3" vs 2.5" exhaust systems

horsepower for each pipe size:
Max HP For
Pipe Diameter/ Pipe Area/ Total CFM/ Max HP Per Pipe/ Dual Pipe
1 1/2 1.48 171 78 155
1 5/8 1.77 203 92 185
1 3/4 2.07 239 108 217
2 2.76 318 144 289
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
3 1/2 8.95 1029 468 935
NOTE: These numbers are just estimates. All pipes are assumed to be 16 gauge steel.

How To Calculate Muffler Size and Exhaust Pipe Diameter

http://www.exhaustvideos.com/faq/how-to-calculate-muffler-size-pipe-diameter/