i dont know what 686 you have seen get beat by a YFZ stock but not around here,they better come with some dollars in that engine to win me,and a little money at that not to get beat too bad.lol.

 

depends on how long the two are racing, and if the yfz is "uncorked" but stock. (meaning baffle removed) Top speed, that 686 I would think would get a yfz. I dont know, never done it. But as far as a race "to" the top speed, the yfz should pull the raptor. Thats my opinion.

 

I crunched some numbers to see what you’ll need to get 70Hp form the YFZ.

I assumed the following. compression ration 13:1, Altitude Sea level, Volumetric efficiency 100% and displacement 475 cc. Lets assume this can be done, this motor have to rev to 10 000 RPM plus to make this sort of power. Most likely it will need to do 11 000 RPM if the volumetric efficiency is not 100%.