"The concept of keeping the pipe size small enough to keep the density of the exhaust gas uniform enough to keep from adding back pressure due to cooling is hard for many to grasp. "
The key is to find a balance between maximum flow and maximum gas speed. The smaller the pipe, the faster the gas is going to exit, but the engine will have to work much harder to push these gasses out. On the other hand the bigger the pipe, to an extent, the easier the engine will breath. Too big a pipe, and the gas will cool and expand, as Dalite said. An example: It would be foolish to put a three inch exhaust on a 1.6L NA Civic. That little engine couldnt pump out enough gasses to take advantage of the pipe, and it would hurt performance. It would also be stupid to put the tiny exhaust from a civic onto your turbodiesel truck. You have to match the flow of your engine to a pipe that will flow freely without causing stagnation. Hector mentioned 2.5 in piping. Like he said, this will be good enough for most of the people on this forum. Personally, I would trade a couple of ft-lbs of tq in the very bottom of the power band for the much upgraded midrange and high end. The loss with 2.5 in probably wouldnt even be noticeable. I would go with 3 in, because if for any reason I am actually making 400 hp one day, I wouldnt want to have to upgrade my exhaust again. Yes, you probably would notice a small loss in bottom end, but I believe the gains would more than make up for the loss. Anyway, thats my $.02
Mike
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90 Eagle Talon Tsi AWD - 300 Hp -K&N, 16g turbo, JE pistons, 3in Turbo back exhaust, mild port&polish
03 PT Cruiser GT - 215 Hp-Airbox and Intake pipe silencers removed
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