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…
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.
Useful Links
Great forum discussion that really discusses the details of the scientific calculations: http://www.eng-tips.com/viewthread.cfm?qid=104735
An interesting discussion of header pipe designs: http://victorylibrary.com/mopar/header-tech-c.htm
A good general article about designing the perfect exhaust system: http://www.popularhotrodding.com/enginemasters/articles/hardcore/0505em_exh/index.html
Trackbacks & Pingbacks
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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.
@ 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.
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
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.
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
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
Great site and helpful
kind regards
Geoffrey
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.
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).
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?
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″).
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?)
Michael – I’m sorry I misunderstood – 2″ collectors are quite big. No restriction there.
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.
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
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
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
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…..
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?
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.
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.
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.
Any advice or comments would be sincerely appreciated.
Thanks, Larry
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.
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
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
Matt – The advantage in increasing pipe size at your HP rating is better performance at high RPMs.
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
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?
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
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?
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 ?
Agreed! Thanks for commenting!!
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
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
justin – I agree with Michael. Better to go with the smaller size if you value low-end torque.
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.