ORIGINAL: Juice
What happens when you over prop an engine in such a way that the max RPM is 20% lower than with a typical prop?
Does it damage the engine? If so, does the damage occur immediately, or does it happen over time with repeated flights with the larger prop?
Thanks,
Juice
The real problem is ignition timing that is too advanced for the load on the engine.
Overloading an engine can advance the timing to the point of preignition. Preignition causes the engine to run hotter than normal, or above the temperature that the engine can safely handle.
There are other ways of accomplishing the same thing. Running too much nitromethane, too lean of a mixture, too much compression, etc. If preignition is too severe, it can lead to detonation. Detonation will destroy your engine fairly quickly.
As others have mentioned, cooling airflow in adequate amounts are mandatory for normal operation when everything is perfect. Flying a model with marginal airflow to the engine and with one of the above causes of preignition present can ruin your engine fairly quickly. Remember to have a large, directed air intake (internal baffles) with an air exhaust opening two to three times the area of the intake opening. This is the rule of thumb that has been proven to be true over the years with engines that are not being driven into preignition or detonation.
If you absolutely must run a larger than normal prop on your engine, for whatever reason, you can do several things to reduce the likelihood of preignition or detonation. One is to reduce the load on the engine by reducing your propeller's pitch. If that is not acceptable, reduce the nitro content in your fuel. Or, switch to a colder glow plug. Last but not least is reducing the compression of your engine by adding a headshim. These measures are only necessary if the engine is exhibiting signs of overheating (preignition/detonation).
It will not harm your engine to employ the previously mentioned tactics if you're not sure your engine is experiencing preignition or detonation. The engine will just produce a little less power. However, if your engine improves with power production with these changes, you are simply confirming that the engine was overheating due to advanced ignition timing and you did the right thing.
Good luck and tell us how things turn out for you.
Ed Cregger