Muffler options ?....

Pipe Diameter (inches) Pipe Area (in2) Total CFM (est.) Max HP Per Pipe Max HP For A Dual Pipe System
Pipe size-area(in2)-single-dual hp max est.
2 1/4 - 3.55 - 408 - 185 - 371
2 1/2 - 4.43 - 509 - 232 - 463
2 3/4 - 5.41 - 622 - 283 - 566
3 - 6.49 - 747 - 339 - 679
3 1/4 - 7.67 - 882 - 401 - 802

All taken with a grain of salt. A little more or less, a few cfm is OK for a street system. A lot less or more will hinder or hurt. These pipe sizes are for max output. So if you have a 400hp engine at max power level, a 2-1/2 free flowing well designed system is what you need to extract the most from your set up. This is good for drag racers. In the street, to could live with less since your not at 400hp much. (Thinking of cruisers here with a WOT application every once in a while, not a often application like in street racing.)