Well, you can kind of generalize it like;
For the small block
2-1/4 good for stock or minor mods, and headers, maybe a small cam as well.
2-1/2 good for 300 HP to approx 450 HP
3 inch best for 405+ and up
Engines looking to whoop some *** somewhere someplace anytime (And a nice freindly word to the comp. "Is now is good for you to lose?")
Like I said above, around 450 HP there is a gain, but this tube size should be very good to really chase down some real decent times for a street/strip car or track car.
At this size, your looking to maximize power without concern to noise level. Baby's and old ladys will love you for it....if you don't have it coming home at midnight. :rock:
Hows that? Help ya some?