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      03-14-2013, 11:14 PM   #18
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Drives: '08 IB E92
Join Date: Sep 2011
Location: OC

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Originally Posted by M3takesNYC View Post
Hmm to me that does not make any sense. The rear exhaust piping is where the gas velocity is the slowest, especially if coming from a smaller diameter pipe. It essentially would "hit a wall" if you will and come to a halt in terms of velocity. Many tuners actually would make the rear exhaust piping smaller to actually help keep the velocity up from the inherently slower exhaust by the time it reaches there.

Not sure there is any logic in that at all or its way beyond anything I have ever understood about physics and fluid dynamics
High velocities are more important near the cylinders where high velocity help scavenging by creating low pressure that "sucks" the exhaust out of the cylinder. This also creates unwanted back pressure because pressure loss in a pipe is a function of velocity. Near the muffler, high exhaust velocity isn't as important. In fact, the exhaust cools quite a bit and slows down (because it's volume is reduced when it cools) back there. Near the muffler, you simply want to reduce pressure loss/back pressure. Reducing the amount of bends and velocity (larger pipes) are both ways to reduce back pressure. The gases do slow down but there isn't "a wall", there is less restriction.
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