For exhaust, you need 2.2CFM of flow per HP made to end up with zero backpressure. At 8psi, figure around 400hp (crank) so you need atleast 880CFM of exhaust flow. A 2.5 straight pipe flows around 560cfm. Duels would flow 1120cfm. But as b18cls1 mentioned, number of bends and degree of bends will change that number...but not by alot. You may loss 100cfm at most.

 

Getting all scientific.. I LIKE IT!

 

The turbo car that I know of that went froma 3" speedcraft duals (with apex-i 4" outlet mufflers) to the UR dual 3" with 2.5" tips was making over 400whp. With both exhausts.

 

Here is a shot of my car with the Speedcraft 3" exhaust Apex N1's cans.