Quote:
Originally Posted by JephryB
+2
Pipe diameter will affect velocity of the hot gas as it travels down the pipe- this is why people sometimes wrap the exhaust componants with "header wrap" as it keeps the gases hot and the flow up.
Exhaust diameter isn't a case of bigger-is-better, it's a case of optimizing the pipe size to the application to maximize flow. Pipes too large will result in a loss of torque on the lower end- the turbulence and loss of velocity aren't mitigated until a lot of air is being pushed through the pipe, so you lose out down low and gain up top  |
That is completly incorrect.
With FI cars, the bigger the better, no exhaust is even better. Having dual 5in exhaust will have no negative effect on torque. To have an exhaust system that is big enough to cool the gases to the point where ANY power would be lost, more than likely wouldnt even fit under the car.
A good guideline to follow with FI cars is to keep the exhaust piping as large, or larger than the turbine wheel. In the case of the N54, dual 2.5in exhaust is more than enough with the stock turbos. If someone were so inclined going to dual 5in exhaust would not hurt the torque what so ever.