The Tiger 60 is a big airplane. With a .61 it will be fairly docile. It really peps up with a .91. The newer .91s, like the OS FX and the Magnum XLS, are build in a .61 case so they weigh the same, but put out more power. You can run your .91 on a 12-8, which will really turn up. It'll be like a fast turning .61. The people at Performance Specialties, who make the Ultra Thrust tuned mufflers, run OS .91FXs on 12-8 or 13-6 props at 14,000+ rpm so the engine will take it.