[jessrayo]

Originally Posted by silverds

This statement is inaccurate. Usable boost pressure is not positively affected by positive exhaust manifold pressure.

Ideally, the exhaust valve should be fully closed when the intake valve opens. So exhaust manifold pressure would be irrelevant during the intake stroke. If there is overlap in the timing between the closing of exhaust valve and opening of the intake valve, then only negative exhaust manifold pressure (vacuum) is benefitial. The optimal amount of vacuum needed in the exhaust manifold varies with engine speed.

Properly tuned headers are so valuable because they reduce the backpressure and heat that the exhaust manifold normally retains.

I hate getting into these debates and to be honest, I used to be one of the guys that said all exhaust backpressure is bad. I'm now on my third supercharged engine with an extensive list of mods and I can tell you that a boosted engine likes some backpressure. A turbo self regulates, the restriction produces the boost. Supercharger needs some help. It is a fine line... My car has the switch to run the muffler baffles open or closed. Clearly there is more backpressure running the exhaust through the full muffler but if the car makes multiple dyno runs you can't tell a difference between open and closed. It you run through a cut-out, it will be louder and sound meaner but the dyno is not going to show a significant change on a supercharged engine. I would even go so far to say that if you put on headers and remove the cats then hit the cut-out you will lose power because you won't have enough backpressure. If the stock manifold and cats are ahead of the cut-out they will give you plenty of restriction and you will be fine.