These are user friendly and long lasting engines that are heavy and make average power. The 90 came before the 108 and had a different looking case but is nearly identical dimensionally. The 91 shares the same case and head casting with the 108 with different identification. All 90/91 parts are interchangeable with the only difference being the case and head castings. Contrary to popular belief the 90/91FSR have a 61 size crank with an even smaller gas passage. Only the marine version could rev. The carb is same between 90 and 108 making the smaller engine require more rpm and nitro for good fuel draw. Prop for 11k rpm, a 14x5/6 worked well for slow planes and a 13x7/8 for sport planes.