? about Exhaust Backpressure

[inferno9898]

This is more relevant to naturally aspirated engines i think, but it still matters in the turbos and i figured the guys here could answer it. Anyway, I always hear (especially in reference to V8s) that when you go too big on exhaust size, you suffer in low rpm torque. How does that work? Why does backpressure actually help the engine to create power? Can anyone help me out. Thanks
-Scott

 

[Black95GSX]

Back pressure actually does help a turbo car, in the pre-turbo RPM range it gives you more torque.


Once the turbo starts spooling though, the less back pressure the better.

 

[Turbo Guru]

Short answer
Actually, turbo cars do not require any additional exhaust back pressure from the exhaust system. This is because the turbo creates more than enough by itself (being that the turbo itself is an exhaust restriction.) A turbo car will run best with no exhaust (after the turbo) but that is not feasible for several obvious reasons.

In regards to naturally aspirated engines, backpressure is undesireable (sp?) to a certain extent. No back pressure on a NA motor cause it to lose the scavenging (sp?) effect. This effect is is that one cylinder's exhaust pulse actually pulls the next cylinder's exhaust pulse out (kinda like a successive vacuum effect.) If the exhaust is too large, eliminating backpressure, is also causes the loss of that beneficial effect, therefore decreasing performance.

Joe