012

Michael – I’m sorry I misunderstood – 2″ collectors are quite big. No restriction there. π

I can’t comment on Delta 40 vs Super 44’s from experience, but I’ve heard the same thing. My honest opinion, however, is that you won’t notice enough difference to justify the expense. They’re both going to sound great.

• Michael Walton
Jan 10 2012

Jason, So basically unless im going to run the car at max revs the dual 2.5 inch system will lose nothing to a twin 3 inch system ? and may even be better for low down torque (Although the car makes massive torque anyway) thanks Michael

• Jason
Jan 10 2012

Michael – You’re going to lose something at 6000 RPM using 2.5″ pipes – the CFM requirements are higher than your current exhaust will allow at that RPM. However, in low to mid RPMs (4000RPM or less) you’ve got more than enough pipe capacity, and just as you say it’s actually a good thing to have smaller pipes in terms of off-the-line performance.

So, if you use the vehicle as described, than you’re better off with your current setup than a set of 3’s…assuming all the assumptions we’ve made are correct. π

• Kurt
Feb 25 2012

Hey Michael,
Your set up sounds nearly perfect! the one thing I can think of that will up your horsies across the entire rev range is to try siamesed pipes instead of a balance/cross pipe. Siamesed pipes are basically where the two pipes merge into one pipe for about a foot before splitting again. However on alot of cars it can be impossible to get room for two 2.5″ pipes next to each other at all!! Just a thought π

7. patrick holt
Jan 13 2012

thanks in part to the information you have provided ihave decided to go with a single 3 lnch exaust instead of a 2 or 2.5 inch dual setup all the information i have been able to find says i will gain torqe with a single 3 inch system thank you much

8. nick
Mar 21 2012

Hi there Jason,i’m running a toyota celica gts with rev at 8700 rpm,1796cc and the car has a valve lift at 5700 rpm.horsepower is 210,What is the best dimension for this car?the car has 3 inch air intake pipe from 2.75 and tuning ecu with lift from 6200 to 5700 and rev from 8200 to 8700 and my exhaust is 2,25 inch!!!nice article it helps my a lot but i have comfused…..

• Ivan
May 26 2016

I have a 2006 chevrolet equinox and I’m planning to dual exhaust, can you tell me which diameter i should get?

• Jason
Jun 2 2016

Ivan – Check the chart. π

9. Anthony
Apr 4 2012

I have a 1993 gmc sierra 5.7 350. I didn’t understand how to do this so i was wondering if you could do it for me. I plan on putting a dual exhaust.Can you tell me which diameter i should get?

• Jason
Apr 8 2012

Anthony – Is it stock? Factory HP was probably around 250…which means a couple of 2″ pipes should cover it. Now if you start doing mods and crank it up over 300hp, a set of 2.25″ pipes will work.

10. Francis
Apr 24 2012

Hello can u help me whit exhaust pipe size. I got 2,0 16v , 136 HP
car, whit cylinder size 1998 ccm, what size of pipe i have to put on for more power.
Help.

11. Larry Steller
Apr 26 2012

I recently bought a 1975 Mercedes 450SL as a classic but it idles rough. They have replaced plugs, fuel injectors and the catalytic converter in the exhaust system. None of this changed the rough idle. It wants to stall if the idle rpm is reduced below 1100rpm. Now they tell me that the exhaust manifold on the right side of the engine needs to be replaced. I understand that this car uses a “catalyst” as part of the exhaust manifold. Could this catalyst be blocked like the catalytic converter was thereby causing the engine to run rough? There is no missing at highway speeds or over 2000 rpm so I can’t quite grasp what role the exhaust manifold plays in this problem.

Thanks, Larry

• Jason
Apr 27 2012

Larry – I gotta tell you I have no idea. However, this guy – http://daveknowscars.com/ – is an MB expert. You might try looking over his blog and seeing if he’s got the answer to your question.

• Apr 28 2012

It’s been a few years that i last worked on a old classic merc.
Some of the fuel injection systems are trigerd by an old fashioned points system in the distributor, there s a block of them that comes out of the side of the distributor.
This may help, everyone overlooked the one i repaired and he had been to about 30 different mechanic’s including a merc dealership

Kind regards

Geoffrey

12. Matt
Apr 29 2012

Hey, just wondering if you can help. I have a clevo running 350-400hp, it currently has twin 2′ pipes out of headers. Would there be any advantage upgrading to 2.5′ Thanks

• Jason
Apr 30 2012

Matt – The advantage in increasing pipe size at your HP rating is better performance at high RPMs.

• Michael Walton
Apr 30 2012

I cant see where there would be any advantage even at high revs by running a twin 2.5 inch system with only 350-400hp on tap 2 1/4 would be more than enough surely

13. justin
May 5 2012

I tried doing the math but I’m falling between 2.5″ and 2.75″. Was looking at 2.75″ system but would like your advice. I have a 4.2L V8 FSI with 420hp that redlines at 8250rpms. I plan on doing a few bolt modifications that will bring the car around 480hp. What is your suggestion?

• Michael Walton
May 5 2012

HI Justin, See my discussion with Jason above im running 620hp on a dual 2.5 inch system so i think 2.5 inch max for you as a twin 2.5 is good up to 500hp I would run a twin 3 inch on mine if I was taking it to max revs but I seldom go about 4000rpm

• justin
May 5 2012

Michael,

Thanks for the response. Does the fact that my car redlines at 8250rpms make a difference? I did the calculations with stock numbers and im getting 610CFM which is right on the cusp on 2.75″. I usually shift between 3k-5k rpms but get close to the redline fairly often on my drives. (not a DD car) From factory, it comes with 2.5″ dual exhaust. Do you still think that 2.5″ max is right for me if I plan to do mods?

• Michael Walton
May 5 2012

Hi Justin, I can see no advantage having a dual 2.75 inch system in fact most of the time you would be losing hp and definately low end torque, im making massive torque right through the rev range on a dual 2.5 inch but lose hp at max revs but then were comparing a 4.2L to a 9.3L im producing 500hp at 4000rpm and my system if great even at max revs your not producing more than 500hp. What size header pipes and collecters do you have ?

• Jason
May 7 2012

Agreed! Thanks for commenting!!

• justin
May 9 2012

Not sure. I looked around for it but couldnt find any dimensions. I know the dual stock downpipes and exhaust pipes are 2.5″. No idea about the header size. I know they are 4-2-1

• Michael Walton
May 9 2012

Justin, dont worry about headers im sure they are fine dont waste money on going to a 2.75 inch system and then end up losing hp and torque the opposite of what you are trying to achieve

• Jason
May 7 2012

justin – I agree with Michael. Better to go with the smaller size if you value low-end torque.

14. Kyle
Jun 20 2012

ok guys I need help . Ive got a 67 olds with a 350 ci motor and 455 edelbrock heads. motor should be pushing 460 to 500 hp. I want louad but deep sound and I want the best preformance i can get . if it matters ive got a set of hooker headers on it now. what muffler sould I go with and what size ? and should I go center to center or a off set chambered muffler….. I wandt the sound
thanks guys

• Jason
Jun 21 2012

Kyle – You can calculate size by looking at the CFM figures on the mufflers or contacting the manufacturers. As for offset vs center to center, I don’t think it makes much difference…but if I had to choose, I’d go center to center.

15. Nqobile Nkomo
Jul 14 2012

I have a rammer that melts plastic close to the exhaust. Could the engine be running rich or lean ? Or the mufler is restricted

• Jason
Aug 20 2012

Could be that the mix is off, but it might also be that the plastic is too close. Is this a factory part that’s melting or an after-market part?

16. Brendan
Aug 20 2012

The sizngs are so incorrect it’s not funny.. my car has about 270hp stock and has a 2.5″ with a crush axle back.. even a better (catless etc.) 2.75″ exhaust is good for 330-350hp N/A.. should see 350 *W*HP out of the OEM exhaust even with forced induction.. and that’s with cats in the headers and 4 cats total -_-.. Evo’s run 2.5″ up to about the same and even a single 3″ is fine for 450-500hp

admittedly the OEM exhaust i run is twin 2’s (or 1.75 or something) off the headers into a single at the diff.. but still. you should mention more about the design not neccessarily needing a consistent piping size to the end like this.

• Jason
Aug 20 2012

I don’t follow your comment – not sure what you’re trying to say.

Also, bear in mind that the article says best way to figure out the proper size is to do the math…the chart is a shortcut.

17. akshay
Sep 15 2012

Sir,
I m interested in silencer . but I am having very little knowledge about silencer . I donβt know how to calculate pipe size and holes maid on pipe of silencer and how to calculate the chamber in silencer and all regarding silencer . I m having esteem car . and I have to modify the silencer and make it more silence instead of noisy . can please guide me for this silencer developing and if there is a book covering my requirements can you please suggest me . I will be thankful to you .

18. Ej1 Coupe
Sep 20 2012

I have a 93 civic about to have 300-350 hp on my d16z6 will i need a 3″ exhaust or can i get away with 2″ 3/4″?

• Jason
Sep 20 2012

I would go with 2.75 inch rather than 3 on a 4cylinder. It’s better to preserve low-end torque.

19. Kirt Peters
Oct 13 2012

hi,i’m looking into a catback system and i’m looking at the chart and were it shows the max hp for a dual system is it true dual from the manifold back with 2 converters and 2 mufflers or is it dual catback?

Kirt

• Jason
Oct 14 2012

Kirt – As far as the chart is concerned, it’s not specific to cat or muffler back – it’s based on volumetric gas flow rate.

• Kirt Peters
Oct 15 2012

thanks jason, so are we to assume the chart is assuming the flow rate using just straight pipes not taking into account it has any converters,mufflers or am i just totally missunderstanding the chart altogether?

Kirt

• Jason
Oct 15 2012

Kirt – The chart shows you what approximate size of tubing you need for a specific engine HP, based on the amount of exhaust gas an engine typically produces for any given HP. Since it’s based on the amount of exhaust gas that has to flow through the tube, mufflers and cats don’t matter…it’s just flow.

Think of it as a water pipe – a 1″ pipe is only going to flow a certain amount of water at any given pressure. If you want to flow more water, you have to make the pipe bigger. It also doesn’t matter what you put on the pipe anywhere in the line either. You can add valves and tanks to water pipe, but those things aren’t going to increase water flow because the only thing that matters is pipe size. Make sense?

• Kirt Peters
Oct 16 2012

Jason, yeah it makes sense and thanks again for the explanation…always better to have it explained more in depth or a different way since we all don’t have the same translation abilities.

Kirt

20. Kevincarothers
Oct 16 2012

Have cad eldorado with 3″ from engine back! No cat no mufflers with 5 inch outlets! Might loose back pressure, who cares, NORTHSTAR comes to life!!!!!!! Try it, best of both worlds!!!! Thanks kev

21. Jason 2
Oct 25 2012

So i was wondering what formula did you use to get the pipe area. I want to see what the larger diameters flow. I may have to use a 4″ exhaust to achieve the power goals that i want but i wanted to do the math to make sure.

• Jason
Oct 29 2012

Jason 2 – Sorry man, I don’t have a simple formula to calculate flow by diameter. It’s actually quite complex, which is why it’s not listed here. If you’re looking for an easy way to calculate pipe area, it’s Pi times the square of the radius (remember Pie – R – Square from school?) π

22. Oct 30 2012

Hey, ok so I’m not fully understanding, this but I’ll tell u my set up and maybe u can help me! I have 2008 dodge ram 1500 with a 287ci motor stock hp 310, stock tq 330 but I have about 40 hp/tq extra of bolt ons. Now I installed a 2.5″ pipe to a y pipe to twin 2.5″ pipes going to five inch tips. I don’t know why but I personally feel like I lost hp/tq after this install? Would it be better to just go single 3″ since I’m steady adding more performance parts also is why I’m asking I’m not trying to rob performance, over sound? Hope I explained this good enough, thanks Donald

23. JAN
Nov 12 2012

I HAVE MORE OF A QUESTION, I HAVE A 99 DODGE DURANGO AND INSTEAD OF THE PREVIOUS OWNER PUTTING THE CORRECT SIZE MUFFLER ON IT HE WENT WITH A MUFFLER THREE SIZES SMALLER. SHOULD I CONTINUE WITH THAT OR GET THE CORRECT SIZE AND WHAT IS THIS DOING TO MY TRUCK?

THANKS 4 ANY SUGGESTION

• Jason
Nov 12 2012

Jan – A small muffler could be restrictive relative to a normally sized muffler, only it depends on design. If it’s a flow-thru design muffler, size isn’t a huge concern.

• Jan
Nov 12 2012

Thanks Jason, I guess if its not broke dont fix it.

24. John
Nov 15 2012

I have a stock 4.8 gmc sierra with 2 3/4 pipe exhaust. I has told the muffler robs my truck of mpg and horsies. I want to put a new muffler only on and can’t find any after market exhausts that are 2 3/4″ should I replace the muffler with a 2.5″ or a 3″ and when doing so should I keep the 2 3/4″ tail pipe or replace to fit the new muffler. will going from 2 3/4 pipe to 2.5″ higher flow muffler back to 2 3/4″ tail pipe work?

• Jason
Nov 15 2012

John – I think I’d go the other way (up to 3 then back down to 2 3/4) but I doubt it makes much of a difference either way. Whichever muffler is cheaper. π

Having said that, you might check the price of necking up and down vs down and up. Could up that up and down is cheaper.

25. Jason
Dec 6 2012

I’m currently in the process of swapping my 97 Civic 1.6L SOHC engine to a 2.0L DOHC engine approximately (170hp @ the flywheel) and I am in need of a custom exhaust but I want it to be a very quiet exhaust. I will have a header with a 2.5″ collector and I was thinking I could go with a 2.5″ non-mandrel bent exhaust (essentially 2 1/4″ right?) into a restrictive muffler to keep it all quiet. Would it be best to go with a multiple baffle muffler or a turbo style muffler? Also is my exhaust size right or should I smaller or larger?

Thanks,
Jason

• Jason
Dec 6 2012

Jason – Your plan sounds good to me. I might think about going with a slightly smaller pipe (2.25) that was mandrel bent simply because a smaller pipe will perform better…part of what you’re trying to avoid is having the exhaust gases cool too quickly. If you go with an oversized pipe (even if the bends are a little smaller), you’ll get some of that negative cooling effect you’re trying to avoid. This can reduce low-end power.

As for muffler style, I don’t have any guidance other than to look at Hushpower mufflers. They claim to be quiet yet still high performance. The trouble with after-market mufflers is that they’re almost always loud.

• Jason
Dec 10 2012

I can get crush bent exhaust for a very good price. Would I hurt my fuel economy and/or power production too much by going with 2.25″ crush bent piping with an 18″ inch resonator and a 2.5″ oem replacement muffler (since it’s going to really restrictive)? Or should I go with mandrel bent exhaust at a bit more costs?

• Jason
Dec 10 2012

Jason – There’s not a HUGE difference between crush-bent and mandrel-bent tubing, but mandrel-bent is always better. I’d say go with whatever makes the most financial sense, especially if you’re going with the OEM muffler.

26. Ray
Dec 7 2012

Great website, ive been looking for ta resource like this.

I have a question. Ive built a 4 cylinder MGB GT engine 142bhp@ at the back wheels, its a very tuned engine std they were 95 at the flywheel which probably gave 65-70 BHP at the back wheels. Im currently running a 2″ freeflow system with a rear muffler (Moss Tourist Trophy system). Id like more of a low down throaty sound (MG`s tend to sound like farts in drain pipes).

Would a larger backbox help? Or would that two box system have helped with that? Perhaps a 2.5″ system would have been better?

I chose the single back box as I had read somewhere that the rear muffler gives more throat and the front creates a higher pitch?

Ray

• Jason
Dec 8 2012

Ray – The very best sound I’ve ever heard out of a 4-cylinder exhaust is fairly muted and likely restrictive. If you’re looking to get good power AND good sound, I don’t have any ideas. I’m not aware of any tricks to make a 4 or 6 sound like anything other than a 4 or 6, at least outside of idle.

27. Jason
Dec 15 2012

I currently have a CRV engine going (approx 170hp at the crank) into my 97 Civic and I was considering a custom dual exhaust (1 3/4″ or 1 5/8″ into a dual inlet and dual outlet muffler. Would the dual exhaust help quiet my exhaust any?

• Jason
Dec 15 2012

Jason – Probably not. The frequencies that generate the most noise aren’t really attenuated by additional tubing. I’d also say that you’re trading exhaust noise on just one side of your vehicle for noise on both sides.

If you want to reduce noise, a good trick is to point your exhaust tip(s) down towards the ground, and to run them out of the side (behind the wheel) rather than straight out the back.

• Jason
Dec 15 2012

but would my car be quieter with two 1 5/8″ pipes run into one muffler or maybe two mufflers rather than one 2 1/4″ or 2 1/2″ pipe run into a similar sized muffler?

My goal really is to maximize flow/velocity without too much or too little restriction but most importantly I don’t want my car’s sound to be loud or droning at all.

• Jason
Dec 16 2012

Jason – If I understand you, you’re contemplating a custom dual inlet and/or dual outlet exhaust. I’m also guessing you have a V6. A dual outlet won’t be any louder or quieter than a single outlet if it’s coming from the same single muffler (see my previous comment, where I was assuming you had a 4cylinder). As for a dual outlet coming from two separate mufflers (i.e. a “true dual” system), than a dual exhaust will probably be harsher than a single.

This is because the V6 exhaust note is inherently unbalanced, as opposing sides of the motor are detonating an unequal number of cylinders at any given time. Therefore, splitting this exhaust up into two completely separate tubes is going to give you a raspier, harsher sound than you’d get from balancing both sides by running them into the same muffler and/or using an x-pipe.

Based on the fact that your vehicle has a relatively small V6 (I’m still wondering if you have a 4cylinder, quite honestly), I’d run a single muffler, single exit exhaust system and then just try to buy a good muffler that isn’t too loud. Hushpower is supposed to be a quiet performance option, but I don’t know if they have anything that fits your CR-V. Good luck!

P.S. If you have a four cylinder, there’s nothing to talk about here…single in and single out is the only way to go. Anything else would be silly. π

• Jason
Dec 17 2012

My car is and will be a 4 cylinder from a CRV.

My line of thinking is that if I split my exhaust after the header then I have smaller pipes which in return I think should be quiter then I add two resonators which in return makes it even quieter then I can two mufflers to each pipe and now it’s even quieter but I was also thinking that I could run both pipes into a dual inlet and outlet muffler and hopefully still keep a quiet exhaust.

• Jason
Dec 17 2012

Jason – As I’ve said, there’s no benefit to extra tubing. One resonator and muffler will work just as well as two. It’s not a “bigger is better” situation because a 4 cylinder doesn’t pump out enough exhaust to require two of anything. You’re over-thinking it. π

• Jason
Dec 17 2012

I think you misunderstand because I’m thinking smaller pipes versus one larger pipe. Your chart shows that two 1 5/8″ pipes flow the equivalent of one 2 1/4″ pipe and the 1 5/8″ is what I’m getting at.

• Jason
Dec 19 2012

Jason – I’m not misunderstanding. You want to take the single exit on the stock manifold, split it in two, and somehow reduce the noise level without effecting performance. Unfortunately, it just doesn’t work that way – one pipe (and one resonator and one muffler) is all you need. No amount of tubing is going to reduce noise any further than a good muffler (the frequencies that are attenuated by extra tubing aren’t impactful on overall sound levels), and whatever extra tubing you add will likely hurt performance too (if exhaust gases cool too quickly, you’ll kill your low-end torque). Don’t screw around with it – focus on finding the best muffler you can.

28. rob
Dec 19 2012

so explain this to me please anyone. if these car geniuses calculate this the why does my suburban and dakota have stock 3 inch exhaust. suburban 454 about 350 hp. understandable. dakota 3.9 v6 180 hp. shouldnt there be a 2 1/4 to maybe 2 1/2 in. please explain.

• Jason
Dec 19 2012

Rob – Who knows but a Dodge engineer? I could guess that a) only the tip is 3″ or b) that the pipe size from the manifold to the muffler inlet is much smaller, but the fact is I have no idea.

If it helps you, Chrysler is a three-time loser. They went under in the 80’s, then went under in the early 2000’s when Cerebus Capital bought them out, and then they needed a federal bailout. Chrysler didn’t fail on three separate occasions just because they had some bad luck…

• rob
Dec 20 2012

thats why the dodge costed 500 bucks. its 3 inch from manifolds to muffler and 2 1/2 out. i kinda wanna do a 2 1/2 all the way through but really how much money do i want to throw away. i mean dodge really isnt what you want to tell people you own but the fact of the matter is its 4×4 and works good 92000 miles and no mechanical problems. MIGHT last another 8000 miles.lol.

29. Darrell Smith
Dec 22 2012

Hi
I was just wondering, I have 2000 Mercury Grand Marquie that I bought used but in great condition with only 140,000KM on it. I installed a K&N recommended custom cold air intake with shroud and I also installed a Helix throttle spacer on the throttle body. The car came with a single exhaust from factory so I had a custom dual put on by the performance shop in my area. We cut the original Y and made 2 separate exhaust by adding the left drivers side. We put a regular muffler on the left side and kept the original on the passenger side. The performance was excellent and the car would squeal the tires even with the traction control engaged. We kept the pipes and everthing normal diameter. I missed my performance mufflers ( magnaflows ) that I had on my 1993 mercury 4.6 ltre that I finally scrapped at 357000km. I decided to put the same type magnaflows (tube type) on my 2000 Mercury. The car sounded really deep in tone but when I started driving for about a week I noticed that I didn’t have the same power and my fuel consumption increased. Is this because of not enough back pressure? I am seriously thinking of going back to the normal standard mufflers if this is the cause. The performance guys say sometimes you comprimise power for nice sounds but I am not sure? Any ideas would help. Car runs perfect otherwise. Thanks Darrell

• Jason
Dec 23 2012

Darrell – There are a lot of reasons that you could feel as if your car is slower and getting worse gas mileage. First, it’s the winter time, which means that fuel economy is always lower (cold engines are less efficient, and winter fuel mixes aren’t as good as summer fuel mixes).

Second, the “butt dyno” – i.e. your personal assessment of power – just isn’t very accurate. It’s hard to tell how fast something is just by feel.

Still, any time you hack together an exhaust system, you risk creating something that doesn’t perform as well as the stock system. This is why I recommend either a) going with a quality after-market cat-back kit or b) working with an experienced performance muffler shop that knows about tuning.

If your current muffler shop says that “you have to compromise power for sound,” find a new shop. That guy couldn’t be more wrong.

• Darrell Smith
Dec 23 2012

Hi Jason.
Thank you for the reply. I just wanted to give you a little more detail to avoid confusion as maybe I didn’t explain enough. The guys that did my 2000 Mercury Grand Marquie are the same custom shop that did my 93 Grand Marquie. They have been doing every type of car out there and have a great reputation as far as their work and critical details involving all their installations. All their systems are welded and in most cases better than factory. They do not install junk but only top of the line products. The comment about sometimes you sacrifice a bit of performance when you change some of the original factory equipment. They did all of this work in the summer and even though the 2000 Mercury I have now feels much quicker than the 93 Mercury, I only seemed to notice the feel of the car being different since I went over to the recommended Magnaflows that were installed. Is it possible that because the 2000 Mercury is more solid and quiet that when I switched exhaust I became more aware of the engine sound which I never really heard with the factory single exhaust. I know when they installed the duals they cut off ( which seems to be standard ) the damper weight that was just after the Y. When the duals were installed and perfectly matched except for an additional new standard muffler and the original factory I was getting a vibration at idol in Drive with the brake on. The opinion of a few custom shops was that I shouldn’t need these weights and even they cut them off when they insall duals. My performance shop thought it could be because of the mismatch between mufflers. I bought new weights and had them installed on both sides of the duals and this solved the problem. I don’t know if the match mufflers would have made a difference without the weights as I never bothered after the problem was gone,
My 93 Mercury also had the original 4 converters removed and we put on the mustang cats (only 2 ) and then took everything to the xpipe and then to the magnaflows (oval ). My 2000 Mercury sounds really good right now but I don’t know if some back pressure is needed as they say most of these cars have tuned exhaust when they are designed and is there any possibility that maybe I have disturbed this equation since installing the magnaflows? Jason I am just trying to figure out if it is me or has the car lost some additional punch that I noticed when I went from the duals ( standard mufflers to magnaflow tube type). We also added an H crossover connection between left and right but it made very little difference. If you get to know how a car runs and feels as we all do after time I’m sure if something changes you will notice it. I felt right after I drove the car away from the performance shop that it just didn’t seem to have the same snap. ( Maybe I’m just losing it but most of the time I have always been correct when something changes in the characteristic of the car that I am use to driving ). This definetly changed something but no one ( other shops ) have any idea? I would like to know if any of your members have ever come across something like this when they modified their cars from muffler changes be performance or otherwise! Thanks Darrell Smith Toronto.

• Jason
Dec 24 2012

Darrell – I hear what you’re saying, but in all honesty I’d find another shop. Here’s what I know:

1. There’s very little reason that removing the factory exhaust and replacing it with after-market components should reduce power. At worst, it should make no difference…and on a Crown Vic/Grand Marquis, you should be able to add power with an exhaust relatively easily. This “sometimes you lose power” comment is alarming.

2. Connection quality (i.e. how great they weld things) has very little to do with tuning ability or performance.

3. Adding an H-Pipe to a V8 makes absolutely no sense…if you’re trying to get rid of a vibration you could try it, but I would focus elsewhere first.

4. If your car felt fast when you first had it, and is now slower, it could be that the mufflers have become clogged. Did they check that?

5. Swapping out the stock catalytics for Mustang catalytics doesn’t make a lot of sense to me either. Modern cats aren’t very restrictive at all…unless you’re strapping a supercharger onto the engine, you don’t need to touch the cats.

6. There’s a whole lot of rigging and trying going on…good shops don’t need multiple tries to get it right.

Again, for almost all the reasons listed above, I’d say your shop isn’t very experienced. I’d contact your local racing clubs and ask them for a recommendation.

• Darrell Smith
Dec 24 2012

Hi Jason.
I guess there is still some confusion with my explanation so I will try to simplify what has been done.
Just the 2000 Grand Marquie
1. Car was bought with factory single exhaust and was very responsive.
2. Added K&N cold air intake complete kit in summer (good response in performance and much improved.
3. Added Helix power tower to throttle body. (better mid range acceleration)
4. Had factory Y pipe cut and kept original factory right side exhaust complete and factory weight damper was removed.
5. Had left side exhaust installed without H pipe just straight through.
6. Now car had dual exhaust with no H pipe and the only thing different was original factory muffler on right and a new satandard muffler on left side. Car was responsive and only thing that changed was that at idle and in gear there was a small vibration at a certain frequency when stopped,
7. We installed the H pipe at my request thinking that it might help the small vibration but didn’t.
8. I decided to buy the left and right damper weights from Ford dealership andd had them welded on in the same place as the original factory exhaust.
9. Problem solved vibration gone!
10. Car running very well and performing excellent and very responsive accelleration.
11. Missed my nice Magnaflow sound so in about a month later I decided to install the new magnaflow tube type reccommended by Magnaflow for my car.
12. I got to hear them before they were welded in place and was very impressed, Low nice rumble but much louder if you hit the pedal.
13. Drove away after every thing completed and felt the car sounded great but it seem to lack the same punch. I thought maybe it is just me adjusting to the new system but again I can’t explain why the car just doesn’t feel the same.
You are correct in the fact that you can’t measure power and speed from your butt and I’m thinking maybe it’s the difference in the acceleration noise between original quiet mufflers to the magnaflows.
The comment about sacrificing power for sound was taken wrong. What I mentioned to one of the installers was that I felt my gas mileage was a little worse than before with the factory installed single system. His comment was more that like any HP improvement
sometimes you have to sarifice a little more fuel to get more response which makes sense.
I have a lot of respect for these guys and they do have an excellent reputation and I probably drive them a little crazy when I ask them why the car just feels slightly different since all the work. They even test drove it and inspected all the work and said it was running very well for a Mercury Grand Marquie and couldn’t see any problem.
I still love the sound so I don’t want to go back to the quiet mufflers. I also know Grand Marquies love cold weather and perform very well the colder it gets and yes fuel burns better in summer for sure.
Thank you Jason for your feedback and like I say it may just be me.

regards Darrell

p.s. A very Merry Christmas to you and your family!

• Jason
Dec 27 2012

Darrell – I understand more now. It sounds like you had a semi-customized factory exhaust that you replaced with two high-flow mufflers.

If the Magnaflow mufflers you added were designed for a single-exit exhaust, they’re likely too big for your dual system. THAT could cause the performance to fall off a bit.

30. Patrick
Dec 27 2012

I’m having to have a custom exhaust done to my car which will be making around 175hp around 7500rpm. I want a quiet exhaust but not too restrictive. I was thinking about going with an oem type muffler but I figure it will bottleneck the whole exhaust.

Would it be best to go with a 2.5″ piping out to a restricitive 2 1/4″ muffler or go with a 2 1/4″ piping out to a 2.5″ muffer? In other words where’s the best place to put my exhaust restrictions nearer to the header or near the rear at the muffler? Oh all piping will be crushed bent so the actual restriction size will be smaller.

• Jason
Dec 27 2012

Patrick – If we’re talking about a quarter-inch here or there, it’s not going to matter. I would go with a mandrel-bent system, however, as that does matter. I’ve seen data to suggest mandrel bending is just as important as finding the right size.

My general guidance: the factory exhaust system tubing is very close to the right size. When in doubt, stick with the same size.

31. bryanh
Jan 9 2013

very interesting. but how about pipes below 1.5 inches. im building a header for my 1.3l ford aspire motor. and i want it to be maximized for cruise rpm. i know your pipes hp rating were for max hp numbers, and cruise hp numbers are maybe 1/3 of the rated wot for that specific rpm. any idea on what size i should run for my primary pipes?

• Jason
Jan 9 2013

bryanh – I’d suggest going with the same size as the factory exhaust system – that’s size is usually optimal for less than maximum power.

• bryanh
Jan 9 2013

i cannot let myself believe that factory exhaust is maximized for cruise on any car. we as americans want to go fast, so our cars are designed with flowier than effienct exhaust systems. i may just have to do this test myself with some sort of pipe sizes, like 1.2, 1.1. 1. .9 and .8 inches. itd be a blast building 5 different but similiar headers like that. do you think those size primaries could get me the results that i need. reading about that tri y design got my hopes up a bit on wanting to try that on my car for fuel economy. in that case, can lower torque are lower rpms get me better fuel economy?

• Jason
Jan 9 2013

There are a lot of factors that go into header design besides tubing diameter.

My suggestion is to find a successful header design on a similar yet larger engine, then scale it down. If you try a bunch of different diameters without considering other factors (tube length from head to collector is really important), you’ll be spinning your wheels. π

32. rob
Jan 15 2013

bryanh actually jason is correct. the stock pipe is optimal for less than max hp. the factory stuff is meant to maximize power for normal driving conditions. although if you still build those headers would you be so kind to post those results so we can see what works and what doesnt? losing hp and torque doesnt mean you will lose gas mileage. alot of the reason people lose mpg with better hp is because the lead foot that comes with the power.

33. taylor
Feb 10 2013

Having got caught up with all the “bigger is better” hype I went with a 3″ dual (with x pipe) exhaust system on my full sized chevy Caprice. I built a 355 SBC that made 400 flywheel HP on the dyno and have a set of tri-y headers with a 2.5″ collector. The system has already been purchased and installed on my car. After reading lots I have a terrible feeling that I made the wrong choice on going with 3″. For one, the car is too loud for my tastes. I actually have a pair of resonators on order that I am going to install in hopes bring the volume down some. Hindsight I would have gone with a 2 1/2″ system, but I dont want to spend another \$800 on exhaust.

Can I except to lose a lot of low end power? Any advice?

• Jason
Feb 11 2013

Taylor – The headers are probably going to impact you more than anything, and they don’t sound like a bad choice. However, the only way to know what impact the dual 3″ pipes have is to test the vehicle with 3″ pipes and then 2.5″ pipes and see the difference. My guess? You’re losing a little bit of low-end torque. How much is hard to say.

• taylor
Feb 11 2013

Jason, between the headers and crossover the pipes are removable. I believe the drivers side is ~2′ and the passenger is 1.5′. Would it be be beneficial to knock these down to 2.5″ before the 3″ crossover? I picked the headers as they have good low end and midrange power, just not to sure why I went with the 3″ now that I think about it. The guy that manufactures the headers and exhaust system usually recommends people switch to 3″ around the 400FWHP mark. Live and learn I guess…

• taylor
Feb 11 2013

Forgot to mention I will need to be doing emissions testing where I live next year so I will be adding cats into the “hook up” pipes I mentioned above.

• Jason
Feb 11 2013

taylor – First of all, cats don’t effect performance nearly as much as people think they do, especially if you purchase racing cats.

As for dropping to 2.5″ before going to the crossover, I’d suggest getting rid of the crossover unless you’re trying to mellow out the exhaust note. Its not necessary on V8’s. That way you can run the right sized pipe all the way.

Still, if this system is all done, I don’t know that I’d bother. The slight loss of low-end torque isn’t going to make much of a difference on the street.

• taylor
Feb 11 2013

Thank you for your input Jason. And here I thought the crossover was the “way to go”… I think I will just leave it and the next time I build another car will do my research before hand rather than after!

34. Frank
Feb 11 2013

Hello
Love the topic and conversations i have read!!!!!
My question is on a 99′ regal gs 3.8 s/c. Currently stock, but finally have the money for mods. Car is rated at 240hp 280tq, planning on getting custom pcm, ported throttle body with spacer and a whole bunch of little stuff to help reduce the common knock retard issue with this engine. Planning on doing full exhaust, front p log to replace manifold, rear ported manifold, crossover tube, they offer a 2.5″ and 3″ down pipe with and with out cat, prob gonna go with out out cat and an o2 simulator. The only full exhaust replacement i can find is 3″ stainless with muffler and resonator optional. Will probably go with the resonator to kill that annoying rasp 6 cyl are known for. What do you think of the 3″ down pipe with 3″ mandrel bent exhaust, i will be running a smaller pulley not sure what size, guess it depends on where i go with the exhaust. I can buy all the bends and do a 2.5″ exhaust and get the 2.5″ downpipe…….after the mods prob lookin 330hp but with forced induction not really to sure how the math equation helps…..guess i will be around 12 psi, stock is at 8psi and im not sure what size factory exhaust is either. I do know the factory down pipe is extremely restrictive and just that swap alone is good power gains. What do you think……

• Jason
Feb 11 2013

Frank – There are lots of questions there, none of which have easy answers. Going with an after-market downpipe is smart, but going with a 3″ vs 2.5″? No idea. You can’t really trust the downpipe manufacturer’s dyno testing, but that’s probably what you’ll have to do to get an answer to your question.

As for a single 3″ tube system, I say go for it. That seems about right for a forced induction motor pushing 300hp.

• Frank
Feb 11 2013

Thanks for quick response, lol….i did get a little carried away i guess. Goin 3″ all the way!!!! Sad part is i just wish it wasnt gonna cost a 1,000.00 for a single pipe set up:(

35. Mihir
Feb 26 2013

i’d like to know the minimum pipe wall thickness which should be used so as to avoid drumming/vibration. also would this principle apply to motorcycle exhaust design?

• Jason
Feb 28 2013

Mihir – I don’t know if tubing thickness has much impact on drone…I’m sure there’s an effect, but I’m not sure that it’s the most economical way to solve these problems on a car (thicker tubing is both more costly and heavier, so using it for an entire exhaust system is economically out of the question).

However, many vehicle exhaust systems are 16 or 18 gauge steel, and I’d guess that this is a good thickness.

36. darren
Feb 28 2013

Hi,
I have a 393 cleveland with 600hp 7200rpm street strip what do you think??

37. Mar 24 2013

Hey Jason,

I have enjoyed visiting your site and reading your thoughts. I have a Q for you. I own a ’92 Ford Ranger w/ the 2.3 L, 4 ( and I LOVE IT )! It came w/ a single 2.25 pipe on it. When that wore out, I replaced it w/ a split/dual cat ( 2 pipes @ 2.25″ ) back system and a FLOW MASTER 40 muffler. I desired the look of the “functional” pipes as well as the deeper, and louder tone.

It sounds GREAT when idling and cruising but, when I “get on it” it trumpets because it’s a stock, non loping 4! I am getting ready to lift the body to make room for larger tires. This means, that the exhaust system will have to be adjusted so it looks good again.

This is the time to make mods! I want to use a HOOKER AERO FLOW muffler and 2, 1.25″ pipes. The engine rarely turns above 3000 rpms ( it has a 5 speed ) and am seeking more low-mid power gains.

Later this year, I will bore the throttle body, spacer, port and polish the intake manifold and install higher PSI fuel injectors. This will increase the flow into the motor. Do you think a larger 1.25″ pipe, might be a better choice?

Thank you for your time, just reading this Long Winded letter! I look forward to your input.

Sincerely,

• Jason
Mar 25 2013

Alan – If you’re most concerned with low-end power, than 1.25″ pipes are likely fine no matter what you do. I’d rather have a smallish system on a rock crawler, b/c smaller pipe diameters create more of a scavenging effect at lower RPMs…they just rob HP at higher RPMS. So, I’d leave it alone if I were you. π

38. Mar 24 2013

PS: In the letter I sent you a few hours ago, I think I may have said, going to a smaller 2.25″ 2.50″ exhaust pipe size. I meant; 2 @ 1.25″ or 1.50″ w/ future performance upgrades. Sorry for the confusion.

39. Mar 26 2013

Does the chart take in account for standard crush bending or mandrel bend pipe?

• Jason
Apr 1 2013

Michael – Excellent question. Mandrel bent, if I recall…I’d say crush bends are good for a 1/4 inch “hit”.

40. brian
Apr 6 2013

I have a colt rodeo 2.4 i (4g64) engine . viper branch 4into1 , and 50 mm pipe with free flow. it has lost power since putting the 50mm , and seemed to be a bit heavy on fuel. it had a 48mm pipe before . would this 2mm make such a difference? what size U recon / the 48mm sounded great and had good low down torque

• Jason
Apr 7 2013

Brian – Can’t imagine that 2mm would make such a big difference, but if that pipe included a new header, than all bets are off. Changing the header/exhaust manifold can make a big difference, especially if the header/exhaust manifold is tuned for top-end performance.

I don’t have a recommendation for a Colt Rodeo – no idea. π

41. jose
Apr 11 2013

THANKS FOR THE GREAT INFO!!!

Iam thinking of buying an exhaust
for a 280hp inline 6 bmw, its an after market from a company in itally “Ta Technix Sportauspuff “.

The exhaust starts out at dual 2in then to a resonater continues out with dual 2in to another resonater but it comes out to a single 3in to muffler with dual 3in tips.

Would this be an upgrade? or our they just trying to cash in?

Thanks Jason

• Jason
Apr 11 2013

jose – I’m not sure if the system you’re looking at is worthwhile or not, but there’s nothing about the description that gives me pause.

However, I’d hesitate to buy any system that didn’t come with some sort of expected performance improvement and a dyno graph to support that claim. Meaning, they should say “our system adds up to 10hp – here’s a dyno sheet to prove it.”

• jose
Apr 12 2013

Thanks Jason. They claim 30bhp π but no dyno sheet :(. Just wanted your thoughts, and thankyou.

42. Hagen
Apr 11 2013

I had a muffler shop splice in a pair of 2.25in magnaflow (11225) on my 2012 mustang 5.0. The pipe is 3in duals but he said I had less than 2.5ID. I know I have a pair of 2.25ID resonators as well. Did this guy just give me one heck of a bottleneck or should the mufflers be ok with my 412hp?

• Jason
Apr 12 2013

Hagen – I think it’s going to be just fine on the street, but it probably doesn’t help your quarter mile time to go this route. Might even have taken a little bit off (a tenth or two I’d guess).

• Hagen
Apr 12 2013

That’s good to hear. I’ve noticed an increase of lower end tq but haven’t had a chance to make runs or test the top rpm range yet. I do enjoy the added lower end though! Thank you for the info and piece of mind, it is greatly appreciated!

43. Frank
Apr 11 2013

You probably dont remember, but you replied to my comment awhile back. I have a regal gs with the 3800 series s/c, currently stock. We talked about a full 3″ mandrel bent system from manifold through tailpipe and decided sence there was boost involved it was good to go. Do to finances i had to cancel full 3″. From the resonator back it is 2.5″. The down pipe is the most restrictive part of exhaust along with a u bend that was designed for 2nd o2 sensor just past the cat. I ordered a true 2.5″ downpipe without cat and an o2 simulator. I was thinking instead of the 3″ downpipe being knecked down to 2.5″ it would be just as benificial just having 2.5″ all the way. With the pulley ill b putting out 11lbs of boost. This should b an ok setup? I really dont have any plans on very much more, except for the 4″ cai and custom pcm currently on order and mayb a throttle body soon.

• Jason
Apr 12 2013

Frank – Sounds good to me, but I’d recommend posting this question on a forum for Regal GS (or 3800 S/C) vehicle owners to see if anyone has any specific insights. However, as a general rule, 2.5″ vs. 3″ isn’t likely to make a big difference in the way your vehicle performs on the street. If anything, it will perform better at the lower RPMs with this setup.

44. Kim
Apr 15 2013

It is an interesting topic, But i have a feeling that it does not take into consideration for backpressure that are needed in non turbo petrol cars(too low backpressure = damanged valves and loss of HP). I am currently “making” my own custom, exhaust for my 206 2.0 GTI 159bhp@6700rpm(with the old worn out system) and are wondering if a 2″ is not better than the 2.25″ after the race cat, for the sake of backpressure and decreased volume of the colder gas? Or am i completely on a sidetrack here?

Is there any golden rule on how to calculate the proper backpressure?

• Jason
Apr 15 2013

Kim – It’s not really about back pressure, it’s about exhaust gas velocity. The heads and ECM are tuned for a specific ideal velocity, and your goal is to make sure the exhaust system hits that ideal velocity figure at the point you want it to.

Quite frankly, the headers/exhaust manifold is far more important here than the exhaust tubing. Does your header manufacturer have a recommendation? If not, you might try to match the size of the stock system, test it, then increase diameter and test again. Testing is the only way to know for sure, as I’m not aware of any good (yet simple) methods for calculating the ideal setup…only if you had the right modeling software like the exhaust system manufacturers do, I bet you could get very close.

• Kim
Jun 3 2013

You are right, there probably is not any easy way to find the proper backpressure, it is a topic i had no luck finding good info about.
The header i am using is slightly a modified 4-1 from supersprint(from old system), to fit the cat and middle muffler piece. I have a feeling that it is pretty optimal for the car.
I am still unsure if 2″ or 2.25″ is better, but for now i welded on a original middle muffler and it has a 2″ exit pipe.
If i can get info on if a bigger diameter can give alittle more bang for the buck, ill probably swap it to 2.25″ π

45. Tracy
Apr 20 2013

Jason, I see where you said “A good section of straight pipe will flow about 115 CFM per square inch of area” but how do bends in the exhaust pipe such as where it goes over the rear axle affect flow? According to your chart I have my exhaust pipe sized pretty close (460 hp into a dual 2-1/2″ system with an X pipe). The pipes are also mandrel bent. Do mandrel bends present any appreciable back pressure to the system that could skew the results shown on your chart? If so do you think going with a larger pipe (such as a 2.75″) would make up for it, or might that cause turbulence? What are your thoughts on a H or X pipe? Do you think they lower system back pressure any due to each side now having 2 exits to flow through instead of just one? Thanks for your time, Tracy

• Jason
Apr 22 2013

Tracy – Mandrel bends shouldn’t effect flow too much, as fluids “roll” along the surface of the pipe they’re in. As long as that surface remains smooth, bends don’t have a significant effect…so I’d stick with the suggested sizes.

Crush bent pipe, however, can flow much worse than mandrel bent pipe – as much as 20% according to some things I’ve read. While you could go with bigger pipes to compensate, it’s better to go with mandrel bending.

As for cross pipes (x-pipes, h-pipes, etc.) they’re useful on V6’s for increasing the lower frequency sounds, and some say they work quite well on smaller V8’s from a performance standpoint. However, they have almost no effect on exhaust flow, so my guess is that most cars don’t need them.

46. david
Jun 6 2013

I have a 2007 Chevy silverado 2500 HD newer body style with a 364 6.0L it’s a gas truck. I was wondering what aftermarket exhaust system I needed? I have a cold air intake system on it dose it matter?

• Jason
Jun 6 2013

David – Any of the popular systems will work, only you might think about going with a single side exit rather than a dual, as they tend to produce a little more usable torque (sometimes, depends on the system).

Whatever you do, make sure you buy a system that is tuned to your truck and offers dyno testing as proof.

• david
Jun 6 2013

So what sizing of an exhaust do I need for the truck how it is right now?

Jun 6 2013

I am now changing my entire exhaust system on my truck.
I have a Toyota tundra 4.7 I force 245 crank HP stock

I am putting headers, Cold air intake and electric fan. which gives me around 55HP to the crank. which brings me to est 300BHP

Now this is a V8 so the down pipe is connected through a Y or X.

According to your calculator i should be using a 2″ pipe
now is this the down pipe or the entire system?

because i was thinking 2″ down pipe and into the y then 2.5″-3″ cat back
or into a the X and back out 2″

• Jason
Jun 6 2013

Adrian – Dual 2″ or 2.25″ pipes all the way back will be plenty. Additionally, if you go with 2.5″ or 3″ duals after the cats, you may find that you lose a bit of power, as the exhaust gases will “backfill” that extra pipe diameter.

Or not – just depends.

I’m also thinking that you should dyno-test before you mount a system, as these accessories don’t always bring the promised power on an older engine.

Good luck.

48. david
Jun 6 2013

Ok so what sizing do I need for and exhaust?

• Jason
Jun 6 2013

49. David
Jun 21 2013

Gday, interesting read, but 1 thing I did not see, tho i never read the whole comment section, is distance.

A 400hp mini minor compared to a 400hp cadi would require a much different size pipe, yet you have not touched upon this.
Are you just talking larger American size vehicles, or just a general all round sort of set up.

Im a diesel man in reality (boats) and our exhausts are sized not so much by power, but by distance from engine to outlet or tip.

• Jason
Jun 21 2013

David – The tubing diameter recommendations are based on gasoline vehicle implementations, where it’s understood that the system must be long enough to convey the exhaust gases from the front of the vehicle to the rear. Thus, system length is really not important (once it gets longer than 4′, it doesn’t make a difference).

Additionally, motor displacement doesn’t really matter either. Most engines burn about the same amount of gas to generate the same amount of horsepower, which means the volume and temperate of exhaust gases on a 400hp 4.0L are roughly the same as a 400hp 6.8L.

Of course, having said all of this, there’s value in calculating things exactly…which is why we say the chart is a shortcut.

• David
Jun 21 2013

Gday Jason, thanks for the reply.
The analogy between the two cars was more in that one will have an exhaust pipe as long as your forearm, the other a mile long.

One little thing tho, a 400hp big block will use a hell of a lot more fuel than a very modern 400hp fuel injected computer controlled car.

Im not pickig at all, just trying to work out the best for my toy, there are so many different aspects to think of, and some, (proven) go against every thing that seems rational.

In my mind, I see the very lowest back pressure as being the best, well designed will cause a vacume during cam overlap, and so gain best performance with a full charge of clean fresh air, any form of back pressure will negate this. Im having a hard time following the accepted norm, that a smaller pipe, so restricted, makes for better bottom end torque.

I see individual, tuned length pipes as being by far the best, (zoomi pipes) but sent to the back of the car, way over the top expense, and fitting would be a logistical nightmare, but if do-able, surely the best. IF you could tune these so that a pulse was leaving the tip as the engine was lifting the exhaust valve, perfect.
Understanding that this would mean a small rev range of perfection.

• Jason
Jun 23 2013

First, a note: You’re 100% correct that older and less efficient engines will push out more exhaust per HP than newer engines. However, in my experience, it’s not significant.

Second, the reason smaller diameters work best at lower RPMs has to do with exhaust gas cooling. If you dump hot exhaust into a 2″ tube, it’s not going to cool as quickly as it would if you dump it into a 3″ tube. Cooler exhaust is more dense (and therefore heavier), which means an over-sized tube at low RPMs creates pressure.

The perfect exhaust system would vary tubing size with RPM…growing from a 1.5″ tube at or near idle to a 4″ tube at WOT. Since no one has come up with a good way to create this sort of system, we’re stuck having to choose one size.

Therefore, you go with something a little small if you want low-end performance, and something a little big if you want high-end. Make sense?

• Sam
Oct 9 2013

Jason, Actually I think it does.Thanks for taking the time to help me understand exhaust a little. Sam

50. Mitch
Jul 11 2013

So i never got a response on my question about my 4cyl Hilux Diesel. I have ruled out the Cone filter idea due to the amount of dust in this region but are there any benefits to be gained from Straight pipe or highflow exhaust replacing the huge factory muffler. Sound, performance, MPG?

• Jason
Jul 12 2013

Mitch – You never got an answer because the info you seek is readily available on the site.

The short answer is that there are benefits, but that they aren’t dramatic. Most people who add after-market mufflers see a 1-3% increase in power.

As for going with a straight pipe, it’s not recommended.

51. Jozeph
Jul 15 2013

Great stuff here, need some advice. I have a 86 Chevy K10, 4×4, engine is a early 90s 350. Engine is only slightly modified. Looking for the most performance, HP, Tourqe! From reading I’m seeing I should be looking at maybe 2″ pipe or pipes? Not putting in a cat just pipe and muffler or mufflers. So question is 2″ or slightly larger and dual pipes and mufflers or go into a single muffler? Looking for the tourqe performance. Also what muffler do you recommend? Not concerned with it being to loud.Thanks in advance!

• Jason
Jul 15 2013

Jozeph – I don’t have any muffler recommendation for you outside of looking at Flowmaster, Magnaflow, and Borla.

The best arrangement from a torque perspective is a single exit design.

52. Jimbo
Jul 16 2013

This is a great write up I would like to pick your brain though. I have a 402 with kooks headers 1 7/8 primary’s to 3 in collectors then my y pipe goes into a single 3 in pipe back over my Rearend to the muffler. I just SC the car and its putting down 792 to the wheels at 6800 rpm. Should I go to a 5 in mid pipe when the two 3 in pipes Y together then just take it to a cutout? Have no room for duals and can’t make it over Rearend with the 5 in

• Jason
Jul 18 2013

Jimbo – No idea. The best answer I can give you is to test a few different configurations. At those RPMS and HP figures, there are a lot of variables.

I’d also suggest your tuning is done with the track in mind (assuming you’re racing this thing). Tuning an exhaust for the top-end can hurt ETs.

53. Garret
Aug 6 2013

Hi.. I have a 2004 pontiac Vibe with a 1.8L 4 cylinder pushing out 123 hp.. I currently have 2 1/4 pipes from the cat back and a magnaflow muffler, with that current set up am I losing any power? What would be the best pipe diameter for my car with only 123 hp?

• Jason
Aug 6 2013

Garret – Can you see the table in the post above? It shows you how to determine pipe size by HP.

Suffice to say, I’d guess that you’re hurting performance with such an over-sized setup.

• Garret
Aug 6 2013

Do you know mucb of a performance gain I would get from adding a header and is it possible for me to remove the two cats from my car without having any performance issues?

• Jason
Aug 7 2013

Garret – Removing that cats might effect performance, mostly because the exhaust manifold is designed to accommodate them. If you swap out the stock manifold for a header and remove the cats, you might see a small gain – 5% seems like the upper limit (6hp, give or take).

This is why I say that removing the cats is a waste of funds: Contrary to popular belief, modern catalytic converters aren’t restrictive. They flow pretty freely, in fact, and unless you supercharge or turbocharge your vehicle, there’s very little reason to mess with them.

54. john
Aug 21 2013

I have a 2000 Chevy silverado 5.3 v8 all stock. I have true dual striaghts 2 1/2 pipe an 5 inch tips with no mufflers an cats an it’s the loudest truck so far. I wanted to make it as loud as possible any suggestions in pipe size or secrets ?

55. Robert
Aug 29 2013

hi there Jason
I have a Volvo s60 t5(250bhp) that has factory 2.5″ single piping.. I have removed the centre box(22″ long) and replaced it with a jetex straight pipe now this might sound dumb im stuck with the rear box
can I reduce the size of the factory piping to say 2″ and if I do will it have any effects on the performance seeing as I have removed the centre box…????
the original box is over 28″ long and the replacement I have been looking at is 16″ (I will also be using mandrel bent tubing)
I don’t really drive the car hard I just wanted to let the 5 cylinder turbo roar a little better
big thanks from a confused scots man

• Jason
Aug 29 2013

Robert – I wouldn’t recommend reducing the size of a factory exhaust pipe, as they’re typically right-sized. I’d say you need to stick to that 2.5″ diameter.

• Robert
Aug 29 2013

thanks Jason
I will keep look out for a bigger bore then

56. john
Sep 1 2013

hi jason
I have a 2000 Chevy silverado 5.3 v8 all stock. I have true dual striaghts 2 1/2 pipe an 5 inch tips with no mufflers an cats an itβs the loudest truck so far. I wanted to make it as loud as possible any suggestions in pipe size or secrets ?

• Jason
Sep 3 2013

John – Not really, sorry. Never tried to make them loud. Just fast.

57. Jari
Sep 6 2013

Hi Jason,
I have 355sbc.It has about 400fwhp.Exhaust is SLP tri-y’s(1 5/8 primaries to single 3″ collector.Rest exhaust is single 3″ pipe to single dynomax dual outlet muffler.Do you think is exhaust on small side or not?

• Jason
Sep 6 2013

Jari – As always, the best way to answer your question is with testing. However, a single 3″ pipe is likely just fine for normal street driving.

If you’re trying to maximize your 1/4 mile time, I’d test adding a bigger (3.25) section of tubing between the collector and muffler. It will probably reduce performance off the line but improve flow at higher RPMs. But again, if we’re talking about a street car, I wouldn’t give it another thought.

• Jari
Sep 7 2013

Jason thanks for the answer.Yes it is street car,I keep it as is.
Ps.measured backpressure at wot(6500rpm) was 15kPa (2.17psi)

58. Jaxon
Sep 23 2013

Currently I’m driving a Nissan Versa 1.6L.
Current setup is 4-1 Extractor > 1 3/4″ pipe > middle straight thru bullet > 1 5/8″ pipe > 2″ muffler.

There’s torque/pickup during the first 2000rpm.
After that flat torque in between 2000rpm – 3000rpm.
3000rpm above start to accelerate.

It is too much back pressure in between?

• Jason
Sep 24 2013

Jaxon – No idea. Can’t diagnose backpressure using any of the info you’ve given.

My advice is to test a larger setup and see if it helps…or just stick with the OEM size.

59. Sam
Oct 9 2013

I don’t know if you can help me but heres my question. I have a suzuki hayabusa. It came with duels from factory but now has a single exhaust.I’m trying to get a descent sounding shorty muffler for it, but dont want to hurt the bike in anyway. The bike has about 158 hp and the header pipe size is 2 1/4 in. I’ve been looking at a Coffman exhaust but they only come as pairs. I was wondering if I bought the pair but only used one would it be safe for the bike,also what might be the limition that I need to know so I can address them with the company? Thanks Sam

• Jason
Oct 9 2013

Sam – Why does your bike only have one exhaust pipe now? If it’s a relatively standard mod, than you should have no trouble finding a muffler designed for a bike with a single exhaust…

• Sam
Oct 9 2013

Jason, At some point before I bought the bike someone changed it over to a single exhaust header. When I got it,it had a yokoshima single exhaust . I just put on a 300 tire kit with a 12in stretched swingarm. A standard exhaust can not be used now because the swingarm is in the way and it can not be mounted as it was, so I need a shorty exhaust. I’m just trying to find something that won’t hurt the bike. If I understand things,as long as the muffler has a connecting pipe the same size or a little bigger than the header pipeit will be ok. Is that right?

• Jason
Oct 9 2013

Sam – A good rule of thumb is that an exhaust system can’t hurt your bike at all mechanically, and that going with whatever tubing diameter you have now won’t change things much.

I’d say that you want to make sure whatever muffler you buy is designed for a bike like yours with a single exit…most muffler manufacturers can recommend a couple of options (either on their website or by calling their 1-800 number).

• LC
Jul 21 2014

Im in same situation…but i have stock header with duel outlet and i caped one of outlets off, so now only one is used with an stubby can i bought. Will there be any issues running like this till i can afford a single sided header? or is there no need can i just run as is and just do PowerCommander tune?

60. Yuki
Oct 19 2013

Hey Jason, I recently bought a resonator and a muffler without doing much research on diameters. The resonator was 2.25 in and the muffler was 2.5 in what effect would that have on my car if I welded it on there with the stock piping being 2in? Thanks!

• Jason
Oct 21 2013

Yuki – Shouldn’t have a ton of impact, but I’d try to get the right size if you can. It’s cheaper and faster.

61. Frank
Oct 21 2013

Hello Jason,
Back again, we have discussed my regal and now its time for the toyota pickup. Im setting a 6.0 ls1 bored and stroked to 409, square port 2.06 heads and supercharged. Low boost around 600hp. According to ur chart dual 3″ is good for just over 600. Not quite sure what its gonna b putting out exactly yet, but im looking forward to that 650+ mark. Dual 3″ should b enough Right, around 7,500 rpm is the range im looking at. According to the mathematics i should b ok, mayb a little under sized for the top end being forced induction. But i dont want it too loud. Maybe go a little bigger on the pipe to run a quiter muffler to make up for loss of flow?

• Jason
Oct 21 2013

Frank – My only thought is that you supercharger will hide any potential problems with exhaust gas restriction at the lowest RPMs…going to be very hard to notice the difference between a 3″, 3.25″, and 3.5″ tube (at least in my opinion). As always, testing both a 3″ and 3.5″ setup would be ideal, but I don’t know how feasible that is.

If you’re looking for a shot from the hip from me, I’d say buy whatever size you can get that matches the muffler you want. π

• Frank
Oct 21 2013

Jason,
Thanks for the quick response. Im gonna try the 3″ starting off and c how that works. Get some hp #s and go from there, here is my hardest decision…..WHAT MUFFLERS TO GO WITH:/
Frank.

• Jason
Oct 21 2013

LOL – I hear you. Good luck.

62. Anthony
Nov 7 2013

There should also be a section for single exhaust pipe systems.

• Jason
Nov 13 2013

There is. Check out the table again. π

63. Praisedeath
Nov 19 2013

Currently have a 2008 Pontiac G8 GT with the 6.0L Bought the car with headers w/o cats. I had a exhaust shop in town to take out the mid muffler and added a x-pipe (the butcher it all up) Had the magnaflow mufflers taken off and add flowmaster 40 series, I know that most people wont look under the car, it looks like crap. I was wondering if i should keep it (i think its 2.5″) or go up to 3″ exhaust pipe. Currently exhaust is a true dual setup. Hoping for a different cam and such in the future.

• Jason
Nov 20 2013

Praisedeath – Seems like your exhaust is big enough right now, but you can read the chart as easily as I can. π

64. Mike
Dec 2 2013

I’m currently building a 383 for my truck and I have a few questions. Idle to 5000 rpm motor and I’m anticipating the 350 HP range and 450+ tq. I plan on using 1 1/2″ primary port matched customized headers (mid length) and dual 2 1/4″ mandrel bent exhaust with magna flow center/offset straight thru mufflers (I want plenty of low and midrange power but I don’t want to fall off up top).
1) Is 2 1/4″ gonna be big enough in towing/hauling situations?
2) Do you see any benefit in a merge style collector (2″ outlet transitioning back to 2 1/4″ or 2 1/2″ final outlet? I’ve been told they increase low end torque.
3) I planned on installing an H-pipe behind the collectors. It is my understanding that they help balance the uneven pulses in the v8 firing order and also give a deeper, throatier sound to your exhaust but I get the impression from reading your responses that you’re not very supportive of a crossover on v8 dual exhaust. Can you expand on this?

Thx, Mike

• Jason
Dec 2 2013

1) Probably so…the times you need power are lower RPMs.
2) Yes. I’ve seem some data to suggest these 4-2-1 collectors facilitate scavenging at lower RPMs (depending on the length of each tube).
3) I’ve never been a big fan of h-pipes on V8s.

First, the pressure waves are coming at the same time from both sides of the motor on a V8, so the sound is basically balanced already. There’s probably a slight diff. in the distance each wave travels, but it’s got to be miniscule in such a short span of tubing.

Second, the h-pipe is only going to have meaningful airflow if the pressure on each side of the pipe is different. Since the pressure waves are basically coming down each side simultaneously, there’s little to no flow…and thus few benefits in terms of sound.

So I say skip it.

65. Alberto AAA
Dec 8 2013

BMW 3.0 straight 6 (S50) with about 270 crank HP at 7000 RPM. I currently have stock exhaust manifolds and OEM dual cat/pipe setup with a gutted muffler. I will be installing a factory header and would like to switch to a single high flow cat and pipe setup to a single Borla muffler. Would cat back 2.5″ pipe be suitable?

• Jason
Dec 8 2013

Alberto – Can you see the chart in the post?

66. Hoss
Jan 9 2014

I am trying to figure out what size of exhaust pipe to run on my 3.4 6cyl. it is stock out of a 1995 Firebird. The stock header pipe is 3″ To me that’s a little big.
It comes together into a single pipe. I am not running a cat. and am wanting to go with a 40 series flowmaster muffler.
Would I be best to go with a 2.25 pipe into the muffler and out as well?

• Jason
Jan 10 2014

Hoss – The chart is pretty clear…you look up the horsepower of your engine (which, btw, I have no idea what you’re running) and then the recommended size is shown. If you’ve got a 6 cylinder, a 3″ is probably too big and a 2.25″ muffler is probably OK. If you’ve got a V8 (or you’ve hopped up your V6 a lot), than 3″ might be OK and 2.25 might be too small.

67. Jan 12 2014

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68. Michaeltherod
Jan 12 2014

alright,i don’t get this,i have a ford 460 modified,with 650 carb and i produce 550 hp. what size pipe do i need? please help

• Michael Walton
Jan 12 2014

Hi Michael,I’m running a 572ci Chev producing 650hp and run twin 2.5 inch pipes, unless your racing yours using twin 2.5 would be ideal as you won’t be able to push your car on the street beyond that pipe size capability anyway and going bigger will cost you low end torque not to mention the cabin noise problem and if you do drag it you can just remove exhaust from collectors back anyway.

• Michael Walton
Jan 12 2014

Hi Michael,I’m running 650 HP through a twin 2.5 inch system and would suggest you do the same unless its a drag car but then you can just remove exhaust from the collectors back,you won’t be able to push your car on the street beyond the capacity of a system that size and will have optimal low end torque

69. onnaj
Jan 21 2014

Hello,

I’m wondering what exhaust i should take for my Toyota 4AGE 20V Blacktop. It’s a 1.6 litre with a max stock rev of 8200 rpm.

I changed to this engine from a stock 1.8 7AFE engine with 115 hp

The setup i have now is:
Header: 4-2-1 design, 1.75″ per pipe
Downpipe: Stock downpipe from the old engine. Ends in one pipe with diameter of 1.5″
Catalytic converter: 2.5″ Stock Celica
Middle section: 1.7″
Rear section: 1.5″

I did 148 crank hp with this setup, but the engine normally should produce about 165 (though this might be a bit overrated by Toyota ;))

I was thinking about the next setup:

-Header -> Stock is just fine
– Downpipe -> Use the stock 4AGE 20V downpipe, 2.25″
– Catalytic converter -> Stock 2.5″
– Middle Section -> custom 2″ stainless steel
– Rear section -> 2″ stock muffler from the 2.0 3SGE Celica. Fit’s 1:1

My question is would this setup be sufficient for the Blacktop?

My first idea was to make the middle section 2.25″ and the rear section 2″ so the velocity in the exhaust will be good. (I heard more often that a smaller rear diameter will be good for that purpose,)
Problem is that over here the 2.25″ isn’t common, so it’ll be either a 2″ or a 2.5″. I think 2.5″ is too much while it’s a daily car and i don;t want too much noise. Besides that i’d like to keep some torque available for the daily rides.

Cheers

• Jason
Jan 21 2014

onnaj – I like your ideas…stock exhaust tubing is generally just fine unless you’ve really tweaked the output of the motor. Since you’re rated at about the same HP as the factory setup, I’d say factory spec sizing is best.

Additionally, I’d go with 2″ instead of 2.5″ in this case. 2.5″ is big for a small 4-cylinder. π

• onnaj
Jan 21 2014

Thanks for the quick reply! π

I don’t know the stock exhaust size of the 1.6 20v blacktop, it’s an engine which was only sold in Japan. The piping i have now was from my old 1.8 16v engine.

I miss 20 HP when compared to the output that Toyota claims, but i think the biggest restriction will be the 1.5″ downpipe?

I found out that the head flows about 180-190 cfm, is it possible to calculate the best size with that number?

Thanks again!

• Jason
Jan 22 2014

Onnaj – CFM can help, but it’s about temperature too.

If you look at the chart, 180-190 CFM corresponds to a 1 and 5/8ths diameter. This is close to 1.5″. If you’re trying to maximize top-end production, bigger tubing (2″) will do it. If you’re trying to build up the low end, 1.5″ is fine.

However, you also want to make sure the header tubing is larger than the exhaust outlets on the head. There should be a short “cliff” or “step” between the head outlet and the header inlet. This acts as a barrier against exhaust gases flowing back into the head.

• onnaj
Jan 23 2014

Thanks for the quick and detailed reply. No doubts any more that i’m going for 2″ but use the stock 4AGE 20v 2.25″ downpipe!

70. Joe
Feb 5 2014

First off thank you for this information.

My car is a 2009 Dodge Challenger R/T. The engine is a 5.7L V8 but is supercharged, cammed, and has worked heads.

My exhaust appears to have several changes in sizing. I am running 1.75″ JBA long tube headers going into JBA catless mids at 2.5″. They then go into my DynoMax VT catback with a 2.5″ in then 3″ out x-pipe. The remainder of the exhaust is 3″ all the way back until where the tips clamp on. At that point the pipe is reduced down to 2.75.

I am looking to buy a set of catted mid pipes from JBA. They have a set that are 3″ instead of 2.5″. I can also get another X-Pipe from DynoMax that has a 2.75″ in and 3″ out.

So should I buy the 3″ catted mids as I exceed the horsepower of 2.5″ currently. Also, does buying an X-pipe that is 2.75″ in versus my current X-pipe having 2.5″ in and both are 3″ out or basically a .25″ increase on the in side warrant the purchase of another X-pipe? Does an exhaust system come down to it’s lowest denominator? Meaning the catless mids are 2.5″ mating up to a X-pipe @ 2.5″ in. So does the fact that the X-pies is 3″ out and 3″ rest of the way matter? Is it ultimately a 2.5″ system? Even if I bump my catted mids up to 3″ and it then necks down to 2.5″ or 2.75″ make the rest of the systems diameter a moot point?

Amazing how I can be so long winded over ultimately a .25″ increase in size at only one junction of the exhaust system. Devil is in the details though.

Thank you.

• Jason
Feb 25 2014

Joe – First, I wouldn’t worry about the x-pipe. On a V8, that pipe is pretty much meaningless…each side of the system is flowing at nearly the same rate at any given point in the RPM curve. There’s little to no reason to balance the tubes on each side of the system.

As for replacing the 2.5″ mid pipes with a set of 3″ mid pipes, I’d say you’re one of the few people who should seriously consider it. You’re running a heavily modified high horsepower V8. You may have a flow restriction at higher RPMs if you’re pushing 700hp.

If you’re closer to 500hp, I’d say you should stay with what you have.

71. Jun
Feb 7 2014

Question of a newbie π
i just had my exhaust pipes replaced yesterday and I felt that i just rob some hp to my engine. I’m not yet sure if I got the wrong feeling, maybe because of the noise or the vibration or whatever but I think the response and the peak power of the engine is not as powerful compare to the stock exhaust.

I have a 1995 Civic Eg D15 engine
1.5L 130hp (stock)

this is what I did:
directly from the stock headers, I replaced the stock 1.75″ pipes, 2 stock resonators and the muffler to a customized 2″ inlet, 2″ outlet, 24″ muffler

the sound is great (deep and bassy) but I want to know for sure if I gained a little power or just the same as stock or unfortunately ruined my engine performance.

please enlighten me, is it an improper customization? or did I miss the proper calculation? or do I also have to change my headers first to match the muffler?

my thought is replacing a larger diameter/aftermarket headers, pipe, and deleting some restrictions will give the exhaust air more room to flow. why does it still need a proper size/diameter while logically the larger it goes the better air flow?

• Jason
Feb 25 2014

Jun – It’s not just about removing restrictions – it’s also about making sure exhaust velocity is high enough to facilitate scavenging at lower RPMs. The only thing you mention that concerns me is replacing the connection between the stock manifold and the after-market muffler. The stock exhaust between the head and the end of the catalytic converter is usually sized perfectly for your engine. If you replace that tubing with something larger (and omit the catalytic converter in the process), you can see big changes in performance (usually in the wrong direction).

What about replacing the stock manifold with a header? You’ve gone this far, why not go a little further? There are lots of headers out there, some of which are designed to work with larger tubing.

• Jun
Jul 28 2014

Hi Jason, I really appreciate your time responding to a begginer’s question.

Well you just enlightened me with your answer. I was almost thinking the same logic behind what happend to my car’s performance. Its basically a missed match between stock manifold and a customized muffler. (Do it once, do it right) i should have considered replacing the whole exhaust system with aftermarket parts if im after the performance than the way it will sound.

However, Im now willing to go further… Hahaha
Can you help me again doing the right choice this time?
I need to know the perfect headers for an all stock D15 vtec engine (P08 head, intake and box) (20 camloabs) (130hp) the options in the market is a 4-2-1, 4-1, tri Y and side exit headers. Which one will be the most effective considering that my engine is stock.
I really like the sound of a straight pipe, is it also applicable to my engine?
Future/next upgrade will be cam gear and box tuning.
What is your advice by also considering the future upgrades that i can do with the engine?

I hope to get an answer from you before I commit the same mistake again hahaha
God bless!

• Jason
Jul 28 2014

Jun – Most of the exhaust systems for the Civic are designed for heavily modified motors…if you put a header designed for a turbocharged engine on a regular engine, you’ll probably be disappointed. My advice is to a) only consider header manufacturers that provide dyno data/charts and b) make sure you know what your plan is. If you’re saving up for a turbo kit, you might want to wait to buy the headers until then.

If, on the other hand, you’re just going to do a few minor upgrades, I’d look for a header that shows improvement on a stock engine. It doesn’t really matter what the design is either – the key is the length of the section between the head and the collector.

72. Dean Allan Richardson
Feb 24 2014

First let say I’ve really enjoyed reading the science behind the equation. .I would appreciate your thoughts on what I would like to do ..I have a 2000 sts cadillac with a few upgrades for hp running approximate 330 hp and stock exhaust… what I am thinking is custom made headers by Carroll customs and after the cat …remove the stock 3in pipe and mufflers to y off the cat meaning from 3″to dual 2 1/4″ pipe and where the exhaust divides to the mufflers add a x pipe to pull exhaust then go to dual magnaflow 11225…I’m hoping this will not be to much and loose power …I’m looking for a more deep throated sound without the tickets as was mentioned in your article. ..lol

• Jason
Feb 25 2014

Dean – I’m not 100% certain on the exact arrangement, but it sounds like you’re mounting a dual system to a V8, with headers and everything. First I would advise that you ask to see a ‘before and after’ dyno chart for the headers. Sometimes headers take power from the bottom end to add power to the top…it raises peak HP, but it makes your car less fun on the street. Second, I would say that V8s don’t really need h or x-pipes. They’re essentially balanced, so adding a tube connecting each side isn’t really necessary. Three, the mufflers will do more to dictate the sound level than anything else you buy. Make sure you’re OK with those Magnaflows, as they’ll probably be loud for a Cadillac. π

73. John
Mar 1 2014

Hello,
I currently have a 02 Nissan Xterra Stock Supercharged (for now) and am looking to swap out the exhaust. I would like to eventually swap out for the smaller 2.35 pulley instead of the 2.64 on there now. Also stock is only about 210 hp, I have an upgraded CAI. Headers would be a consideration in the future as well. There are currently 4 cats on it. 2 primary and 2 secondary. I would like to buy the Exhaust pipe to replace the secondary cats as they are sold legally for a 00 to 01 xterra. Looking into the the exhaust diameter, would you recommend the 2.25 or bump up for the 2.5? I would be replacing the very restrictive y-pipe that is currently on there.

Thanks
John R.

• Jason
Mar 2 2014

John – A quarter inch change isn’t going to make much difference. If it’s cheaper/easier to find parts for the 2.5″ size – or if you know you’re adding more power later – go for it.

• John
Mar 3 2014

Would there be a problem if i went 2.5 into a 3 inch collector back into 2.5 duals? I would be doing the exhaust custom i understand just wondering if that would kink up the exhaust too much. Right now my y pipe is perpendicular and very impeding to exhaust flow. I would pretty much run the pipe as a dual set up with a collector pipe. I appreciate your feedback!

• Jason
Mar 5 2014

John – Probably not, but expanding and then immediately contracting hot exhaust gases isn’t going to improve flow. It’s basically creating a restriction.

Of course, it’s not much of a restriction, and probably won’t have a measurable impact unless it’s very close to the manifold (I’m assuming this is happening after the cats).

• John
Mar 6 2014

You are correct. For some unknown reason, Nissan put four cats on some of the xterra/frontier. I would love to rid of all of them, but as I have read from your earlier posts, they shouldn’t be too constricting compared to hiflow as they are pretty good compared to days of old. But, I would be doing it possibly from the first set of cats and getting rid of the secondary cats, as I have found that Walker makes Federal legal pipes. so i would get those and finish out the exhaust as mentioned. I just one would like the sound to be ddecent, but two increase the flow for future mods, as mentioned. Thank you Jason its been great help so far!

74. Ed
Mar 2 2014

Hello,
I have a 1999 Ford F150 with a 4.2 v6. The stock HP is at 205. The previous owner threw in a K&N air filter and a 2.5 inch cat back dual exhaust. It is stock from the manifold through all of the cats,then to one stock 2.5 inch y pipe, then to one 2.5 inch pipe to a hi-flow muffler and splits to to end pipes measuring 2.5 inch throughout. Since it is all rusting out, My idea is to go with a direct dual exhaust throughout the entire system after the cats on each side of the manifolds. Your page says I should be at 1 3/4 inch throughout for a 217 Max HP. Would you recommend both pipes to share one muffler and exit out dual or go with a muffler on each pipe of its own?
Ed

• Jason
Mar 5 2014

Ed – Yes – a dual in / dual out (or dual in / single out) makes sense on a V6 pickup. I sent you an email about this too.

75. Ed
Mar 3 2014

Hi, I did some more research and learner the stock set up for my F150 was a v6 dual manifold feed into one pipe and muffler all through a 2.5 inch tube. This all corresponds with your chart above given the fact that the stock HP is 205 ponies. Reading some more feeds on your site tells me you don’t recommend H or X pipes for your typical HP recipes. Doing the math and my research brings me to a calculation if 1.58 inches for a dual pipe set up. Now my question to you is, since I will be going with a dual 1.75 inch set up, (1 3/4 is easier to find parts for) would you recommend bringing the two pipes into one muffler then two pipes out (since the pipes are slightly larger than recommended) or going with a muffler on each pipe? Any help would be greatly appreciated.
Thanks,
Ed
PS
I thought you would appreciate me doing some homework on my own LOL

• Jason
Mar 5 2014

Ed – I very much appreciate you reading thru the other comments. As you can see, I get asked the same questions a LOT. π

• Ed
Mar 5 2014

Jason, I’m still tossed on a true dual with a muffler on each pipe or dual in dual out muffler. Everywhere I have read I get mixed reviews. Some say true some say DI/DO. In your honest opinion,(given your knowledge) of the two, (a) What would be best for performance? (b) What would give me the best sound? What direction would you choose?

Thanks again,

Ed

• Ed
Mar 5 2014

Jason, my apologies for running you through the department of redundancy department. LOL! When I read the thread I did so on my smart phone an the replies were reversed so I only saw your second reply. Just goes to show modern technology at it’s best. Ha ha! Great site, a ton of info here! Thanks again. DI/DO way to go.

Ed

76. Chris
Mar 7 2014

I bought a custom f150 94 model 302 coyote motor in it the cat was already gutted I put a flowmaster muffler on it with 3 inch tip

77. sid
Mar 13 2014

Hi,

I am trying to workout the diameter for my muffler for a yamaha YZF-R6 engine (599 cc). This engine is being used to power a race car within my university.

I did the math to workout the CFM and got these values with the thermal expansion and combustion by products

1336 and for the throttle one-third open, the exhaust flow will be 441cfm

From the table above I can’t seem to understand what sort of diameter to use as these cfm are unavailable ?? any help would be highly appreciated

Thank you

• Jason
Mar 13 2014

Sid – Those numbers sound awfully high to me, considering a 5.0L V8 only takes in 440CFM worth of air.

By my calculations, a 600cc motor at 8,000RPM is going to suck in 85CFM (I know, I’m mixing units). Even with fuel, I doubt that engine produces more than 100CFM of exhaust…and that’s at or near WOT (I’m assuming a redline around 9k?)

Another thing to remember is that the chart above isn’t really relevant to race scenarios…I’d suggest cracking a textbook, talking to another race team, or getting with the prof. on this one.

OR, even better, put whatever you think is right on the motor, test it, and then compare. That’s the beauty of being in a university setting – you’re free to think “outside the box”.

• sid
Mar 13 2014

Hi,

I am not involved in the team itself, I am just designing a muffler which they could use next year. So design is just conceptual at this stage.

I was given the wrong details with the bhp and rpm. So after I changed my calculations I have a 599cc 70bhp engine at 11, 500 rpm.

I based my calculations from this forum post by 2SBblue”

Amount of air pumped is 3444 lt/min
Exhaust gas volume = 14414 lt/min
at 1/3 throttle the flow is 4757 lt/min

one cubic foot conversion = 509 CFM and 168 CFM for 1/3 throttle

After thermal expansion and by products
509*1.0756 = 547 CFM
168*1.0756=181 CFM (1/3 Throttle)

In the post above it doesnt mention how we can determine the actual diameter of the muffler with the CFM except from the table. In this case we could manufacture so stock diameters are not really needed if a specific diameter can be calcualted but I am unsure of the next steps?? could you please assist me on what the next steps are??

Thank you
Sid

• Jason
Mar 13 2014

Cool engine!

Next step: Assuming a pipe will flow 115CFM per square inch of area, back into your pipe diameter.

Then, find the closes off-the-shelf pipe size. Since your motor is all top end, I’d hedge towards the larger size of pipe.

In other words, if your calcs indicate a 3.65 inch diameter tube is needed, go with 3.75 or 4. This is OK on a high reving engine that makes all power on the top-end (at least assuming the engine spends most of its’ time at high RPMs).

78. Sid
Mar 13 2014

Sorry if I am sounding dumb but this is where I am unsure about what to do.

i am guessing the muffler diameter has to be between 2 1/2 and 2 3/4?? from the calculated cfm and looking at the table above? or do I do 547cfm/115cfm = diameter?? am I right?

Thank you
Sid

• Jason
Mar 13 2014

Right – here’s how you do it.

The area of a circle is Pi * the radius of the circle squared. You know that exhaust tube flows about 115 CFM per square inch, so you take your CFM (547), divide by 115 to get an area, and then back into a diameter.

The radius is the square root of (547/115)/3.14159

…the answer is 2.46″ in diameter.

79. Sid
Mar 14 2014

Hi Jason,

Awesome, thank a lot for the help :D.

Just one last question, this outer diameter is it irrelevant to what type of muffler it is?? as in a straight through, chambered or turbo? all could have the same outer diameter for the calculated CFM.

80. Matt
Mar 18 2014

hey jason,
for some reason my comment didn’t post on this thread yesterday. I have a 2013 tundra completely stock. They make about 381 engine horsepower. I looked in to getting dual exhaust and the shop recommended a cat back dual in / dual out magnaflow. I was wondering if it would be better to actually spend a little more money and use two mufflers instead of one? The stock tubing is about 2.75in and the exhaust shop uses 2.5 inch. I noticed above that you said a 400 HP engine only needs 2.25-2.5 in tubing but I was wondering if going to a single dual in/ dual out muffler would be a good idea. Or should I get two single in single out mufflers? The shop I went to said it would make no difference. Thanks in advance, -Matt

• Jason
Mar 18 2014

Matt – Provided the dual in/dual out is properly sized, it will make very little difference whether you use two single mufflers or one “combo” muffler.

Having said this, if you ever decide to install a TRD supercharger on your truck, you’ll wish you had two separate mufflers. Otherwise, I say save money and go with a dual in dual out.

• Matt
Mar 18 2014

Jason, Thanks for the quick response. I guess the second part of my question was is 2.5in tubing too small to be considered properly sized? Like my earlier comment said, I found out stock tundra tubes are 2.75in. How can I ensure that this system is not too small or too large in terms of tubes?

• Jason
Mar 18 2014

I just happen to know in this situation that the exhaust system on the Tundra was very carefully designed. I would absolutely make sure that the tubing coming into the muffler didn’t change in diameter.

However, once you get out of the muffler, the tubing size isn’t too critical.

81. kris
Mar 19 2014

i have a 08 chevy silverado 5.3L it is a double to single exhaust with a 3″ exhaust can i drop the exhaust from 3 to 2 1/2 and be fine note it is magnaflow glass pack muffler

82. jon kow
Apr 10 2014

Jason..
I have a 1998 Chevy Silverado with a 5.7liter I wan yo replace the stock exhaust with another set up not quite sure what’s the best size for efficiency the truck is stock but I know I need a new system due to melted cats I would like yo leave room for performance because I’m in school at the moment so I don’t want to have to re do my exhaust again I was thinking 2 3/4 any suggestions?? Thanks

• Jason
Apr 10 2014

Jon – My best suggestion is to buy a tuned cat-back system designed specifically for your vehicle. They offer the best combination of performance and sound quality, and they’re relatively affordable (\$600-\$800). If you don’t have the money to do a full cat-back, than I’d just replace the mufflers and leave the rest alone. Increasing tube diameter isn’t usually impactful on stock trucks.

83. Ron
Apr 12 2014

So, going from long tube headers with a 3″ collector to about 24″of collector length to a 2.5″ reducer to terminate the collector length effect, then going to a single 3.5″. On a built dodge 5.9 at 9.6 comp. It has a bit of a lag at 1500. Set up to shift at 5500.
This is what I am considering. Any feedback?
Thanks
Ron

• Jason
Apr 15 2014

Ron – Does the header manufacturer have a recommendation? What you’ve suggested seems reasonable, but I’m wondering about the value of a reducer that’s 2 feet down the line. If the collector is 3″, and it’s designed specifically for your engine, than 3″ is probably the right tubing size.

If the headers with a 3″ collector are not designed for your engine (and honestly that sounds very big), you’re probably not going to like the final result. Over-sized headers usually zap power.

• Ron
Apr 16 2014

Jason, Yes the headers are for the rig. 1 ΒΎ prime tube with 3β collector, with a 3β Y pipe going to a single 3β I have a Hughes airgap intake as well. Sound is a bit of an issue as well as power at around 1500. I have done some research on this topic and it would seem like collector length is a major factor in creating balance between the two sides of the motor, and moving the power down to where it is needed. 24β seems to be the target for my RPM range of a 5500 shift. As it is now I feel like it is unbalanced with the driver side twice as long to make the trip over to the other side.

• Jason
Apr 16 2014

Ron – Unless we’re talking about a diesel 5.9L, those headers sound like they’re for a race motor. They’re too large for a street motor, at least if you want to make power at lower RPMs.

When you have headers that are that massive, they tend to “back up” at lower RPMs. The engine is probably producing 80% of its’ torque at 1500RPM, yet you’re noticing a lag. My guess is that the exhaust velocity in your headers is too low at these lower RPMs.

Now, if you’re trying to maximize your quarter mile time, or if you’re running your vehicle around a track, this type of header design is A-OK, as the engine spends most of it’s time at higher RPMs. But if you’re putting headers like this on a pickup you drive to and from work every day, you’re going to notice power problems.

In short, I don’t think collector length is your problem, at least at 1500RPM. I think the problem is that your exhaust velocities are too low at low RPMs, which means you’re not getting the scavenging you want.

84. David
Apr 15 2014

Jason, I have an Old’s 455ci auto, 320Hp 460lb-ft with the original 2into1. The left pipe 2″ crosses under the sump to the right manifold and a 2 1/4″ pipe runs from there to the muffler then a 2 1/2″ pipe to the rear. Its too quiet, like silent, so I was thinking a true 2into1 to free up some ponies and let it sound more aggressive. My plan is a 2 1/4″ pipe from both manifolds back say 36″, then both into a 2 1/2″ for about 24″ into a free flow muffler. From there a 3″ pipe the rest of the way mandrel bends over the diff and out the back.
I think this will provide good scavenging without losing bottom end torque and make it sound more beefy. Useful rev range and power is for street driver, idle to about 4 grand.
What do you think, am I close with this? Any suggestions. Cheers, Dave.

• Jason
Apr 15 2014

David – Sizing sounds right to me. The only tip I have is that you want to design the exhaust to go with whatever manifold or headers you’re going to install later. If you’re going with some performance headers down the road, I’d recommend building things with that in mind.

85. Danny
Apr 19 2014

Hi Jason
Firstly, great info to simplify things for us laypersons…I know you are busy with all the posts so I’ll keep this as clear as possible.
I live in the UK and have a 2006 1596cc NA engine Ford Focus…stock power is 100bhp with 1.75″ tubing, centre resonator and chambered rear muffler….and now my exhaust needs changing. The only full SS cat-back system in the UK is (Piper Exhausts) is 2.25″…which according to the chart is a bit too big…However I can get a custom one made to my sizing…since I’m planning on upgrading the intake, ignition, replace exhaust and get a re-map at the end…it may take the car to 115-120bhp…so my questions are these:

1) Do I stick with 1.75″ or go 2″ with tubing as with the extra power its kind of in between sizes (according to chart) but nearer the 1.75″?
2) I used to work with office and industrial air-con systems and there the ductwork used reduce in cross sectional area the further it got from the fan….so an idea someone above suggested how about 2″ to the resonator (about half-way) and 1.75″ to the rear silencer in the interests of flow velocity? or stick with one of the sizes in Q1?

Many Thanks

Danny

• Jason
Apr 19 2014

Danny – I’d go with whatever option is less costly, and put your money towards a turbo. π

1. I usually go smaller when it’s between sizes, especially on smaller displacement enignes.
2. Flow velocity is also dependent upon the temperature of the exhaust gases. If the exhaust gases were staying at the same temperature as they traveled thru the system, necking down a section of tubing might be a good idea. However, they gases are changing temperature as they move, and – typically – necking down the tubing has minimal effect on velocity.

The best way to keep velocities up is to keep the tubing just barely big enough to do the job. Hence, my suggestion to go with a 1.75″ instead of a 2″ if you’re between sizes.

• Danny
Apr 22 2014

Cheers Jason…excellent!

86. Ants F.
Apr 20 2014

I had a quick question regarding this topic that I was hoping one of you pros can help me out with:

I recently installed a new Cat. Converter to my 2000 Chevy Cav. Z24. Im in California so I didn’t want to throw down a good wad of cash to only end up having problems when it came time to get it smoged. I went with a 50 state legal direct fit Magnaflow cat instead of getting a way cheaper but more likely to give me problems Universal cat. so this Cat came with the flex pipe/down pipe, honeycomb Cat., and the pipe at the outlet of the Cat which houses the O2 sensor and the flanges at each end of the unit for easy no weld bolt on installation. the problem im having now is the rest exhaust system(resonator pipe, muffler). initially I was planning on purchasing a obx cat-back system they offer which includes the new N1 style muffler already welded and ready for install. the mechanic who helped me install the Cat Converter did a quick check of the old resonator pipe and told me it was still good so that made me lean towards only purchasing a performance muffler now… only thing is that my exhaust systems pipes are 2″ inlet and outlets. the smallest inlet on a good (50\$)muffler has been 2.25″. is this common and there is there a way that a professional makes it work? and might it effect my cars performance? what do you recommend? im open for going with the obx cat back if that would give my cars performance a substantial boost in highs and lows, or would getting a new muffler give me about the same boost in performance?

-pheR

• Jason
Apr 20 2014

The 2.25″ inlet and outlet can be necked down to 2″ with some simple parts. The muffler shop will have no trouble.

As for performance gains from a cat-back vs. a muffler, a tuned cat-back will be better. But not a whole lot better, especially on an older car with a small engine.

I’d say buy the muffler and put the rest of your money towards a bigger upgrade. π

Apr 29 2014

I have a 2006 ford fusion V6 with a Steeda cold air intake and soon a tune with Steeda custom tunes and a Steeda throttle body spacer. It’s stock 221 hp but with all that it’s got around 280-300 and don’t know what size piping to get. The flow master cat back is like 2 1/2 but I’m having a buddy’s step dad do it. They work on drag cars a lot and was gonna have a custom system put in to get some deeper idle tones and a bit of a bark when I step on it. I don’t want to go to big because the back pressure. But need to know a good size to up it too. The pipe looks really small just haven’t had time to measure or anything. I had the resonator deleted off of the car which have it a deep tone on idle but other then that nothing. Help?

• Jason
Apr 29 2014

If you’re going to put a Steeda tuner, throttle body, etc., on your car, why not buy the Steeda exhaust to go with it? It was probably tuned as a package.

If you’re not going to do that, than buy tubing that’s the same size as the Steeda exhaust.

If you’re not sure what that is, than buy tubing based on the chart above. Or install the same size tubing that comes from the factory.

But whatever you do, don’t strap some big 3″ dual exhaust on your Fusion just because your uncle says that’s what you need. You’ll regret it.

Apr 29 2014

It’s single all the way back to I’m pretty sure right before the axle then it y pipes into two But I don’t have money for te Steeda one. Plus I was looking at flownaster or Magnaflow and it’s like 2 1/2 or 2 1/4 which is what I’m thinking about matching. With some flow master super 40’s

88. mohamed
May 8 2014

can anyone tell me how much will be the average size or specification of the motorbikes silencer? because i am working on the project where i am designing one. but it a air preheater which will connet to the exhaust pipe. so ideally i want the specification for air preheater(heat exchanger) for a bike which will look like silencer.

89. Ray
Jun 2 2014

You guys are about performance, but I need guidance about not damaging the engine or clogging up the cat.
I have 2006 Ford Freestyle with a 3L 6 cyl that had the only muffler available in a tiny town 1,000 miles from home put on (Not the right one!). It is about 4″ shorter, and they had to put on a 45 elbow that necks down to 2″. The muffler inlet and the pipe from the front both are 2.25″. The outlet is two 1 7/8 pipes so together they have way more area than the inlet pipes.
My concern is not HP, it is damage or long term harm to the engine, or soot collecting the the cat. As you can tell, I have no experience, I just read that a restricted flow is not good, but I have no way to gauge if this is a non problem, small problem, or big problem. I want to baby the engine because I need this vehicle to last. Do I need to replace it with the right muffler or am I OK?

Thank you very much for any help or advise you can give me.

• Jason
Jun 3 2014

Ray – Damage? No worries. Reduced fuel economy? Potential false readings from oxygen sensors that trigger check-engine lights? Yes.

The good news is that a muffler that’s actually designed for your vehicle can be purchased after-market for \$75 (give or take), and most muffler shops will install it for about the same. I’d guess that the \$150 you’ll spend buying the right muffler will save you some gas money every time you fill up too.

90. Dom
Jun 7 2014

I have a question I have an audi 2001 s4 it makes 500hp at the crank, my question is how big of dual exhaust do I need ? would a 3″ downpipe to a dual 2.5″ ehaust be better than a 3″ downpipe reduced to 2.5″ then bolted to a 2.5″ dual exhaust be better? to me i think they basically are the same because either way its being reduced to 2.5 inches but am I wrong? is this big enough for my horsepower? oh and the car is twin turbo and not remotely stock it is rs6 conversion.

• Jason
Jun 10 2014

Dom – When we’re talking about forced induction, a lot of the concerns about exhaust cooling and “stopping up” an over-sized exhaust pipe go away. Not only are exhaust gas temps generally higher on a vehicle with a turbo or supercharger, but scavenging is less important.

Basically, if you’ve got a turbo or supercharger, you can go bigger than you would otherwise go. I’d go with the 3″ downpipe to the muffler, then whatever you want from there.

91. Dom
Jun 10 2014

Yes agreed and thank you for the response. The 2 down pipes is full 3″ then into a 2.5 ” full dual exhaust should I go with full dual 3″ or does a 3″ tapered to 2.5 downpipes to a true 2.5 dual exhaust the same as tapering from full 3″ downpipes to a 2.5 exhaust?

• Jason
Jun 10 2014

Dom – Going with 3″ all the way to the tailpipe seems aggressive to me – I’m not sure you’d suffer any power loss, but you might have a little more punch off the line if you go with a 2.5″ exhaust from the muffler back.

As for the downpipes, I’m a fan of bigger on this sort of application…only the *best* way to know is to test both the 2.5″ and 3″ downpipes and see which does better.

92. Jerry
Jun 29 2014

I have a 4.6 liter marauder.
18lbs of boost via a procharger
626 rwhp with pump gas
791 (645tq) with a 175 shot
Obviously with a boat, low end torque is my biggest concern.

It currently has 2.5 dual with a Siamese xpipe. Local shop is suggesting I go to a 3″ Your thoughts are appreciated.

• Jason
Jun 30 2014

Once we start talking about heavily modified engines and/or engines with forced induction, the concerns about low-end torque loss due to over-sized tubing diminish.

Modified engines, turbocharged engines, etc. run hotter exhaust temps, and those higher temps prevent problems with gas cooling at lower RPMs. I think 3″ duals sound just fine.

Jul 1 2014

Hey guys, wonder if you can help me, I have a measely little 1.5 Suzuki Swift, and I’m looking to change the exhaust, it makes 75whp haha and redlines at 6200 rpm, the stock exhaust size is 2″ from what I can see, would there be any benefit going to 2.25″, or would sticking with a 2″ stainless system work better?

• Jason
Jul 7 2014

Chad – The smaller your engine, the less sense it makes to mess with tubing size.

A new muffler, however, could make a difference. The less expensive economy cars tend to have the cheapest, most restrictive mufflers.

94. Andy
Jul 8 2014

Help me out here please. I have the cummins isx 14.9-15L turbocharged diesel motor with 515-550 hp – almost 1950tq. – help on the exhaust size please.

Jul 17 2014

Thanks Jason, will just do a mandrel bent 2″ stainless system with some decent mufflers, thanks again

96. Dave
Jul 31 2014

Hi there – I have an annoying problem with my toy..

BMW 2014 X6 5.0L bi-turbo putting out 450BHP/480ftlbs, anyway – I swapped out the too-refined factory muffler (2 pipes from the engine to a single muffler) for a Meistershaft EV system (not cheap at all) which has a pair of independent mufflers per pipe (2 tips per side, 1 from each can).

Problem is that at low RPM (1000-1500 ish) I’m getting a “pulsing” sound some of the time. Once gas starts flowing the nice noises start, so my question is.. any ideas?

The system also has valves on 1 of the mufflers on each side that can close one of the tips – if I do that the pulsing is much more pronounced .. reminds of of how a V8 VW bug might sound.. not at all a nice V8 growl..

I’ve been wondering about X-tubes, H-tubes (before the mufflers) anything that might merge the gas flow into a more uniform flow..

I know this is long shot, but I’m desperate :/

thanks,

dave.

• Jason
Jul 31 2014

Dave – A cross pipe could help I suppose, but the fact that it goes away once things are warm – and that you can make the sound worse by closing a valve – suggests that this is a defect somehow.

However, this seems like something I would complain about to the exhaust kit manufacturer. They should fix this problem or make it right somehow.

• Dave
Aug 8 2014

Hey Jason, figured I’d give an update .. An exhaust shop created me a custom X-pipe installed between the resonators and the mufflers – it has completely fixed the problem, the sound is a nice even rolling growl and with the EVs closed the system is close to stock sounding.

X-pipe FTW π

thanks.

• Jason
Aug 8 2014

Dave – Awesome – glad to hear it.

97. Austin
Aug 3 2014

Will a 2.5″ xpipe fit on my 5.0 v8 limited explorer

• Jason
Aug 3 2014

Austin – Why put an x-pipe on a V8? The exhaust is naturally balanced…connecting the two sides with a pipe shouldn’t make any difference.

98. joe
Aug 6 2014

my 93 chevy trucks 4.3 needs a new exhaust system it has stock everything 2.75 cat to 2.25 pipes. can i use a 2.5 muffler+tailpipe or duals without any loss in power or just keep it stock with a good flow muffler?

• Jason
Aug 6 2014

joe – Look at the chart man. Dual 2.5″ pipes is a lot for a 4.3L V6, unless of course you’ve got a 300hp supercharger bolted on top.

99. Leroy Oller
Aug 6 2014

Jason Help if you can
I have a 93 Dakota 4×4 from my fathernlaw kept burning up transmissions ,Exhaust crossed within 1/4 inch of tranny pan bad design baked fluids in pans I now have cutoff the exhaust so drivers side comes straight back instesd of by pan and over to Y on passenger side will 2.25″ work to run it back and around tranny for dual out ok & I welded up the Y xtra Hole but the 2 in outta the manifold runs into 3 in at the Y with oxygen sensor that wires were cut off but 3in runs into a old cat burned up can I eliminate that cat and run 2.25″ straight back also or should I get a new cat and sensor & if I get the new cat will just runnin it on one side be ok ??/ got six kids Low budget doin myself need a little advice Guys Thanks for any Help, Bubba

• Jason
Aug 7 2014

Leroy – Aside from the fact that eliminating the cat could cause problems when its’ time to pass emissions, eliminating the sensors will probably cause the engine to run poorly. You can buy oxygen sensor simulators to try and “hide” the fact that the cat is missing, but it’s not a bulletproof solution.

My honest suggest is to try not to put much money in it. I personally owned a 92 Dakota, and it was by far the worst vehicle I ever had. π

• Tracy
Aug 8 2014

Leroy, I know Jason knows more than I do about exhaust but just thought I’d give you a real world experience from a previous 93 Dak owner. I bought a 93 Dakota brand new. Had the 5.2 with A518 trans. First off the crossover is not what is/was burning up your transmissions. I’ve been rebuilding torqueflite’s for nearly 33 yrs. and have seen many crossover pipes pass right by the trans. pan and they never burned up the trans. My guess is they are not being rebuilt correctly. So many rebuilders just yank stuff apart and slap new clutch plates in something and call it rebuilt without even checking any clearances or replacing bushings or steel plates. Some don’t even service the valve body. IMO they just “patch up” the trans, not rebuild it. That aside lets talk exhaust. When my Dak was about 5 yrs. old I decided I wanted to have dual exhaust on it and had the same concerns as you but I took all the factory exhaust off and had 2.25″ duals ran and installed a single exhaust bung in the right side pipe only and it ran just fine other than fuel mileage dropped slightly.

BTW: funny Jason’s Dak was so bad. In addition to using mine in everyday service I raced it a whole bunch. I’m sure I made at least 500 trips down the 1/4 mile drag strip. Wasn’t real fast but I did get it down into the 14.40’s. Sold it with a little over 90,000 miles on it with the original engine (never even took a valve cover off) and original trans with just a Trans-go shift improver kit and it still worked fantastic the day I sold it to a tree trimmer and he ran it until it had 130,xxx miles on it before the trans gave out. Other than general maintenance and 2 wiring issues and 1 blown heater hose it was a fantastic vehicle. Unfortunately yours has time (and probably a lot of miles) against it so I agree with Jason in not putting too much money in it. Good luck

Jason I hope it was ok to post this

• Jason
Aug 8 2014

Tracy – Always OK to leave a useful comment. Thank you!

• Leroy oller
Aug 8 2014

Thanks for the response Jason ,no emissions to worry about here so Im gonna try with just the sensor have a great day!

• Leroy oller
Aug 8 2014

Also thanks for your input Tracy appreciate all pointers I can get you also have a great day!

100. Tyler
Aug 7 2014

I have a 1991 gmc Sierra with no cat or muffler and it feels a little more gutless than what it used to be. It’s set up with 3 inch piping all the way back to where the muffler was and then 2.25 the rest of the way. Is there any way or suggestions on what I could do to gain back power without putting my cat back on and muffler? Thanks

• Jason
Aug 8 2014

Tyler – First, if putting your cat back on your vehicle would increase power, why would you be opposed to it? Power is what we all want, isn’ it?

Second, a single 3″ exhaust tube is probably too big for your 91′. A dual is absolutely too big. See the chart above as to why.

101. Porkchop
Aug 20 2014

I recently built a carbed 383 stroker for my 94 chevy fullsize truck. I didn’t get a chance to put it on the dyno before I had to get the truck going but, by my best estimations and calculations it’s making around 410-420hp. I use it as a daily driver and for pulling a fairly heavy toy trailer on a lot of weekends. I had a built 4.3 v6 in it before this and I had dual 2″ exhaust ran from the manifolds to behind the rear tires which was plenty for that motor. While this works, I notice when I step out to pass someone going down the road it builds hp fast ans strong up to around 3800-4000rpms and then it seems like it really isn’t gaining anymore power. I’m looking at a 2 1/2″ mandrel bent header back system that I would end up building/installing myself and if I understand the chart right this should be a good size for my application correct? I plan on using Flowmaster’s deltaflow 40 series or their super 44 for the quiet down parts. BTW, have you had a chance to hear the difference between these two yet?

• Jason
Aug 22 2014

Porkchop – It sounds like you might benefit from bigger pipes, especially if you’re losing power at the top end. It certainly can’t hurt, as the engine makes plenty of power for a dual 2.5 system.

As for deltaflow 40 vs super 44, the super 44’s are louder I believe.

102. Nate
Aug 31 2014

So I got a question for my 95 civic dx (I think 1.6L or .5 not sure at the moment) but I just installed a fairly cheap exhaust with headers, cat back and everything else but I broke a bolt on my cat converter cuz some dumb ass tacked all the bolts on the stock exhaust so I just have a test pipe til I fix the cat. The new exhaust is 2″ with a 4″ exhaust tip. I recently replaced the spark plugs and the only other upgrade I have is a spectra cool air intake.

So my questions:
1) Is the pipe too big for a 4 cylinder civic?
2) Since its aftermarket and more air flow, will the intake have more effect on hp? (With a 4 cyl I’m not expecting much but I’ll take wat I can get.)
3) Idk if it’s true but better overall performance? And maybe a little more fuel efficient?

Anything else u wanna add is greatly appreciated!
Thx for takin the time to read this!!

• Jason
Sep 1 2014

Nate – I’m not sure what you’re asking re: intake and #3, but i’ll say this: you’re not going to find a lot of power on a small 4cylinder with intake and exhaust. You’ll want to bolt a turbo onto that engine to get any real power, and you’ll want to pair the turbo with the exhaust system and intake.

If I were you, I’d save my pennies for a turbo kit and stop with all other upgrades. Unless you’re willing to strip down the motor and get a new cam, upgraded internals, etc. They you might be able to get some more power.

103. mike
Sep 23 2014

I keep coming back to this page, but something critical has not been covered and I can’t find anywhere.

Given an exhaust gas outlet temp from a cylinder head, and say 1.5″ pipe, how do you figure the loss of volume as the exhaust cools?

This is necessary information if you are going to properly size pipe for good velocity (and post-catalytic converter) throughout the system, but I’ve not found anything that even remotely covers that.

• Jason
Sep 25 2014

mike – You’re right. I can’t find that info either…the folks who design headers, etc. don’t like to share. π

104. KcD
Oct 1 2014

My ex worked at a muffler shop and a guy brought in his truck for a custom muffler. He brought home the original. I have talked to a couple people that have told me it’s worth money. My question is how do I figure out how much to ask and what it goes on? There are no numbers anywhere!?!?!?

105. Layne
Oct 13 2014

I am bringing a 62 cutlass back on the road. It has a alum 215ci 4bbl with 10.25 compression which ran factory hp at 185bhp. A better cam, and carb, and balanced it looks like around 200hp at 5000rpm reving to 5500-6000. Nothing much but I want it to breath well out the back with dual exhausts. Your table looked like 1.75″ dual exhausts and I’ll try to fit a cross over in. Does that sound right?

• Jason
Oct 19 2014

Layne – Seems reasonable to me.

106. Lou
Oct 27 2014

Jason, thank you for the very informative article. I have a question, I have stock 454 out of a ’72 passenger car. It doesn’t have any smog equipment on it. It has the stock manifolds and a 650 Street Demon carb. I rebuilt the motor and left everything stock except the cam. I put a very mild cam that’s supposed to increase torque. I have this motor in a 58 Cadillac and all I do with it cruise. I wanted a super torque monster motor for my application. This motor will rarely see anything above 4000 RPM’s. I also have a 700R4 tranny and this thing cruises around 2200 RPM’s at 75 mph. I opted to put 2 inch straight pipe all the way to the bumper instead of 2.5 due to wanting to maximize the torque. I don’t really care about the horsepower. Question, is the 2 inch pipe appropriate for my application? I’ve read a lot of articles about 2.5 inch stuff going on big blocks but they’re more geared towards higher horsepower and performance. I just want to cruise and keep her at low RPM’s. I plan on eventually adding an EFI system but that’s going to be geared more towards economy and just over all running better. Once again, thank you for your time.

Lou

• Jason
Oct 28 2014

Lou – A dual 2″ system is probably fine for a dedicated cruiser. That’s about what GM put on those engines at the factory (give or take), so you should be just fine.

If you’re trying to extract every bit of power, a larger diameter exhaust pipe might be beneficial…but there’s a lot of other work you’d do too.

107. Lou
Oct 27 2014

Just to clear one thing up, I put 2 inch “dual” pipes to the back of the car. Once again, Thank You.

108. Lou
Oct 28 2014

Jason, Thank You for your quick response. It was greatly appreciated.

Lou

Nov 6 2014

I want to design a small size muffler for 5 hp diesel engine then give me the ideas of different mufflers to reduce the noise with minimum back pressure.

• Mike
Dec 2 2014

Problem found with most small motors, even gas (such as generator) is that the noise does not come from the exhaust. The noise is from the workings of the engine itself. Many “silenced” generators are enclosed in boxes with soundproof materials on the inside. You can see results of people trying to muffle small engines all over on youtube, and as far as I’ve seen, it’s all fruitless.

110. Andy
Nov 24 2014

So I have a β96 Ford F250 with the 7.5L 460. Being the fool I am, since there are 2 pipes coming out of the muffler(stock), I assumed there are 2 pipes coming into the muffler. Unfortunately, after purchasing a Cherry bomb dual-in dual-out, I learned there is only 1 pipe coming from the engine. Anyway, my question is this. What Y-pipe do I need, and would a dual-in dual-out cherry bomb 2.75β³ be risky?

• Jason
Mar 3 2015

Andy – Definitely don’t want to put a muffler on unless all the ends are capped/attached to a pipe. But beyond that, you can probably hook up what you have and move on (again, provided you weld a cap on one of the intake inlets). The cherry bomb mufflers aren’t exactly restrictive.

But won’t whomever sold you the muffler take it back?

111. Chris
Dec 4 2014

Hi there if my engine produces 550HP@4700 RPM and 570TRQ@4300RPM 1200 CFM 2X4 600 holey carbs I have FPA headers in mind. With more HP on the way. What size pipe would I require?

• Jason
Mar 3 2015

Chris – See the chart?

112. michael
Dec 18 2014

Good morning,

So, I will assume the table is gross hp? How much difference does bing off a size make really? If you have 3 inch instead of 2.75 inch, can you wrap the pipe to keep it from cooling down as much to balance out the loss.
I’ve got a V8, DOHC, 4 valve engine makes about ~ 430 hp. I look at it’s exhaust. Which goes goes into 2X 3inch pipe due to headers, then marries back w 2.5, through a resonator and exits as 3.5 inch. You list 3.5 inch as 468. So if my goal is 500 hp, what’s the impact of using a single 3.5 versus a 3.75 pipe? That said, the intake I believe is the problem.

• Jason
Mar 3 2015

Michael – Don’t understand why you’d have 3″ diameter collectors on the header and not keep that size all the way thru the system. If your goal is 500hp, than you need to go 3″ all the way (whatever your header collector size is).

113. Danny Fugitt
Dec 27 2014

I have a 2011 silverado 5.3 and had a flowmaster series 40 dual outlet installed; it is 3” inlet and 2.5” outlet. The man used 2.25” inch for the tailpipes instead of 2.5” pipe. Is this enough or do I need to take it back and insist on 2.5”. I called flowmaster and they said it needs to be 2.5” tailpipes for the muffler to work properly. I have noticed their catback systems for this truck all have 2.5” tailpipes, but I have seen other brands and some are using 2.25” tailpipes. I assume all the numbers are outside diameter. My tailpipes are marked 2.25” and inside measures about 2”. Thank you. Danny.

• Jason
Mar 3 2015

Danny – Don’t worry about it. 2.25″ pipes aren’t hurting you, contrary to what Flowmaster says. Their systems are oversized anyways.

Dec 28 2014

I have a 350 in a 77 Chevy pu 4×4 at 4700 rpm my hp and torq meet at 400 can I run the factory manifolds with a single 2″ system or is it to much back pressure if so what do you suggest

• Mike
Dec 31 2014

Your manifolds will be the restriction no matter what size of pipe you run (within reason of course), so pipe size is irrelevant. Remember…the closer to the engine you make changes, the more effect you will have. Since manifolds already suck at scavenging (which is why headers are worth so much power on a stock small block chev) replacing them with long tube headers should be your first consideration. Otherwise, forget replacing the pipes with anything larger than what bolts to the manifolds, it would be pointless. You can never flow more than the largest restriction…and manifolds on the SBC are it.

1.5″ primary headers are about ideal for your engine and application (400HP divided by 8 is 50HP per cylinder in a truck, which needs low end torque) but 1-5/8″ is the common size.

The chart above clearly spells out what you need.

2.5 or 3″ collectors merged into a single 3.25″ pipe will be the best solution if you can find it. Duals are REALLY a dumb idea, since they add cost and complexity (both in added components and for working around it) for no benefit. They were ok back in the day that 2″ pipe was all you could find, they aren’t today.

• Jason
Mar 3 2015

Mike – A very interesting point about duals no longer being necessary…never occured to me that large diameter tubing was expensive/hard to come by in the
“old” days. Thanks for commenting.

• Jason
Mar 3 2015

Hog head – Not to be catty, but I suggest remedial English. You’ve got a heck of a run-on sentence there and it’s hard to take you seriously.

115. Mark Mitchell
Jan 2 2015

I have a stock 5.7 from a 92 silverado in my 89 Astro van the stock Y pipe is dual 2.5 to single 3 going into a 3 inch catalytic converter then out to a 3 inch sonic turbo then it drops to a 2.5 tailpipe (largest I can find for now) my question is if I put an after market Y in front then another at muffler with dual 2.5 tail pipes will it make enough difference to make it worth it? My goal is to freshen the engine in a year with better cam, intake, and all new bearings etc. I plan on retaining the TBI system all this to tow a camper. Any ideas are greatly appreciated .

• Jason
Mar 3 2015

Mark – If you’re building your own system anyways, why not size it correctly from end to end?

Also, this 5.7L Astro – will it be racing? How fast can it run a quarter? Sounds like a cool project.

116. Jim
Jan 19 2015

Somebody please explain to me why so many factory V8 single exhausts converge their flows at a 90Β° angle? In other words, a “T” pipe. It’s not even a “Y” !

I have a ’78 Buick Electra with the Buick 350. The driver’s side pipe crosses over and the passenger’s side comes straight down and connects perpendicular! That seems like the WORST way to converge exhaust flows yet, it’s quite common.

Is there some mysterious acoustical or pulsing gas flow physics behind this?
Or, is it because it’s cheap and easy for the assembly line?

117. Raul
Jan 30 2015

Jason,
Iβve been back and forth through this site and many others trying to make sense on how to finish off my custom exhaust. Iβm running a 2GR-FE from toyota, 3.5 liter V6. Stock hp is about 270, looking to squeeze about 300 crank or about 255-260 at the wheels. The stock, constricted, crush bent Y/J pipe was swapped in favor of a custom fabricated MKC Y pipe that starts off connecting to the factory exhaust manifold flanges and are 2.25β³ on both sides merging into a single 2.5β³ flex unit that measures about 6 inches then straight 2.5β³ pipe, effectively eliminating the third cat. The factory 24β³ resonator links up there and then then the midpipe takes over up until the second merger where the 2.5β³ splits back to dual 2β³ to the mufflers. The stock setup has two cans out back but is effectively a single exhaust in the middle.
The intake side is already altered with a 3β³ CAI, and now that Iβve installed the custom J/Y pipe up front, I need to decide what to do. According to the flow chart, numbers, and max hp per 2.5β³ pipe is about 232 hp and 283 for 2.75β³ from what I understand. Assuming they are a guideline, are those HP numbers more or less determined as max crank or max wheel hp? I donβt want to overshoot and loose power, as the overall objective here is to gain maximum hp without giving up too much lower end torque. I am planning to keep the resonator to keep the rasp from the V6 in check, I want a throatier-deeper sounding exhaust rather than loud-raspy and annoying.
I was planning to modify the fabricated J/Y pipe up front, by replacing the 2.5β³ flex unit at the merger of the dual 2.25β³ coming in from the exhaust manifolds with a 2.75β³ (70 mm) flex unit and 2.75β³ pipe through the next flange and into a custom ordered 7 mm 24β³ resonator. I could do this or just do a 3β³ flex and pipe back to a 3β³ Vibrant super quiet resonator. Since this is where most of the real exhaust tuning takes place and where most hp is achieved specifically at the J/Y pipe up front, should I go 2.75β³ or 3β³ from the dual 2.25 manifoldβs? The other thing thing is the midpipe after the resonator, there isnβt a shop anywhere in Texas that Iβve found with a 2.75β³ die for their machine, mandrel bending would have to be 2.25/2.5/3β³ respectively, this implies either welding in a cone to gradually decrease to 2.5β³ which is oem by the way, or step up to a 3β³ midpipe from the 2.75β³ resonator with a transition, unless of course if you recommend I do a single 3β³ from the flex unit back all the way to this point. The end of this midpipe will merge back from a single to a dual again, where I currently have a unit installed from mandrel-bends.com. These units are top notch as the cuts are nice and clean, without obstructions that cause turbulence. I could do a 3β³ to dual 2.25β³, mandrel bent 2.25 axleback pipe to my dual inlet outlet magnaflows. The other thing is to leave it all as a single 2.5β³ all the way back to the axle back merger and split it off into the dual 2.25β³ pipe and magnaflows.
The numbers are clear, I just want your input please, Iβve pulled the trigger on most of these items, part of which are installed, part of which Iβve yet to under hesitation. My gut is telling to do a single 2.75 split in the back to the 2 cans, but I canβt, well unless of course if I get 2.75β³ (4) 45 degree mandrel angles and pipe and custom weld the whole thing to make the mid pipe and y merger. Help me please, I need to make an educated decision.

Kind Regards,
Raul

• Jason
Mar 3 2015

Raul,

I’m pretty confused by your comment, but I think I can help you anyways. The difference between 2.5″ and 2.75″ pipe is relatively small (~20%) as is the difference between 2.75″ and 3″ (~20%). If you have to go down to a 2.5″ section from 2.75″, you’re not going to run into any performance hindrance unless you’re running WOT. Since 2.5″ is the OEM size anyway, and your vehicle is mostly stock (just a CAI and an engine tune, I assume?), 2.5″ is fine.

Also, the hp figures in the chart are at the crank.

118. Renato Piereck
Feb 1 2015

I got a Peugeot 205 GTI 1.6 with 105 hp. I am upgrading the head to a model bigger valves that will put me at 115 hp, on the same size stock pipes. I also have a French downpipe that I want to install, which is know to make good horsepower on this car, but the rear end of this downpipe is 2.25″ size. Should I mount 2.25″ pipes all the way back, or should I downsize the pipes with a restrictor to a 2″ size?

• Jason
Mar 3 2015

Renato – Normally, a 2.25″ pipe is severely oversized for a vehicle with 115hp. However, turbochargers change the rules a bit. Feel free to keep the 2.25″ all the way back, as the concerns about exhaust gases cooling and slowing down are overcome by the forced induction.

119. Eli
Mar 2 2015

Hi Jason – Wow, I read through this entire thread. I commend you in being able to keep your cool while answering, technically, the same questions over and over.

I’ve been researching exhaust for a year and am glad to finally have found some actual demonstrable math and engineering sequences behind this. A lot of my searching has been on mustang message boards and it appears to be many people simply repeating what they read on the internet, without attempting to apply reasoning, science, or hands on experience. After visiting 6 exhaust shops in person, I was met with the same ignorance. And right on par with what I would expect, only 1 mustang owner inquired on this great write-up. With that said, I completely trust your advise.

Now to get to my question:

First of all, I inherited a 2007 v6 (4.0) mustang, which I read has 200 stock HP. Judging from your chart up there, when using dual, my pipes should both be no more than 1 3/4. Magnaflow has a similar chart involving their mufflers: For my engine, using duals, they say the muflfler in/out should be between 2″ and 2 1/4″ (reference: http://www.magnaflow.com/wideopen/performdata.asp) which is different, if ever so slightly, than the pipe sizes on this chart.

Adding to that, ALL of the aftermarket v6 “dual conversions” (Most of the big name muffler companies sell these) come with two 2.5″ pipes. I understand that they say the tests show these pipes to increase high end torque/horsepower, and they market them as doing just that, but being that these pipes go against the mathematical equation, my question is: will using two 2.5″ pipes, or even two 2 1/4″ pipes, really be all that noticeable regarding low end torque. ie: daily driving take offs, in regards to the whole velocity bit?

I just wonder… is it easier for these companies to package the 2.5″ (Most mufflers and tips on the market are easily obtainable and work with this piping size.. as opposed to trying to find something that fits a 1 3/4″!) and feed it to people who don’t bother asking the “whys”… or is it really just such a slight difference in low end torque loss that the average consumer would never notice?

Basically, if the car is a daily driver and has got a 4.0 v6 engine/200 ph, is equipping it with 2.5” piping really all that bad, and if so, what is the motive behind these companies selling their “true dual conversion” kits in that size pipes?

• Jason
Mar 3 2015

Eli – First, thanks for the kind words. I’m not sure why I kept answering the same question over and over…I guess I’m a glutton for punishment.

On to your question – if the chart suggests 1.75″ duals, why are manufacturers selling 2.25″-2.5″ dual systems??

You’ve basically answered it. First, they’re trying to prove horsepower gains, so they’re intentionally designing their system to sacrifice some low-end for some top-end. That way, they’ve got a nice dyno chart to show their customers.

Second, the loss of low-end power probably isn’t too noticeable to the average consumer, who “hears” the power more than they feel it. If you did a series of tests before and after adding the exhaust, you’d probably notice a small increase in 60′ times. But how much I can’t say. I doubt it’s more than 5%.

Three, it’s about packaging. The 2.5″ systems are perceived to be better, so that’s what they offer. Even Ford Racing offers a system this size, which makes me question their product line a bit.

The good news is that these are all cat-back systems, and the most critical portion of the system is from the head to the end of the 2nd catalytic. As long as that chunk of the system is dimensioned properly, things will mostly stay the same in regular street driving.

But if it was me, I’d probably go with a muffler upgrade and save my pennies for a procharger or vortech kit. Imagine the look of surprise on a GT owner’s face when your single exhaust wearing Mustang V6 beats him in the quarter mile. He’s going to feel pretty dumb. π

• Eli
Mar 3 2015

Ah, you have beautifully confirmed my suspicions. Now I can stop searching the internet trying to find answers. I can also enjoy a nice sigh of relief. Thank you.

(Reader note: My comment here is specifically in regards to upgrading a 4.0, 200 HP mustang, to dual exhaust in regards to your typical daily street driver. No racing or towing)

Seems we have a case of the age-old philosophical question, “If a tree falls in a forest and no one is around to hear it, does it make a sound?”

It seems to me that the cult belief of, “Don’t put 2.5 inch duals on a 4.0 mustang, go with 2.25 duals to keep low end torque” is ridiculous. You won’t notice any more low end torque being kept with the 2.25″ pipes than you would with the 2.50″ pipes due to them not only being oversized for the engine, but both also being closely related in diameter to (a) each other, and (b) the recommended 1 3/4″ pipe size.

Simply put I would gather that:
(A) Choosing between dual 2.25″ vs dual 2.5″: slight low-end torque differences between these 2 set ups will be picked up on a dynometer but not felt by typical street driver. Apples to apples. 2.5β is more readily available and comes standard in the kits. No use going out of your way trying to locate 2.25β piping because it will feel the same as using the 2.5βs.

(B) Choosing between dual 1 3/4″ vs dual 2.25″- 2.5″: low-end torque differences will be picked up on a dynometer but not felt by typical street driver. If choosing the larger pipe sets over the 1 3/4β pipes, I would assume that a more experienced/aware person may feel a slight loss in certain take off instances between the two set ups BUT there will also be high end HP gains. 2.5β pipes + bolt ons are readily available and marketed to you due to said HP gains. Low end torque loss minimal and still not felt by average driver, thus, not mentioned to you.

This, IMO, explains why there are no complaints about low end power loss on the reviews/community talk of these 2.5β dual exhaust conversion kits, I just don’t believe it is felt. In the end the customer is happy because, with the larger pipes, they get some high end gains and are able to tell all of their buddies about how they’ve just, “added 10 HP!” to their vehicles.

I look at it like colour shades– Most people would say dark blue is dark blue. But a small margin of professionals (interior designers, for instance) would be able to pick and point out the different blue hues in a single colour spectrum. To their clients.. this is moot and of no interest, as long as they get a beautiful interior to show off. In this scenario, the piping marketers/manufacturers are the interior designers.

Smart marketing, but I can’t help but to chuckle a bit.

Please, please, correct my response in a follow-up comment if I am wrong. I certainly don’t want to feed into any more miseducation on the internet.

Thanks for what you do!

• Jason
Mar 6 2015

I like what you wrote. All I’d say is that the ‘butt dyno’ is notoriously unreliable…I doubt there’s anyone that can feel a slight decrease in low end torque.

Thanks for commenting. π

120. Jose
Mar 18 2015

I have a 496 8.1 with longtube headers. I run 2- 3″ pipes to a muffler now question is the muffler is dual 3″ in/out but in the center of the muffler its only 1 3″ pipe so do you think of it as a Y but at both ends two Y’s together in the muffler. will that be a bottleneck? Or will it speed up the gasses b/c of the smaller diameter pipe something like the burnelli affect, the truck has about 350-400whp. I just want a free flowing exhaust thats all

• Jason
Mar 19 2015

Jose – I do not think of a 3″ dual in dual out muffler as a “y” – it’s more of a junction box. It is not a restriction.

• Mudi
Jul 1 2015

I have a 2014 chevy silverado with a k&n filter and custom tune pushing around 320 hp on wheel. I just installed some kooks offroad long tube headers (no cats) and I was considering a corsa sport catback…but I think going 3 inches from the y-pipe all the way to the tip will be too wide don’t you think? Should I just go with a magnaflow or american offroad muffler? or should I look for a narrower catback?

• Jason
Jul 20 2015

Mudi – The long-tube headers probably make the question academic, as they have a big impact on performance (a lot of truck owners don’t like them because they impact bottom-end performance).

Also, why remove the cats? How are you managing the ECM now – O2 sensor simulators?

There are a lot of moving parts here…you’re asking for advice about something that I would have thought about before I removed cats or added long-tube headers, so my advice is to go find a tuning shop that specializes in trucks and let them advise you. I’m not sure what you’ve done so far is wise.

121. Buz
Mar 25 2015

What is your take for an Explorer with 3.5 ecoboost motor? It is 365hp stock with 2.25″ dual exhaust. I think about 425whp or close to 500 at crank is possible which looks like 2.5-2.75″ pipes. Thinking go 1 size up on a turbo.

• Jason
Mar 30 2015

Buz – I think EcoBoost motors are fun, but I’m worried about carbon deposits on valves and stalling.

Still, there are a ton of after-market parts for these engines. Pretty sure you can find an extra 100-200hp. That will make your Explorer a hell of a surprise for some unlucky Mustang or Camaro owner. π

122. Kenneth
Apr 23 2015

I have the opposite dilemma most people have, I’m looking at building an exhaust system for my 660cc three cylinder Daihatsu hijet. The problem is in building something that is not too large. I have sourced the smallest 3-1 merge collector I can find, a 1.25″ into 2″. Will these three primaries be too much flow and reduce my low end torque? Can I step down the output from 2″ down to 1.5″ without issue? I just don’t know the effect of ‘creating’ an expansion chamber directly behind my collector.

• Jason
Jul 20 2015

Sorry – no idea here. When you get to small displacements, it all hinges on the valve design…can you copy the OE exhaust? I highly doubt it’s a hindrance.

123. frank
Apr 25 2015

What r your opinions on catback systems? I have an 08 silverado w 5.3. It has 3″ from the y to the muffler then 2 3/4″ for the tail pipe. Only thing done is a live tune and I opened up the airbox so it can breath better. Been looking at catback systems……but I’m seeing mixed reviews on performance. Some have said theres no noticable difference in performance just good sound…..I really don’t want to spend that kind of money on something if it’s not going to increase performance or mpg….it’s either this or a CAI…..thanks, frank

• Mike
Apr 29 2015

I’m not Jason, but generally catbacks are garbage for performance.

The closer to the engine you modify the exhaust, the more difference in performance it will make. That is, of course, IF the exhaust is problematic in stock form, and the changes made are beneficial, not negative.

Exhaust is a very complex science. Too much exhaust (in terms of CFM the system can flow) is as bad as too little, but for different reasons.

In the case of factory pipe sizing after the converter? No benefit.

Any modifications made to intake or exhaust airflow MUST be tuned into the PCM unless the system is using wideband O2 sensors…common narrowband o2 sensors cannot modify fuel delivery under heavy load, which is where airflow modifications are most critical to make sure fueling is right.

• Jason
Jul 20 2015

Mike – Great comment. Thank you. π

• Jason
Jul 20 2015

Frank – I’m a huge fan of tuned cat-back systems (emphasis on “tuned”). But can you noitice a difference in performance? Not with the old “butt dyno”, which is what most people are using when they say that they noticed no difference.

If you just want rumble and don’t care about adding a few HP, I’d suggest getting a muffler only and installing that on your OE system. That will sound great and will cost \$200 or less.

124. mike krikun
Apr 26 2015

I have a ’94 GMC Sierra with a 200HP 305. Originally equipped with a restrictive muffler and a 2/1/2″ single outlet behind the rear wheel. Now it has a Dynomax cat-back system with two 2/1/2″ pipes out of the back. I now realize that this is major overkill for this engine and my style of driving. Muffler has two outlets. Can I remove one side of system after the muffler , weld that muffler outlet shut and use the remaining side? What will this do to my fuel economy? Thank you.

• Jason
Jul 20 2015

Mike – It could help to weld one side shut. It definitely won’t hurt. The concern I have is about the part of the exhaust system before the last cat…is that original? If so, it’s probably no big deal either way…

125. Roberto
May 11 2015

Hi jason,

This thread is amazing, Thank you for keeping it alive. I have a civic type R fn2 (4 cyl, 2000cc, dohc.. 201 bhp @ 7500 rpm /142 lbΒ·ft @ 5600 rpm) and i’m seeing that many of the aftermarket exhaust are 3″ and only a few are 2,5″. OEM is 2,25″ (with 4 mufflers, lol, NA muted)

The question of what pipe size is better is over and over asked in the forums and most of the people say that the dyno proves that 3″ is better. furthermore, some custom exhaust made by famous tuning workshops of the civic scene are 3″.

so.. why? i have been reading about this a while and i didn’t found any logical reason for a 3″ exhaust perform better in a 2 liter NA engine.

Maybe the people only want to see the peak HP’s in their dyno graphs..or maybe they 3″ do magic..or they are planning to make their engines work between 7900 and 10000 RPM dunno…

as far as i understand, for NA tuning for ~250-260 HP a 2,5″ pipe is better that 3″. More mid-range torque without sacrificing any or only a few top end bhp’s…

• Jason
Jul 20 2015

Roberto – It’s all about making the peak HP as high as possible. Bigger exhausts sacrifice low-end grunt, but they accomplish maximum throughput at high RPMs. So…the “dyno” says that they’re better, but the race course might disagree. π

• Mike
Jul 21 2015

The unfortunate truth is that the “import tuner” market is based on marketing hype and what looks cool, not what actually works. Without digging into the specifics (“cold air intakes”, “high flow air filter”, larger throttle bodies, bigger injectors, etc) you are correct in your assumption that the bigger pipe diameter is not helping.

The import tuner market by and large is cluttered with people who have bought into marketing hype, then use that hype to advise others on what is supposed to work best. With an entire community that thinks there is something wrong with the OEM setups that can be cured with a few hundred dollar bolt on pieces here and there, with no analysis, dyno testing, and actual tuning, it is very easy to sell components that are questionable if not downright preposterous.

There ARE good modified import cars out there, done by people that actually understand engines, but the majority of what we see is all about appearance, not function.

126. Sam
May 11 2015

Quick question, is the table still accurate when translated to single pipe exhaust? I’ve been looking into upgrading my stock exhaust (single 2.75″ pipe) to headers (LT with 1 7/8″ primaries) and a single 3″ pipe (duals are nearly impossible with my car). I crunched the numbers and found that the cross-sectional area of the pipe would be roughly the same as two 2.12″ pipes. So would my HP be limited to somewhere below 371 (max hp listed for dual 2 1/4″ pipes)?

Thanks,
Sam

• Jason
Jul 20 2015

Sam – There’s a column for singles.

• Sam
Jul 21 2015

Oh that must be the “max hp per pipe” column. I guess I figured there might be some crazy physics involved that would make a single pipe produce different numbers than half of what a dual pipe makes. Thanks for clearing that up!

127. jsmes harris
Jun 5 2015

throuly enjoyed article 85 years old but would like little more sound have 2015 kia optima turbo would like kn typhoon and perfomance mufflers any problems has more than enough power!! thank you very much

• Jason
Jul 20 2015

James – Thanks. You wont’ have any problems adding an intake kit or exhaust upgrade to your Kia. Have fun. π

128. Yash Shindekar (India)
Jun 15 2015

I want the formulas for calculating the exhaust pipe of a two wheeler, it should contain formulas of diameter of pipe, back pressure, length, etc… We need this calculation for manufacturing a Go-Kart which has engine of a two wheeler….

THANK YOU..

129. Mitch
Jul 21 2015

I have a 74 dodge truck with a 318. We don’t know the exact hp but are thinking it should be near the 200hp range. (318 with comp cam (rv) 4 barrel eddy 600cfm and eddy intake with hooked headers)
We are removing the current exhaust and going to redo it. My mech wants to go easy old school and bring two pipes right back with individual muffler. 2 1/4

I was ok with the 2 1/4 until I read this. I looks like I should be 13/4 and I want to install either and H or X pipe but yet have two mufflers and side dump. Any thoughts ?

• Jason
Jul 26 2015

Mitch – There’s some room for error in the numbers, especially if we’re talking about older vehicles…the combustion efficiency can very greatly from one to the next, depending on the type of parts you’re working with (basic carb, or EFI conversion? factory heads or high-perf aftermarket varieties?).

I guess I’m wondering if the 200hp figure is too low. But if not, I’d say back things off to maybe 2″ and call it good…you’re close enough on that old truck.

• Mike
Sep 6 2015

If looks are important to you, the small exhaust looks goofy today, when 3″+ exhaust is normal on larger engines.

I’d tell him not to go easy, I’d want a single exhaust of the appropriate size. It’s easier to work around, and it requires less components.

For anything other than looks or sounds (or packaging constraints, which there is not on a truck), duals are pointless today when larger diameter pipe is easy and cheap enough to find.

130. John
Aug 21 2015

I have a 2001 Taurus w/3.0L Duratec. For true dual exhaust set up, if I ran 1.75 in pipe from front to back with 2 in cats, “X” pipe, and resonator in between, would I lose a lot of low end power?

• Jason
Sep 6 2015

John – It’s possible. Based on what I know about the stock 3.0L, it’s not powerful enough to justify a dual exhaust.

131. Ian
Sep 6 2015

Hello, I have a first gen mazda 3 with the 2.3 litre duratec mazda engine. Stock 163hp. I have vibrant headers installed which are a 4-2-1 design. With the stock exhaust the headers showed a good increase in power, estimated 10 hp. I then installed a 2 1/2 inch exhaust from the catalytic converter back. The stock exhaust was 1 7/8 inches in diameter which reduced from 2 1/2 to 1 7/8 after the catalytic converter. After installing the free flowing 2 1/2 inch exhaust I felt a loss of torque and power across the rpm range. My question is can I install a length of tubing between the catalytic converter and muffler with a reduction is size to say 2 1/4 or even 2 inch. Would this replicate the scavenging effect the stock exhaust had? I enjoy the sound of the free flowing exhaust but miss the low end torque so I don’t want to re install the stock system. Thank you Jason.

• Jason
Sep 6 2015

Ian – It’s possible, but not likely. My best suggestion is to replace the stock system but keep the after-market muffler.

Also, you might check to make sure that the horsepower loss is significant…the old “butt dyno” isn’t terribly accurate.

• Mike
Sep 6 2015

Ian, does that vehicle use wideband O2(s)? If it does not, you may be feeling a lack of fuel (lean) under throttle if the exhaust helped.

How far past the converter did they neck it down? Converter adds more heat to the exhaust, so if the necked down portion is immediately after the converter, it may have been a restriction that results in BETTER scavenging now, and a lean condition. But as mentioned, guessing (butt dyno) isn’t real valid to come to good conclusions.

• Ian
Sep 7 2015

Mike, the mazda 3 from 2006 onwards I’m pretty sure has a wideband primary O2 sensor. Mine is 2007 so I think it does, not 100%. The piping necks down directly after the catalytic converter about a couple inches from 2.5 at the downpipe/catalytic to 1 7/8 all the way to the muffler (stock). Mine is now 2.5 all the way to the exhaust tip.

132. christopher
Sep 28 2015

im doing the exhaust on my 3.6L V6 g8 and i belive the stock pipe size is 2.25 i gonna be running long tube headers with primary pipes at 1 5/8 then a 3 inch collector then H-pipe i was thinking of running the stock 2.25 pipe size with long thrush glass packs would this work good? and support the power im makeing or should i step up to 2.5 pipe?

• Jason
Oct 5 2015

Christopher – Long tube headers trade some low-end torque for top-end power…you’ll probably lose some performance on the street, at least if you’re keeping things near the speed limit.

Unless you’re trying to maximize your quarter mile time or performance at higher RPMs, I do not recommend long-tube headers.

• christopher
Oct 5 2015

im looking to increase my quarter mile time for the car. the real question is off the headers should i reduce down to a 2.5 or a 2.25 pipe after the h-pipe and have the h-pipe the same size?

• Jason
Oct 6 2015

Christopher – Without knowing a whole lot about what you’re doing, I’d say that you want to lean towards a larger size when optimizing for quarter mile time. Whatever low end torque you lose is more than made up for by better flow at high RPMs, at least as far as time and trap speed are concerned.

• Mike
Oct 6 2015

Did my other comment get lost?

Jason, do you mean you don’t recommend shorties except for high RPM performance? The testing I can find shows that long tubes make better power at the low end, but sacrifice some at the extreme upper RPM end of the same engine vs. shorties.

• Jason
Oct 6 2015

Mike – You’re right – I’ve got it backwards. Thanks for catching that.

133. Andrew
Oct 21 2015

I have a 1976 ford cobra 2, this car has been modified to have a 7.0 V8. It currently has corvette side pipes with no mufflers, could it make more hp if i add mufflers?

• Jason
Nov 8 2015

Andrew – Probably not, only it would be street legal…

Once you get to a big enough block (or forced induction) the concerns about exhaust cooling too quickly and “stopping up” your exhaust are minimal.

134. Nov 6 2015

Jason,
I think it’s great that you take time to answer the questions. I’ve read through all the posts and can’t find anything quite like mine.
I have a 1/2 ton Dodge truck with a 440 small block, Indy heads
& intake, larger cam, 2- T60 turbos, huge air cooler. At 12lbs of boost it made 707/685 through an automatic and transfer case and 35″ AT tires.
The exhaust now is a 3″ down pipe from each turbo merged into a single 5″ system. Running 2- 5″ Aeroturbine mufflers in tandem and 5″ all the way out including the tail pipe.
I’m the process of adding a water/meth system with another 5-6 lbs of boost. Do I need a 6″ single or run dual 4″ tail pipes off the present 5″ system? I’m expecting to be in the low 800’s at the wheels, approx 1100-1150 at the crank. I don’t want the wrong exhaust holding me back. Thanks

• Jason
Nov 8 2015

Mark – My best answer is to dyno it with and without the 3″ pipes and see if they’re a restriction.

However, since that’s not so easy – and since we’re talking about forced induction – there’s very little downside to increasing the exhaust size.

135. Brent
Nov 8 2015

Hi Jason, I’m running a 440bhp 2litre supercharged civic type r ep3, with 3 inch catback exhaust and 2 inch 4-1 race manifold, would having a bigger catback help me increase my HP? Thanks

• Jason
Nov 8 2015

Brent – It might, at least at WOT. Since you’ve got forced induction, going with an over-sized exhaust isn’t a big deal…so you can just make it a little big if you want.

However, if you’re streeting this car, I wouldn’t worry about it.

136. Stafohn g
Nov 30 2015

I have 2.25 inch exhaust. What size tips do i need 2.25 or 2.5

• Jason
Nov 30 2015

Whatever size you want. Tip diameter doesn’t matter. Pick something that looks good, pick your favorite number, whatever. π

137. Layne
Jan 11 2016

I am in an area where there are no shops that do mandrel bends, they all do stretch bends.

How does this effect recommended pipe size?

Also, it is good to run a smaller tail pipe from the muffler back?

I have a 215ci, 200hp, V8

Thanks

• Jason
Jan 11 2016

Layne – Crush bends aren’t the worst thing in the world, but they impact flow rates (particularly at higher RPMs).

The problem with increasing tube size to counteract the bends is that you still end up with flow rate issues (the turbulence inside the tube is the issue), and now you’re risking low-end power.

My advice is to install mandrel-bent tubing if at all possible, even if you have to order it online.

138. mike
Jan 16 2016

OK Jason, been reading this blog for an hour & love it. Been fascinated by exhaust for a decade & scooping up all every lick I can.
Quickly, my question’s for a 302 (Low end cam/600 carb) in a ’65 Fastback on its way to Germany to sell one day. Even w/Autobaun, I’m building it for torque/mileage – because they spend as much for a litre as we do a gallon! (k, not quite, but its bad, yea?-lived in Barcelona 2yrs)
High rear axle ratio, so I’m “All about the torque, bout the torque, BIG Bottom!” So LOVIN Small piping!! Just makes so much sense.
So here it is: Ford Tri-Y’s are 1 1/2″, to 1 3/4″ & JUMP TO TWO & ONE HALF!!! Where is the logic in this??? MY best reasoning sz: Cut the Tri/Y’s at the end of the secondaries…& run 2″ all the way to the Bumper!!! (X,H,Muffler,Resonator, even ‘Branch Resonator’ aside. More on this another day: Timing official w/SCCA at Laguna Seca here in Calif. Historic races w/12cyl Vintage Ferrari’s give me wood like nobody’s biz & the closest thing??? “Small block GT350 w/X-pipes, at full song”!!! Don’t Argue, chk ur own wood ;-}~)
Delivered in the kit, is a set of conded reducers: 2 1/2″ to 2″. “D’oh!” I’m not a rocket scientist (but ‘I do play one on TV’) -this just kills me! I just feel Mandrel bends & minimizing expansion/contraction, especially early on’ when exh temp is at it’s hottest/ FASTEST,is the optimum protocol.
K, I’ll quit. Whattya got Jason? throw me a bone, here. -Mike-

139. Jason
Jan 18 2016

Mike – My first question: Is this exhaust setup developed for a specific head? If it’s for the factory head, I’m thinking the size is OK. The 2.5″ tube size is overkill, but it’s far enough back that it’s probably not a big deal. As the exhaust cools and moves away from the engine, the size of the tube becomes less critical. Also, these older engines run rich and don’t necessarily adhere to the table (they’re more likely to come in over than under). They’re not as close to stoichiometric efficiency as fuel injected vehicles.

Mostly, my advice is to find a set of headers that are designed for your specific head. If it’s designed for the heads you have, I think you’re good to go.

Finally, if you want to tinker, you might try putting the reducers on and see how it runs (at least after you’ve got a few hundred miles on the new system.

140. Cordell
Feb 9 2016

I built an 360 magnum for my 01 Ram. Its pushing between 350 and 450 HP. I want to get duals that go out the sides at the end of my cab. Would a 2.25 setup be best? Straight through mufflers as well.

141. Gary
Feb 16 2016

Hi, i want to design a good exhaust muffler for my Ktm Duke 390. i want the bike to make good smooth sound with some base in it to have a feel of big bike. can you please help me design the same

142. Robert
Feb 27 2016

Hello. Ok, I’m getting so many different opinions I’m punch drunk. Here is what I have.

99 Ford Crown Vic Police Interceptor, new Ford crate engine 4.6L. New factory exhaust manifolds, new Walker cat assemblies, dual factory 2 inch exhaust.

Currently running a 65mm throttle body, factory MAF. The near future will be a 80mm MAF, 70MM throttle body and ported plenum.

My question: Is it worth the cost to have 2 1/4 or 2 1/5 inch piping made, will it give me any hp increase or anything? I hear 2 1/4 is the right upgrade, then some say 2 1/5 but may lose low end torque.

Which size pipes??? Thank you.

• Robert
Feb 27 2016

Meant 2 1/2 not 2 1/5, never said I could type…. π

• Jason
Mar 7 2016

Robert – If you’re just changing the MAF and throttle body, I’m not sure that an upgraded exhaust is essential. The factory exhaust does a good job of performing both at low RPMs and high RPMs, all things considered. I wouldn’t mess with it unless you’re going to upgrade power in a significant way (new internals, forced induction, etc.).

Feb 27 2016

I just installed a gmpp 350 ho and am adding headman headers to my dailey driver driver truck. I am using duel turbos as well. Considering the motor will be kept under 5700rpm should i go with 2.25 or 2.5 inch pipes?
Thanks

• Jason
Mar 7 2016

Chad – With turbos, over-sized is better than under-sized. I’d go 2.5″. Sounds fun, btw. π

144. Gabor Kato
Mar 1 2016

HI,Jason!
Please help me with exhaust diameter for my bike..I have a 988cc engine with 12250 rev limit. The bike put out N.A. 160hp(not wheelhp)@10500 rpm.What diameter size i need to 200bhp@12250rpm?
250bhp@12250rpm?
300bhp@12250rmp?
This size work on the supercharged engine too?
Thank You for the answear
Gabor

• Jason
Mar 7 2016

Gabor – Sorry. Can’t help. Try the formulas, test, and see what happens. Bike engine design is different.

145. Alan Tomalty
Mar 14 2016

Hi Jason

I have a 2.5 litre 2006 Altima QR25DE engine that has 2 cats on it. It is a PZEV vehicle built for the California CARB requirements. I got the car 3rd hand and replaced both cats with new after market stock cats along with the 2 O2 sensors and a new stock muffler. I am still scared of the whole problem of exhaust gas getting sucked back into the engine and don’t want to have to worry about that so what header do I get that makes sure that my new precat pipe is larger than my header outlet? Or will I have to get a custom made header. If so how much less in diameter should it be than the precat inlet. I don’t know my precat pipe size.

Thanks Alan

• Jason
Mar 18 2016

Alan – Don’t know. My advice is to try and find a complete exhaust system (headers and cat-back) from one company, then model your system after that one. If you find a company selling a header and a cat-back, and they’re using a 4-2-1 collector (for example), that’s the kind of header I would buy.

You can also try testing a few different setups on the track.

146. Pictsidhe
Mar 25 2016

Hi, read a chunk of this. For all those wanting flow without noise, the walker quietflow mufflers are great. The steel ones are also cheap!

147. Daniel
May 17 2016

Are dual 2.25″ pipes too small for a 440?

• Jason
May 18 2016

Daniel – Maybe, depends on how it’s tuned and what you’re trying to do.

If it’s a truck and you really only care about pulling a trailer, it might be too big. If it’s a standard sort of street 440, it’s probably a good size. If you’re racing the 1/4 mile and you’ve got a high horsepower setup (with racing heads, cams, etc.), it’s probably too small.

• Daniel
May 18 2016

It is in a truck and will bruised to pull a boat on occasion. It is mainly a weekend toy though. The engine is a 71 block with the original pistons and rings still in it. It has a large cam (probably too big) and closed chamber heads. It has a 600 carb on it, but I’ll be putting a 750 on when i get the chance.

148. Michael
May 22 2016

Hello Jason,

I have 3 vehicles and 2 of them I have done true dual exhaust and I would like to ask you some questions.
1st vehicle is a 93 E-350 with stock 460 BB running 3″ dual after the catalytic converter flang. Should I run 3″ from the header to what I have added?
2nd vehicle is a 2000 S-10 with stock V6 running 2.5″ dual exhaust. This was a put together and I used what I could grab and I was thinking that I need 2.5 from collector to cat when it is 2.25 or 2″.
3rd is a 91 Vic all stock. Should I do 2.5 dual exhaust?

• Jason
May 23 2016

Michael – In order:

1st vehicle – 3″ tubes for each side of the motor are huge. Can’t imagine it needs that much flow. The chart will tell you what size to put on each side, but I would say that the stock setup is A-OK unless you’ve made lots of modifications.

2nd vehicle – 2.5″ duals are outrageous on a V6 from 2000…way too big. See the table. π

3rd vehicle – If you’re going to turn it into a 450hp+ speed demon, than yeah, 2.5″ duals sounds great. But if not, look at the chart in the article.

• Michael
May 23 2016

Hello,

I know the chart says that dual 2.5″ on the S-10 is to big, but I have had the truck dynoed before and after. All that has been done is pacesetter headers that run into dual cats and dual glasspacks which run on the outside of the frame and dump in front of the tires. The hp and tq before were 141 and 166 and after were 155 and 200. I tend to believe results. I have upgrades that will push the power closer the the 500 mark.

Once I rebuild the 460 BB. It will be a 502 and I will be shooting for 600-750 all motor.

Not sure what I want to do to the Vic yet.

I’ll keep your mind going wtf

• Jason
May 25 2016

Michael – There’s no substitute for testing – the chart is just a guide. If you’ve dynoed the engines and seen improvements, that’s great. I’ll never argue with data. π

However, I would suggest that you check the HP and TQ output in the low RPM range where you drive on the street. Often times, I see decreases in these lower RPM ranges with over-sized exhaust systems. Headers can often cause this problem too. This is because exhaust gas velocity is better in smaller exhaust tubing at lower RPMs.

If you’re not concerned about the performance at these lower RPMs, than by all means put whatever size you think is appropriate.

149. Vincent Law
Jun 9 2016

Hello, what a great read with alot of helpful info. Iβm about to own a KIA Sorento with a 2.2L turbo diesel engine that puts out 147kw/3800 rpm and 441.3nm from 1750/2750rpm.
Test drove the car and its a no brainer for a 7 seater with lots of options thrown in. My only beef with the car is that it feels great off the line but I feel its lacking in the 50mph-65mph range especially when I want to overtake. Based on your chart, its it recommended that I go with a mandrel bent 2.25 inch pipe or should i go with 2.5 inch to increase the performance in the 50-65mph range without sacrificing off the line performance.
Also, from what I read from other sites, some seem to follow the mantra that bigger is always better for turbos as all the effective inertia is used ip by the turbo and getting the the exhaust out asap, allows the turbo to spool up earlier. what are your thoughts on this?
my other option is to chip the car to generate around 245hp and upgrade the exhaust if money was no object lol. now i have to choose between the two.

• Jason
Jun 20 2016

Vincent – The sizing charts only apply to gasoline powered engines – they’re not applicable to diesel.

I’d say that the best way to squeeze more power out of any given diesel is with an aftermarket engine tuner. Air intake and exhaust kits can also help, but engine tuners tend to have the greatest impact on diesels.

• Robert
Aug 5 2016

Okay I have a question about my 96 bronco. Stock 302. It currently runs a single 3 inch pipe from front to back with a flowmaster 40 series. I bought a 50 series 2 in 2 out 2.25 diameter flowmaster. Realizing that it’s a stock engine, will changing from single to double and downsizing the pipe help me or hurt me?

• Jason
Aug 8 2016

Robert – A single 3″ is massive for a stock 302 – I’d guess that downsizing will help you out a lot.

Still, a dual 2.25″ exhaust is awfully big for an engine rated at 200hp. I’d say you’re still using a system that’s oversized.

150. Diego
Aug 15 2016

Hello;
My name is Diego and i’m running an 230 cu inline six cylinder engine. The exhaust system is 6-2-1.
Secondary pipes are around 2″ and the tail pipe after the Y pipe and before the muffler is one 2″ pipe. The car is around 175 horses at 4500 rpm. The engine doesn’t sound harmonic after 3500 rpm and loose power. Do you think the exhaust is too small? Can it cause a noticeable lack of power and bad feeling of the engine?

• Jason
Aug 16 2016

Diego – A 2″ exhaust for a 175 hp engine might be a bit on the small side. Our chart suggests 2.25″ tubing, but in practice I’d say it’s probably not going to make a huge difference.

However, the loss of power after 3500RPM may or may not be related to the exhaust system. Is the configuration of the manifold the same as it was when it left the factory, or is it different? If it’s a custom setup, it may be that it doesn’t work as well as the OEM manifold it replaced…new OEM manifolds are pretty cleverly designed.

The fact that you say the engine doesn’t sound right leads me to believe there’s something else wrong. A restricted exhaust usually just makes for a weak engine, as combustion efficiency drops off due to a “back up” of exhaust gases. That doesn’t seem like what you’re describing.

• Diego
Aug 16 2016

Jason;
Thanks for your reply. You are very clear and yes… something else looks wrong. For one moment I thought the exhaust was part of the problem, but your explanation of a restricted exhaust symptoms make the things more clear.

The exhaust is OEM for the original engine of the car with 155 hp, but I upgraded the setup of the engine changing the whole intake of the car with 3 weber DCOE carbs.
This is the same setup that came in the hot version of my car that delivered 176 horses. The only thing I keeped original is the exhaust. In the hot version, the header is the same but the two secondary pipes are 2″ instead of 1″ 10/16 and the tail pipe consist in two 2″ tubes instead of one.
When I made the transformation I thought that my exhaust was good to deal with 176 horses but now…. this is the reason why I’m asking you.
At this moment, the muffler is a round XR1 borla 2.5″ inlet 2.5″ outlet.

• Jason
Aug 17 2016

Diego – Since we’re talking about a carbureted vehicle, I’d say your exhaust is big enough. They aren’t as efficient as fuel injected vehicles, so the exhaust requirements are a little lower.

I’d say you have what you need in the exhaust dept. Good luck!

151. Samuel
Aug 19 2016

Yo i have a v6 pushing around the 500hp it red lines at 7205 and at the time i didnt know about the exhaust and apparently it has 1 1/2 pipe going down the middle and cutting off into 2 2inch pipes does that mean i need a whole new exhuast system

152. Pete Kerr
Sep 20 2016

Hi Jason,
What’s a good set up for my Ford FPV 2003 BA Pursuit Ute 460hp, Thank’s Pete.

• Jason
Sep 22 2016

Pete – I’m not sure. My expertise on non-US vehicles is quite limited. My standard advice is to get a cat-back system that’s tuned for your vehicle, but I don’t know if those are available.

153. Alex7786
Sep 27 2016

Hi Jason,

Great article! It’s been really helpful.

I’ve got a bit of an issue, as an exhaust shop installed 2″ piping on my BMW 645 4.4l V8, dual system. 333HP at 6100rpm.

I believe this is way too narrow for my car and I’m arguing with them to get a refund.

What piping would you suggest is optimal for my car 2.25″ or 2.5″?

Thanks,
Alex

• Jason
Sep 28 2016

Alex – If you look at the chart, you’ll see a 2″ dual system isn’t completely out of line. A slightly smaller size can really help with low-end torque, and they may have designed your system for streetability.

However, my advice is to ask them to emulate a cat-back system from a well-known exhaust manufacturer (like DINAN or BMW’s M division). If they build your system to match the size/configuration of those systems, great. If not, you can work on them to try and get a change made.

• Alex7786
Sep 30 2016

Hi Jason,
Your chart does specify that 2″ is good up to 289HP, which is 35HP less than my car produces (333HP).

Thanks for the advice on emulating a well-known manufacturers sizing, that might be a good course of action.

I’ll do some research, thanks for your help.

Alex

154. jason emm
Oct 3 2016

I had a flow master super 10 on my 2010 Silverado before. it was a single to a dual set up. loved it. I have upgraded to a 2016 Silverado 1500 z71. My question is, how do I know what size is running on the stock exhaust as far as the in and out diameter. I am thinking of running dual super 10’s this time and want to order the right size muffler. its much cheaper to just buy the super 10’s and bring them in vs having the shop get them.

155. Paul
Oct 10 2016

very good read and I can confirm correct size pipes was a huge improvement.
I was building a engine exhaust based on my head flow 320cfm from a duratec 2.0l na expected just under 300hp.

I sized the exhaust at 1 5/8 primaries into 3″ collector all tuned length. even tho my head flowed 320cfm I didnt take into account my itb’s only flowed 257cfm with CAI fitted it dropped a bit of cfm as well. On the dyno it made a peaky 5700rpm peak tq to 265hp at 8900rpm and off cam was just bad.

Change exhaust to1 1/2primaries, 2 3/4 collector tuned length picked up power everywhere 4400rpm – 8800rpm 275hp. off cam it drove almost stock except idle was a lot rougher than stock engine.

Eventually I will swap up to proper iTB’s that match my head flow and swap back to the larger extractors.

• Jason
Oct 22 2016

Awesome – always good to see these comments.

156. Rick Z
Oct 22 2016

I can’t believe this thread has been going on for 5+years! Thanks Jason! After reading through all of the posts, I don’t think my question has been covered.

I have a 71 Blazer with a 350 SBC that I estimate to be in the 350hp range. I am running an aftermarket ram horn style exhaust manifold that is supposed to flow similar to a header at the lower rpm’s. It has a 2.5″ collector. The chart says 2.25″ dual pipes, but would I be restricting the exhaust manifold. I would consider single exhaust also.

• Jason
Oct 22 2016

Rick – Since the collector is 2.5″, I’d say you have two choices:

1. Neck down that 2.5″ to 2.25″, or

2. Call it good because it’s only 1/4 of an inch.

The chart is a good estimating tool, but if you can get within a quarter of an inch of the estimate, I’d say don’t sweat it. Especially on a carbureted 350, which is probably not as impacted by all of this anyways.