One thing to consider on the larger pipes is heat transfer. The larger the pipe, the more heat it will radiate to the atmosphere. This will cool the exhaust gasses. Cooler exhuast gasses are denser, thus heavier, thus harder to move. This is one reason for wrapping headers. The cooling would negate some of the gain you might achieve by slowing the velocity of the gasses. This is one drawback of the single pipe theory.

Larger pipes also have more surface area which creates more drag to the flowing gasses. And we all know about drag. Run the smallest pipe you can without affecting total performance.

Throwing out pipe sizes without knowing the system the pipe is to be installed on is simple conjecture. No one can tell you what will be the best. Trial and error, or call an exhaust mfgr tech line, and they can get you close.

If you are going for loud pipes, duals win hands down every time. jd

 

thanks for all ur help. im gunna go with 2.25 duals