Would a Rev box help the top speed of the 400ex? What else does it do?

 

pretty much just takes the engine up to RPM levels that honda doesnt approve of with the particular cam/valve train in the 400EX motor.....

It might up the top speed IF your machine is capable of revving out completely in 5th gear as it sits. BUT, I personally wouldnt want to be on top of a motor that is screaming well into its redline going 75 MPH.........

 

The 400EX motor has already reached its peak power, and is actually losing power by the time the rev box does what it is supposed to do. Honda makes it quit reving at a certain point for a reason.

That $100 is better spent elsewhere.

 

A rev box will increase your top speed assuming that you have the power to fully pull 5th. gear. Which a stock 400EX will not do. A rev box is usually not needed until you change cams.