i have a polaris 200cc 05 model and i have changed the exaust to a sports one. than i bought a cdi unit so that from 8.5k rpm it boosts to 10k rpm and from hardly reaching 60km/hr i managde to reach 70km/hr. i will try now to change the size of the rear tyres from 20x10-9 to 22x10-9 to have my quad more faster. hope that this info will help others to imrove their bike like i did