I dont believe this but maybe its true, some one told me the other day that non turbo cars such as a first gen. need to have a cat for back pressure, but i think its the other way around, turbos need back pressure to reduce boost creep, please help me out with this stupid question...Thanks everyone

 

exhaust tuning is a trivial thing. here is the jist of it. many people believe that a little backpressure is needed in a na for effecient exhaust scavenging. i believe this to be true in some aspects but dependent on engine configuration, desired powerband, etc. some people believe the best is no exhaust or no restriction to the motor.

a turbo car needs to be able to breath. the turbo is already going to create backpressure so the piping is just added restriction. boost creep should properly be controlled by the wastegate. 3rd gens tend to creep unless the wastegate is machined out some in order to counter the boost creep that happens due to the turbo's ability to quickly spool with a less restrictive exhaust. the oem wastegate size can't release pressure fast enough to keep up. thus boost creep.

the idea behind backpressure (i believe) is to swirl the exhuast to help pull as much as possible out. this keeps more of the burnt mixture from staying in the chamber and inhibiting the next combustion cycle. it is a tuning game to pull as much spent gases out of the chamber as possible.

 

In the case of the secondary port actuators, they need a little back pressure for them to work.

Also, the backpressure from certain highflow cats is almost non-existant.

Exhaust scavening, I believe, isn't so much a matter of backpressure, but the timing of the exhaust pulses through the length of tail pipe, and how far down the pipe these pulses meet, if they meet. Back 'pressure' sounds contrary to 'scavenging' which implies a vaccum. You want to keep the velocity of the exiting gases fast, no big pipes, and high temps help. The kinetic energy from the previous pulse will help draw out the gases from the next pulse. Since the pulses will change timing through out the RPM band, you can really only optimize the scavenging at one point on the curve. If done right, you'd time it so that this point is at your peck on the powerband. You really can only do this by playing with a dyno.

Yaw has a good write up on this.

 

NO ENGINES NEED BACKPRESSURE TO RUN PROPERLY. PERIOD.

Adding a restrictive exhaust to a turbo car to prevent boost creep is just a crutch, by the way. The real problem is a too-small wastegate.