I too found the 12x4 Xoar better on my OS50FSR; I had been using a 12.25x3.75 APC, and the thrust seemed about the same, but the throttle response/spool up was waaay quicker with the Xoar. Switched to a Saito 82 on the same airframe (GP Reactor), and tried a 14x6 Xoar, but found I liked the 14x4 or 15x4 APC better on the 4 stroke. LHS didn't have a 14x4 Xoar; might have to give that one a try.