The heat rating
The heat development varies greatly from engine to engine. For example, turbocharged engines run significantly hotter than engines which are not charged.
Therefore, there is a spark plug for each engine which can conduct a precisely defined measure of heat to the cylinder head and ensures that the optimal temperature window is maintained.
The so-called heat rating provides information about the thermal endurance of a spark plug. The following applies for NGK spark plugs: The higher the heat rating, the higher the thermal endurance.
The heat rating number is found in the respective type designation of NGK spark plugs.