whats the compression ratio on a stock 400ex ? and how much HP gain will i get using a wisceo 10:1 piston ?

 

I believe 9.2:1 stock. With just a 10:1 piston you should gain maybe 4 hp. You will need to get the engine breathing better to get more hp, such as a K&N air filter and jetting for the huffing and a DMC or WB R4 pipe for the puffing. Just to mention a few.

 

Would you really gain that much power from replacing the piston? I am thinking about getting a high-compression piston, and that extra 4 hp sounds good to me!