The higher nitro content creates a more volatile fuel. As fuel and air is compressed, its potential energy will continue to increase until the point in which it reaches a peak and the mixture will ignite. If it is over compressed in your REAL car, the mixture will ignite BEFORE the spark plug fires. Obviously if this happens too early the piston still must go all of the way up before it can go down, as it is driven by the others rotating the crank. This roughly equivalent to hitting the top of the piston with a hammer. It is heard as a rattling during acceleration sometimes called spark-knock. This is how nitrous oxide with too much timing advance can knock a hole in cast or hypereutectic pistons. In your glow motor it is similar, on a much smaller scale. The inertia of a spinning crank will work AGAINST a motor that is detonating (pre-ignition), so it is working against the burn instead of with it as it should. This can bend or break the connecting rod or the crankshaft. At very least it will seriously rob power.