I have a OS 91fx with a bisson pitts muffler ...apc13-8 ...In mine ...The plane flies great ......an os 61fx and 91fx are the same weight 19.4 0z....the 61 has 1.9bhp@16000rpm the 91 has 2.8bhp@15000rpm and they are the same physical size ......So as far as wing load there is no differance os 61fx v os 91fx ....Mo power = mo fun & speed........OOOOOOOOya give me A 91fx Please.....sorry geebeejim