Quote Originally Posted by bennjamin View Post
2.5 is too big for a "dc2" (assuming its a vtir with the usual mods....) full stop.


IF you have heavy work , or supercharged etc 2.5 is great. But the usual intake/header mods do not justify a larger than say 2.25inch system

Noted that the cat-back piping should match the outlet size of the cat itself - which should also match the collector size of the header too.

IMO if it must be a 2.5 inch , use a standard ITR muffler or similar to tighten the sound and aid throttle response both down low and mid range. Back pressure is real and is needed on smaller engines.
Back pressure is not real, and by smaller engines you're talking 4stroke dirtbikes.
Some sort of flow velocity must be kept however, and if you can't move the gas through the pipe as fast as you should.. Then you're got nothing left to gain out of that oversize piping.