The yoshimura pipe worked great for me, all around great power gains and a great sound for the warrior. I also would suggest a edelbrock carb and a pro flow intake kit for the warrior. The sound and power will finally live up to the quad's name then.

 

get an alba and get a open endcap i suggest seeing if u can order a end cap from the yfz450 pipe and then get a pro-flow intake and ask alba on the jettin or get an edelbrock carb

 

Alien for the warrior is loud, don't know about the power gains though.

 

White Bros E-Series...with all 18 plates! Great power gain, but you'll need to check your main jet and air/fuel srew to run it.....depending on elevation, temp, etc.....