First one must understand the concept of a “hot” and “cold” glow engine. If the ignition timing of a glow engine is correct it is neither “cold” nor “hot”. If the timing is too fast it is hot, a very undesirable condition because it will cause the engine to overheat which in turn would advance the timing even more leading to thermal runaway. IF your engine were running perfectly and you changed to a much bigger prop it will now be running hot because of pre-ignition. You would now have to “cool” the engine, to do this you can reduce the amount of nitro, use a cooler glow plug or ad a head gasket. I have successfully ran my OS91FX with a 15X8 APC prop, tune pipe and 15% nitro but I used a F plug and a extra head gasket. At 9300 rpm it made very little noise and had a very smooth power curve.