[Fast Eddie]

Originally Posted by Spilner521

Sorry Eddie, that's incorrect.

With backpressure in the exhaust, the piston has to work harder to push the exhaust out of the cylinder. The harder the piston has to work, the less efficient it is and the less power the engine makes. With no backpressure, the exhaust is easy to push out of the cylinder.

High exhaust velocity helps the scavenging effect, and any amount of backpressure slows down the exhaust. The exhaust exiting the cylinder at a high velocity helps pull all the exhaust out of the cylinder, and when the valves overlap, helps pull fresh air into the cylinder, letting the piston work less and therefore more efficiently to draw in fresh air.

You say air can escape through the exhaust if the velocity is "too fast". There's not a set amount of air that's going to enter the cylinder. Even if air escapes into the exhaust stream during overlap, there's still air being drawn in by the piston after the exhaust valve closes.

Even on a turbo car you want to reduce backpressure before the turbo, since the turbine itself is a big restriction. You'll want a free flowing manifold and turbine housing with a low backpressure wheel, as well as large, free flowing piping after the turbo. High velocity and equal pulses before the turbo, and low restriction after the turbo help it spool faster and make more power.

Like I said, I've got dyno pulls that show a reduction in low end torque with a higher flowing exhaust, but, your still right