Quote:
Originally Posted by Fast Eddie
Actually, you do want some back pressure. Since there is typically overlap between the time the exhaust valves close fully and the intake valves opening, if the flow out the exhaust is too fast you will pull the intake air and fuel out the exhaust side. That hurts performance, since the whole goal is to have as much stuff blow up in the cylinder as possible.
On turbos the turbine provides the back pressure so everything after that can be as free as a bird, without hurting performance. You also want to have it flowing as fast as possible from there to keep the temp down.
I've got charts somewhere at home (from when I cared) showing the difference from stock to header->larger piping->high-flow cat->performance muff. And there is a reduction in TQ.
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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.