Im getting my custom duel exhaust redone by menekie and was wondering what the benefits of a 3" exhaust system versus a 2.5" is. Im running the pipes from hooker short headers to 2 high flow cats and finally into 2 gt turbo mufflers

 

If its a real dual exhaust system, 2.5" pipes will support over 400hp without issues.

 

Is there a downside to having 3". Someone told me that it reduces the back pressure and thus reduces the torque. But from what I've heard the less back pressure the better.

 

It doesn't reduce torque, just moves your powerband up in the RPM range. Less backpressure = more power higher up.

If you're doing a true dual setup, and you're putting down less than 400-450 RWHP, use 2.5" pipes.