Re: "Performance" Exhausts... A Must Read... (archive)

Great thread and a "hot" topic!
Backpressure helps reduce sound and too much backpressure also reduces horsepower. Backpressure is usually the enemy and that's why 8-cylendar race cars use balance tubes. However, if the exhaust pipes are too big then you can also lose power because the exhaust gasses won't flow as smoothly and continuously. This could easily cost you 4 horses. The proper system is tuned in such a way that it flows smoothly with minimal restriction, gets the stream of gasses moving and then it actually pulls that last bit of exhaust gas out of the chamber so you can burn more gas. A really big pipe just won't do that. I agree with Joe that a stock M6 exhaust probably has the correct backpressure and flow characteristics for the stock engine, but I don't buy the idea that backpressure is beneficial by itself.

correct me if I'm wrong....

I think what you are talking about is the scavenging effect of a properly designed exhaust system. It will pull exhaust gasses from the combustion chamber rather than having the piston "push" the exhaust gases out. This is usally done when a certain cfm of gas flow is reached, correct? This could also be called a "tuned" exhaust.

As for backpressure? If an engine needs to flow say 100 cfm of exhaust gases to provide top performance and the exhaust system is only capabile of flowing 95 cfm --you will end with "backpressure". Doesn't that same backpressure cause a power drop? Since now the engine has to push the gases out of the combustion chamber.

There is a difference between backpressure and scavenging effect of an exhaust. Just because a car is quiet and doesn't make alot of noise doesn't necessarily mean that the exhaust can't flow enough cfm for the engine to
properly reach maximum output. That's why proper size pipes etc are important to tune an exhaust.

Follow Ups:

[ Follow Ups ] [ 6-series (E24) Message Board ] [ Msg. Board FAQ ]

Questions, comments, or problems, please visit the Roadfly help desk.