[Phantom]

NA engines need back pressure?

Actually any vehicle never needs back pressure that is often a misconception made by those who go to to big of a pipe and lose performance. The reason this happens is that going to to large of a pipe exhaust gas cools and then slows. When that happens there is congestion in the pipe and then the motor has to work more to push the slower gases out.

With an appropriately sized pipe it will remove nearly all back pressure and promote the removal of gases since they will be moving faster. When the gas tries to quickly remove its self it creates a pressure wave behind it that works to suck the next exhaust pulse out of the pipe. That is what you are going for with any exhaust system.