Always at the Ideal Operating Temperature
The heat range indicates the spark plug's maximum operating temperature. It is based on the spark plug's ability to absorb and dissipate heat.
Why is the heat range so important? On the one hand, the spark plug needs to operate above a certain minimum temperature to prevent "sooting" (ignition misfires!). On the other hand, in order to avoid pre-ignition, the spark plug cannot exceed a certain maximum temperature either. The heat range needs to match the characteristics of the engine and is determined by the heat range number. This heat range number comprises part of Bosch's spark plug type code.
Low numbers refer to "cold" spark plugs, which have low thermal absorption in hot engines (like in a Ferrari).
High numbers refer to "hot" spark plugs, which have high thermal absorption for cold engines (like a VW Beetle).
Thermal Characteristics of Spark Plugs with Different Heat Range Numbers