I'm Chris front Long Island. I built my first motorized bike 3 years ago for my brother. We shoehorned a 48cc engine int he frame of my brothers 19" mountain bike....without modification. About a year later I decided I wanted one of my owne and installed a 66cc 2 stroke (aka 80cc) onto a junk bike. Power wise they seem simlar, though the 66cc handles hills a touch better and goes a bit faster. Both have seen about 250-300 miles of road time. The smaller one has a catalytic converter on it and it seems to do a pretty good job. I run a 30:1 oil mix in both without issue. Parts stay well lubricated and power stays up. Gone through 3 CDI units between both engines and they have the original spark plugs. I cut the exhaust baffle in mine half way. Little louder, power seems to have improved. Lastly, i'm using a 50tooth rear sproket on both bikes. The 48cc one gets better mileage than mine...oh well. How long do these engines last anyway?