the diameter or the length? thanks

 

I always thought it was the diameter but i could be wrong

 

Both really if you think of it. If you have say a 3" system with a 3.5" tip, the longer the exhaust note has to bounce in a larger area the louder. I don't think it's a huge difference but probably some. Most important is the diameter and then comes length. For kicks try making noise in a few pipes of the same diameter, just different lengths, see what results from it.

 

my vote goes to diameter

 

Longer and bigger in diameter makes it louder. PSS calls my tips megaphones (3.5" slash cut by 18"). Next tips will be at least 4" by 22", slash cut again.