Inliners International Forums General Discussion Engines Exhaust size effect - is bigger always better?

Ah that's interesting. For a 250cid on that chart:

150hp with single 2.25" or dual 2"
200hp with single 2.5" or dual 2"
250hp with single 2.5" or dual 2.25"
300hp with single 3" or dual 2.5"

Looking at that chart, cross-section is only one factor as the recommended dual is not one half of the cross-section of the recommended single. I'm betting that probably has to do with laminar flow of air, with the air contacting the pipe moving the slowest due to friction. A dual 2" has a total circumference of 12.56" while the single 2.5" has 7.85", which is a lot less surface area of the inside of the exhaust pipe that the air is dragging against, even tho the cross-sectional area is a little over 1 square inch less with the 2.5" pipe over the dual 2"