I have heard that for 275 HP or less you should stay with a 2.5" diamater exhaust. And 3.0" for anything over 275 HP. Is this true at all?

 

 

Its not that simple. You need the correct piping size for your power band goals, and your current state of tune. 3'' is widely regarded as turbo only size, since an NA setup would be overwhelmed with that size of exhuast. I dont remember the exact exhuast speed, but you need to do some math to decide where the exhuast speed is going to produce peak torque, and ultimatly peak hp. If you include your state of tune, i will look up the calculations you need to go by and post them here.

 

Exhaust gas flow velocity (or flow speed) determines where peak torque occurs along the rpm range. As a general rule, when exhaust flow velocity reaches the mean value of 240-260 ft/sec., peak torque is achieved.

Taken from team-integra.net

you can see how pipe diameter will affect when the flow speed will reach 240-260.

 

i say, as well as other people say, that 2.5" is good for a naturally aspirated engine. 3" would be losing power on a natural, unless your turbo'd up. and doing something like a dual exhaust 2.5" isn't gonna add hp, it's juss gonna rob you, though some people feel it adds.