Heat Range


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


Zündkerze mit hoher Wärmewertkennzahl. Zündkerze mit mittlerer Wärmewertkennzahl. Zündkerze mit niedriger Wärmewertkennzahl.  Preignition Range
 Safety Range
 Operating Temperature Range
  Misfiring Range

Engine Performance

Spark Plugs with High Heat Range Numbers
("Hot" Spark Plugs)
Spark Plugs with Medium Heat Range Numbers. Spark Plugs with Low Heat Range Numbers
("Cold" Spark Plugs).
Large insulator surface absorbs a significant amount of heat. Low heat dissipation.   Insulator surface is smaller than in "hot" spark plugs. Less heat absorption. Better heat dissipation.  Small insulator surface absorbs a small amount of heat. Excellent heat dissipation.  
Heat absorbing surface
Thermal conduction