The thunder power engine may be less expensive than OS, but I think the overall quality is better, ABC piston and sleave, as good or better bearings than OS, the worst in the industry IMHO. I race a TP 46 pro against a host of OS powered buddies in the T-34 class. No one in the club has beat me overall at the end of the season in the past 5 years. Yea, I'm a experienced pilot, but going on 74, most of the other guys are 40's to 50's. Gotta tell you something about the engines overall ability. I get around 800 rpm more than the OS guys on the same prop. That 800 rpm is not a whole bunch, but enough to make up for old eyes and sort of poor start. Just wanted to let you know that OS is not king in everyone's eyes, and very over-priced in mine for what its worth. ENJOY