skulboep:

You cannot have too much power in an airplane. You can have too much weight if the bigger engine weighs more, particularly if you have to add tail weight to balance it. If the weight of the bigger engine is close to the same, there is no reason not to use it and lots of good reasons to go ahead. Emergency power if you get into a bad spot, larger, smoother aerobatics, and of course just a lot more FUN.

What kills airplanes is not power, it is speed. This is what the manufacturers are worried about. Every airplane, full scale or model, has a speed at which the control surfaces will flutter. When this happens, the airplane will usually self-destruct. By increasing the power you can make the airplane go faster than the manufacturers designed speed, with catastrophic results.

That is why a bigger engine is usually combined with a lower pitch prop. The lower pitch keeps the speed down and turns the higher power into more THRUST, for better acceleration, better vertical performance and more constant speed through maneuvers.

A 61 two-stroke with an 11x7 prop will fly your 4* as fast as it wants to go, but you will not have much vertical or extra power for emergencies. Using a 91 two-stroke with a 14x6 will give you about the same speed but your up-lines will be MUCH better and you can fly around at reduced throttle, which uses less fuel, makes less noise and less vibration, and the engine will last forever.

Bottom line; use the biggest engine you can without adding tail weight to balance, reduce the prop pitch to keep the top speed within the range of the recommended engines and learn throttle control to keep from over-speeding on your down-lines. Your airplane will perform much better overall and will last forever (or until you crash it )

Jim