I was looking in atvsport and seen where it says a D&D pipe and change the gearing gets the 400ex to 78mph??? I don't think thats true at all.

 

I believe thats possible, cause i think the stock countershaft sprocket on a 400ex is a 15 tooth. I've heard people say that they've bumped theirs up to 16, and that it still has tons of low end. So with a pipe, they might have sacrificed alot of low end, and went to a 17(shaving some of the case off so it'll clear), and had a load of top end speed.

 

OR you can just put a smaller rear sprocket instead of screwing around with your case.