An up-to-date spark plug must be individually tailored to the different engine designs and driving conditions. Therefore there cannot be a spark plug which functions without any difficulty in all engines.
Since the temperature development in the respective combustion chambers in different engines varies, spark plugs with different heat ratings are needed. This heat rating is expressed by the so-called heat rating number. These heat ratings represent an average temperature measured at electrodes and insulators, corresponding to the engine load in each case.