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Originally Posted by Sammy
I have heard of and seen this mod done before in the past, but not on a Boxster. I've actually seen where people open up their MAF body (this was done on a supercharged vehicle and everything was opened up as much as possible) in addition to intake kits and a larger throttle body. The idea being that the more air in and out of the motor the more power.
However, despite the theory of more air in and out, the more power, I have experienced where the exhaust on a 4 cylinder (Nick, the red 240SX with the turbo) when opened up to 3" all the way (new header, and full exhaust) actually dropped the power (theory that there wasn't enough back pressure?). This was done before the turbo was added to the car.
I'm hoping MNBoxster can chime in here and explain some stuff...
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Part of making good lower RPM torque is through air velocity. This helps to fill the cylinders and aids in mixing. The larger the intake path, the slower the intake speed, thus reduced low engine speed torque. This is typically most sesitive to port sizing and valve timing.
Exhaust size is tricky. You're not just trying to open the exhaust path from the engine to the air but you also need to take resonant sound pressure waves into account. These sonic waves can hurt or help cylinder evacuation depending on design. Also, valve timing is designed with a certain amount of back pressure. Changing the backpressure can lead to reductions in power and economy without internal timing changes.