RE: .o49 engines by cox
I don’t know much about different plugs or heads. I have read different places that nelson plugs get about 1,000 more rpm. Also all the guys in the know claim that you can get more rpm by changing the number of head gaskets, usually from 1 = 4. Running more head gaskets decreases the compression, and also changes the timing of the engine. If it were possible to change the heat range of the plug this also changes the rpm. Changing the heat range also changes the timing. From what I understand the only way to know which works best is to experiment, and keep a record of the results. Lowering the compression helps as you increase the percentage of nitro, or at least that's what they say.
I am new at making modifications to Cox engines, but when spring gets here I am going to try some of these tricks and see what happens. If I have any success I will share my knowledge. I have no intentions of competing in any way. I simply like the extra power, because it lets me fly a little better.