what kind of bike, cause if its a 08/09 zx10r slip on that doesnt get rid of your pre muffler box on the bottom or as most know of it as ,the "boom box" you will lose hp over the stock can. So if you get a slip on thats fine in the break in period just make sure you dont get a shity slip on that doesnt remove your pre muffler box cause if you do, you basically just wasted money for sound and lost the performance aspect of things. Its a proven fact in tests and on a dyno that I have stood nexted to that the stock can is better then any slip on that doesnt remove the pre muffler box, even arka!!!