ORIGINAL: RVator
I just started flying my first 4 stroke this year, OS 91 on an Ultra Stick 60. It is so smooth. From now on the only two strokes I will buy will be gas burners, for glow fuel it will be 4 strokes. Next plane I wan't to try a YS!!
I have a YS .91FZ. This thing is turning a APC 15x6 at 10,000. This is a serious powerhouse. If you like your OS .91, you will mess yourself with the power from a YS .91 (supercharging all the way). Just have to wrap the fuel tank to keep it from exploding, other than that, it has been no problem. Be sure to keep the valves adjusted to the proper clearance.