Originally Posted by Matt

I think it depends on the engine, the diameter/design of the piping, and other piecs of exhaust in the system.

For some cars, a Y pipe sized correctly will allow for optimum backpressure and power. For other cars, it's the X-pipe.

For our 370z, it appears the X-pipe is providing the most top end hp/torque, along with the most volume. Some people choose the Y-pipe systems, possibly for less weight, lower volume, or in the case of some cars, higher low-end torque (not sure if that's the case with the Z).

None of this is backed up by any facts, whatsoever, but I did stay in a Holiday Inn last night.