Actually horizon suggests a 15cc Two stroke if you go with gas, my 91fx is 6 ounces lighter then the lightest 15cc gas engine which is a RCGF 15bm, I've seen photos and videos of guys mounting a DLE 20 on this bird which is insanity. I considered going with the RCGF 15 but decided I'd rather keep the weight down, and the 91fx likes 10% nitro content so that helps keep the price down per gallon.