I think you'll end up shearing the second gear engagement pin/pin housing with that kind of motor. And if you think it's hard to control it now when shifting, wait until you get a motor with
at least double the torque in there! You'll be
lucky to keep it under control.
TBH, if you want to retain the two speed, a 70mm motor (or even a 60mm motor) would be more than sufficient. The lower gearing of first increases torque, and the higher gearing of second gives you the top-end. If a regular XL motor can handle a single speed mod with tons of starting power AND high speed, an 80mm will do it without sweating at all. The only reason for a two speed is to compensate for nitro's lack of torque. Good luck!
