Ron G

I am looking into get a new exhaust (from cat back) but have heard that opening up the exhaust decreases low-rpm torque. Is this true? I would think it would not be good for autocrossing, as you spend more time down in the lower rpms and need the torque.

hoTTub

normally aspirated engines.....it is very hard (unless you go too big) to screw up with an exhaust on a forced induction car. A good exhaust is a necessity on a turbo car and should give you power across the whole rpm band

smallTTs

especially on a production car. Sure, 4 inch pipes on a small engine reduce velocity and would be less than optimum, but, in spite of urban ledgends, engines do not need 'some back pressure' to produce maximum torque at any rpm.

It is possible that a highly restricted exhaust engine would need different fuel/ignition mapping if back pressure was significantly improved, but a cat-back system with lower total back pressure will increase torque across the rpm band. The increase will be greater at the higher rpm, higher flow range. The important thing is choosing a low-back pressure system. We don't have reliable airflow figures for available systems, so it's difficult to make a choice, other than reports from others.

FWIW: On of the ways Chevy gets 15 more lb-ft of torque on the Z06/405 hp engine for 2002 is to eliminate 1 in.-hg. (about 1/2 psi) back pressure from the exhaust with more efficient cats.

How are you doing in autocross? GS is a hot class.

Ron G

Jon, thanks for the info. Kicking butt in GS (running Novice this year)

Ron G

Sean, thanks for the info