What’s a Downpipe and Why Would I Want One?
What’s a Downpipe?
Technically, the “downpipe” section of an exhaust system connects the headers or exhaust manifold to the catalytic converter(s). For most vehicles, the downpipe section is essentially incorporated with the catalytic converter section, and since there’s very little performance or fuel economy benefit to replacing a modern catalytic converter (they’re just not that restrictive), most vehicle downpipes are never given much thought.
However, on a vehicle equipped with a turbocharger, an after-market downpipe can offer a significant improvement in performance. Because a turbocharger is essentially a pump, it’s important for the pipes entering and exiting the turbo to have unrestricted airflow. On many turbocharged vehicles, the exhaust pipes coming in and out of the turbo contain numerous bends. If these pipes are bent using a cheap, crush bending process (a common situation for many vehicles), they can restrict exhaust gas flow in and/or out of the turbo.
By reducing exhaust gas restrictions, the turbocharger can spool up more quickly. Faster spool-up times mean more power and/or better fuel economy. As a result, many after-market exhaust manufacturers offer over-sized exhaust downpipes with mandrel bends.
Why Would I Buy A Downpipe?
You might want to buy an after-market downpipe if:
- The exhaust piping leading in and out of your turbocharger contains numerous bends. Some vehicles – like large turb0-diesel trucks – have enough room in the engine bay for fairly straight exhaust tubing. Smaller vehicles can have a lot less room in the engine bay and therefore more pipe bends.
- You’ve modified your turbo to run at a higher boost pressure. Some turbocharged vehicles come with a well-designed turbo exhaust system, and in these cases there is very little to gain by adding a new downpipe. However, if you increase the amount of boost on the stock turbo, you may find the increased exhaust gas output demands a larger, smoother downpipe.
- The after-market downpipe is proven to improve performance. Most after-market downpipes are both mandrel-bend and larger in diameter than the stock units they replace. Therefore, they can significantly reduce restrictions and make fairly dramatic improvements in horsepower. Adding a downpipe to an older Subaru WRX exhaust system, for example, can add as much as 40hp all by itself.
Subara WRX’s aren’t the only types of vehicles that benefit from downpipes. Despite having fairly efficient and well-laid out turbo exhaust systems, many late-model diesel pickup owners can add 10-20 hp with nothing more than a new downpipe.
Bottom Line: If you own a turbo-charged vehicle, it’s wise to research your downpipe options. Just be sure that you see a dyno test demonstrating a significant performance improvement before you buy.