[grancuda]

This is out of an old hot rod magazine I had.

The exhaust needs 2.2cfm per hp that your motor produces.
Exhaust pipe produces 115cfm for every sq/in
At 2.25” OD pipe, 16 ga (roughly 0.06” thick) the ID is 2.13” & area would be 3.56 sq/in
so 3.56 sq/in x 115cfm = 409cfm / 2.2 per hp = 185.90 x 2 for duals = 371.8 hp for 2.25” od

At 2.50” OD pipe, 16 ga (roughly 0.06” thick) the ID is 2.38” & area would be 4.45 sq/in
so 4.45 sq/in x 115cfm = 511cfm / 2.2 per hp = 232.27 x 2 for duals = 464.5 hp for 2.5” od


I wouldn't choke it down to 2.25 if I were you.