what is the difference between a zx6r and 636?
Know what you're riding and call the ZX6R a ZX6R. And - the number 636 is motor displacement in CC and is meaningless except to tell you "how big the motor is". Starting in 2003, Kawasaki started making the ZX6R with a 636cc motor. In 2007, they completely changed the motor design and dropped it to 599cc.