Need to rejet



i just purchased a full fmf exhaust for $150 on ebay

going from stock to that, do i need to rejet?
it runs like a champ now but w/o rejetting it, would i gain anything other than sound?
You do need to re jet. You usually need to rejet after most after market performance parts. Even if it is running good, it is probably running lean because the fmf allows the air to flow easier through the engine. If you don't re jet, it will cause your engine to run hotter and it will shorten the lifespan of it considerably.