I think hondarider300 just touched on something that's been bothering me about this whole thread. If there's no need to rejet, then why even bother putting a pipe on? If you can't put more fuel to the engine then your not gaining any performance. Why bother?

When I put my HMF on my YFZ I went from a mid 150s jet to a 178 main......that was worth doing.

I say keep the money in your pocket for something that will net you some performance.

Buck

 

so that means i buy that $200 pipe and dont rejet, no performance..... that sux

 

What are you putting it on? I think the original poster is putting it on a Warrior. I'm gathering from the replies that you don't need to rejet for the Warrior. If you are putting it on one of the Quads in your signature then you will probably need to rejet IMO.

Buck

 

Lets clear this up a little, I hope... You change the jetting when the engine gets more air, you need more fuel to keep the mixture optimal. If you arent flowing any more air into the engine, you dont need to rejet.

Now when you upgrade the exhaust you are helping air get out, this will creat a vaccum and pull more air from the engine. You will be increasing the airflow a small amount, but not a whole lot. You probably will benefit from some jetting changes, but you wont be boosting up to 160/165 or anything with a slip on and a stock airbox with lid.

The biggest gains come from full pipes that can evacuate more air, and opening up the airbox to make use of the pipe.

So yes you probably should rejet, but it wont be major changes.