I started out with this engine in a CAP 232 kit built airplane and the engine ran great but I wasnt wild about the airplane. I have now installed it in a Early models RC repro kit of a Goldberg Sr Falcon. This airplane weighs 6 3/4 ponds and the magnum 70 FS pulls it around with authority!!! I am running a Zinger 13X6 prop currently and am going to try a Zinger 12X8 next for extra ground clearance. I didnt measure RPM as I am still breaking the engine in. I intend to buy more Magnum 4 strokes in the near future, a 91 and a 1.80 and if they run as well as the 91 I will be a customer for life!!! BTW i have owned many 4 strokes through the years, OS and Saito mostly and this engine runs with the best of them at a much lower cost.