Although you can't measure or definitively set when ignition takes place inside a nitro engine, it helps to be able to visualize what's happening when you experiment with different plugs. Let's say, for example, that the fuel mixture is ignited precisely at the moment the piston reaches the very top of the cylinder. This would effectively mean the ignition timing is taking place at zero degrees of crankshaft rotation. Installing a hotter plug in the same engine makes the fuel ignite sooner because less compression is needed to heat the plug's element to the point that the fuel will ignite. Let's say that now, ignition occurs 10 degrees before the piston reaches TDC. In ignition-engine-language, that would mean that the timing is set to 10 degrees advanced, or 10 degrees BTDC (before top dead center). What does all this mean? Simply knowing that plug temperature will affect when combustion takes place and will, hopefully, help you understand why choosing the proper plug will improve performance. Generally, it's best to try to advance the timing or flash point of the fuel—in the case of nitro engines, as much as possible without going too far. If the mixture is ignited too early, then performance is lost and pre-ignition and detonation may occur., because the more nitro you run, the more you advance the ignition point. What's that mean? In an ideal situation the glow plug will ignite the air fuel mixture when the piston is at top dead center (TDC), which will force the piston down and back up for another compression stroke. When you run a higher nitro percentage (and don't go to a colder plug) you'll advance your ignition point (the point at which the glow plug ignites the air/fuel mixture), which will result is less than optimum performance since the piston is still on it's compression stroke when the air/fuel is ignited and not at TDC.

AFM