Rio96 said:
Could some care to offer an explanation:
Why is backpressure EVER helpful to torque?
(at low RPM, or whatever)
There are two important aspects of exhaust:
1. Exhaust gas VELOCITY
2. Resistance to flow (backpressure, if you will.)
Sometimes if you increase the resistance to flow a little (narrower pipe for example) the velocity of the gases will increase.
You want high gas velocity because it will suck the exhaust gas from the compression chamber (called scavenging (SP?)).
You want low backpressure so that the resistance to flow of exhaust gasses is low. If the backpressure is too low, you have lost too much of the valuable velocity.
You trade velocity for backpressure, usually, so you need a proper balance. This is why there are no 3" diameter or larger exhaust parts for non poweradder mustangs. It would slow the velocity and kill the scavenging (SP?) effect in the combustion chamber. You don't see 2" or smaller, because it would be too restrictive to the flow of exhaust gasses.
Just to complicate things, the proper balance is different for different RPMs the enging is turning. You can have an exhaust that is TUNED to give a perfect combination at the High end, but this combo will not have the sufficient velocity at partial throttle to give good scavenging at low RPM (low volume of exhaust gasses). This causes a loss of low end power.
The exhaust system of a stock mustang is so restrictive that there are mods you can do that give accross the board gains, up to a cirtain point. Then, as you get the last little bit out of the exhaust system, you need to start making compromises as to where the system is going to be tuned for.
Hope this helps.