In your car you can alter the ignition timing by rotating the distributor cap, or if it's got electronic timing, a computer will determine the optimum time to fire the plugs based on a number of factors, such as engine speed, throttle position, atmospheric pressure etc. Your car may also have a knock sensor which will automatically retard the ignition if it senses any engine knock.

With model engines, you can alter the ignition timing by changing the heat range of the plug. It's not an exact science, but without changing to spark ignition, it's the best you can do. Essentially, the quality plugs are supplied in the following heat ranges. Very hot, Hot, Medium/Warm, Cold, Very Cold.

A warmer plug will advance the ignition, a cooler plug will retard the ignition. If you run an engine with too cold a plug, you'll get less power, an unreliable idle and an engine that's a pig to start. If you run an engine with too hot a plug you may get pre-ignition which can also result in less power, overheating, damage to the plug and in extreme cases damage to the engine.

The general rules to follow are: The faster you run the engine, the cooler the plug. The higher the Nitro content the cooler the plug. The higher the compression, the cooler the plug. If you use a tuned pipe choose a cooler plug. The Very Hot plugs are for four-strokes. If your 2-stroke engine needs one of these, then it's time for a rebuild.

I'd recommend that you buy three plugs:

An Enya No. 3, No.4 and No.5 and try them all, starting with the No.3 plug, and see which one works best for your particular set-up (i.e. provides the most power at the top end combined with a reliable idle and smooth transitions).

The following is a list of my engines using different fuels and silencers and the plugs I've eventually chosen:

Helicopter:

OS32SH 16000rpm with 17% oil, 10% nitro and a muffler, runs best on a Hot plug (Enya No.3)

OS32SH 16000rpm with 15% oil, 16% nitro and a muffler, runs best on a Hot plug (Enya No.3)

OS32SH 16000rpm with 15% oil, 16% nitro and a tuned pipe, runs best on a Medium plug (Enya No.4)

OS32SH 16000rpm with 17% oil, 10% nitro and a tuned pipe, runs best on a Hot plug (Enya No.3)

OS32SX 17000rpm with 17% oil, 15% nitro and a muffler, runs best on a Medium plug (Enya No.4)

OS61SX 16000rpm with 17% oil, 10% nitro and a muffler, runs best on a Medium plug (Enya No.4)

OS61SX 16000rpm with 17% oil, 15% nitro and a muffler, runs best on a Medium plug (Enya No.4)

OS61SX 16000rpm with 22% oil, 15% nitro and a muffler, runs best on a Hot plug (Enya No.3)

Airplane

OS61FSR(Ring) 11000rpm with 17% oil, 15% nitro and a muffler runs best on a Hot plug (Enya No.3)

OS61SF(Ring) 13000rpm with 17% oil, 10% nitro and a tuned pipe runs best on a Medium plug (Enya No.4)

Worn-out OS61SF(Ring) 12000rpm with 17% oil, 10% nitro and a tuned pipe runs best on a Hot plug (Enya No.3)

OS61SF 13000rpm with Performance Specialties ABC Piston/Liner with 17% oil, 10% nitro and a tuned pipe runs best on a Cold plug (Enya No.5)

OS1.08 10000 rpm with 20% oil, 5% nitro and a muffler runs best on a Hot-Medium plug (OS No.8)

OS1.08 12000 rpm with 20% oil, 5% nitro and a tuned pipe runs best on a Medium plug (Enya No.4)

Ducted Fan OS91 23000rpm with 20% oil, 0% nitro and a tuned pipe runs best on a cold plug (McCoy No.9/Cold Enyas work well too but these engines blow plugs at a phenominal rate and the McCoys are alot cheaper)

-Russell Aisbitt