ok my friend has a 400ex and he put a weisco 440 kit in it ported and polished it.. stage 2 hot cam.. dyno jet kit, FMF Q pipe... white brothers rev box... the works done to the bike i know theres more done to the bike but im going off the top of my head! so how much iz he gonna gain and whats about the top speed?? thanks for the help!! oh also a race clutch..

 

Speed is determined by the engine RPM's. Top speed can only be changed by changing the gearing. Unless you do that, you won't gain anything in top speed. If you ATV has a top speed of 50 MPH and the engine is turning 5000 RPM's, adding more horsepower won't change the RPM's, so top speed will be the same. Now, he will certainly be able to get to top speed much quicker.