Rybred:
Not being an a$$, but in your search engine type exhaust pipe diameter vs. h.p. or horsepower. Several websites have charts that agree with one another.
You can go to flow masters website and look at there pipe dia. vs hp and ci.
www.flowmastermufflers.com and click systems on the left side of the page.
I have not calculated to see if any of these numbers make since, but they look reasonable and the people posting seem to know what they are doing. Plus they are not making any money selling you bigger pipe.