I know there are people out there who would prefer to overpower everything but this particular plane's perfect combination is the OS .46 FX. As someone mentioned before, even with the .46, the US 40 requires a little bit of weight at the tail for balance. I have been flying this plane for more than a year with a .46 FX, Tower Hobbies muffler and an APC 12/5. The only mods I did were reinforcement of the firewall and landing gear. I also fuel-proofed the fuel tank compartment. This plane really shines when you keep it light and well balanced (especially if you're using quad flaps) and the OS .46 provides enough power to hang on the prop, etc.