Backpressure on a N/A car?

[c0dy]

Ive heard people argue that some backpressure is actually benefitial to a naturally aspirated car, and i've heard that its good to have as little as possible. It's hard for me to imagine backpressure actually helping, as i would think it would slow down the movement of the exhaust in the exhaust stroke, and I also see top fuel cars running tubes only a few feet out (and im assuming this is just so the fumes and occasional flames dont go where they shouldn't). does it depend on the engine or is one statement true? Thanks in advance
Cody

 

[FredK'03Cobra]

A certain amount of backpressure is needed to optimize power, whether the car is naturally aspirated or not. The idea is to match the correct scavanging and tube size to maintain proper flow.
Top fuel is a poor example to use. The airflow that they must achieve to reach the 6-7,000 horsepower level from 500 cubic inches, is incredible. And, their tubes are carefully matched -- for that application.

 

[c0dy]

Thanks : )

 

[ka3ak]

Bear with me as I try to explain. During the exhaust stroke, the gas is pushed out of the chamber and into the manifolds and the rest of the exhaust. Since the stock pipes are somewhat small, the flow is not very efficient, and some of the gas seaps back into the combustion chamber. To remedy this, people increase the size of the pipes. By doing so, the flow restrictions are eliminated. However, the bigger the pipe, the more space there is for the exhaust gas to occupy, and the faster it will cool. Hot gas travels faster than cool gas, and cool gas creates another restriction. It creates a sort of traffic effect, and again, some of the exhaust gas seaps back into the combustion chamber. The optimal setup would be open pipes/headers. The closer the opening to the motor, the better. Since that's not legal on street cars (but often seen on top fuel dragsters), we need to find an optimal pipe size than will keep the exhaust hot and at the same time restrictionless. So a 3" midpipe isn't always best ;-) . The "back pressure" people talk about is really the scavenging effect described above. You want your exhaust as free flowing as possible while still maintaining the effect. If someone thinks I'm wrong, I probably am.