Heat rating and heat rating number

A spark plug of today must be tailored to differing engine design and traffic conditions. Therefore, it is not possible to have a single spark plug type which would function trouble-free in all engines.

Because temperature development of the respective engines vary in the combustion chamber, spark plugs are required with differing heat ratings.

This heat rating is expressed by the so-called heat rating number. Previously, with the old single-range spark plugs, two- or three-figure combinations of numerals were used, for indicating differing heat ratings.

These heat ratings, expressed by the heat rating number, are representative of an average temperature corresponding to engine load and measured on electrodes and insulator. At the insulator tip the operating temperature should be between 400° C and 850° C, while temperatures of over 400° C should be aimed at, because at these higher temperatures the carbon or oil carbon deposits dissolve and the spark plug cleans itself again.

However, the temperature at the insulator should not exceed 850° C, because at over 900° C glow ignition occurs. In addition, the electrodes are attacked or even destroyed at extremely high temperatures by chemical-aggressive combinations. All this has not only resulted in technical development to leave the old single-range spark plug for the modern multi-range spark plug, but still more: Particularly the development of new materials, in particular for the insulators or the use of high-grade copper cores in the center electrodes are covering the high quality standards required today for these far-spread heat rating ranges

These technical advantages have led to a change in the type of spark plug designation. Consequently, modern manufacturers like NGK are differentiating multiple-range spark plugs only by one- or two-digit numbers. These do not allow any indication of the "old" heat rating number. At the same time, the sales documentation for each engine contains the corresponding suitable multiple-range spark plug.

For NGK spark plugs there is a simple rule of thumb:

Low heat rating number (for inst. BP4ES) "Hot spark plug".

high heat intake due to long insulator tip.

High heat rating number (for inst. BP8ES) "Cold spark plug".

Low heat intake, due to short insulator tip.

Do you have any further questions? Please contact our product manager Mr Andreas Götz.