do i really need 3" exhaust

Here's a detailed flow test. Lol...in my opinion mandrel bent is more important in this case. A nice 2 1/2" mandrel bent system with an x-pipe placed in the correct location is the best bet, but any decent system will do again, as long as your not racing and trying to eek out every single bit of power.

[ame="https://www.youtube.com/watch?v=CF3RfyD-DzE"]Hooker max flow muffler flow test - YouTube[/ame]