Originally Posted by mew905
We should be able to hit about 20hp from 80cc. A well tuned 80cc scooter I saw a while back cranked juuuuust over 30hp, and was a 3-speed, so... yeah it was fast. I'm not sure at what RPM though, I think it was 12000?
The only gotcha here is that these Hondas are very tall (due to their ohc design). My XR80 motor is 15.5" tall. The XR100 is 16" tall. Note that their height is actually less than the length of horizontal ATV/ pit bike engines. These horizontal engines are typically 17"+ long, precluding them from easy use in most frames.
Both xr's have 5-speed transmissions though. The xr100's gears are spaced farther apart with 5th being around .92:1.
20hp is going to be expensive to hit on the XR80. If there was a small enough turbo, 20hp-30hp might be easier though.
P.S. A while ago, a company offered a 95cc hop-up kit for the XR80. The main issue with go-fast parts is that the XR80 bike is physically so small that few adults modify them. It's usually cheaper just to get your kid a bigger bike. An upshot is that the XR80 engines tend to be cheap and plentiful. I found a nice runner for around $300 with wound stator!