IGNITION TIMING FACTS

·You advance ignition with a hot plug, which will give you more torque or low speed
·You retard ignition with a cold plug, which will give you more rpm’s or top speed

The above is true in both cases if you have the same compression ratio (combustion chamber shimming), and use the same nitro % on fuel.

But if you change compression ratio (combustion chamber shimming, and or nitro %), you have to compromise.

·If you increment compression (smaller combustion chamber = less shims) you are advancing timing, thus incrementing torque, so you will need to put a colder plug to compensate for top end., and more generated heat, which could cause pre-ignition.
·If you reduce compression (bigger combustion chamber = more shims) you are retarding timing, thus incrementing top speed, so you will need to put a hotter plug to compensate for torque, and less generated heat.

The ideal combustion chamber for .12 engines at sea level is:

· For 16% nitro = .015 inch or 0.381 mm
· For 20% nitro = .016 inch or 0.410 mm
· For 25% nitro = .018 inch or 0.457 mm
· For 30% nitro = .021 inch or 0.533 mm

So you have to shim the engine accordingly to get those total combustion chamber sizes

AFM