Demands on the spark plug:
The heat range describes the ability of a spark plug to conduct and dissipate heat. Heat supply to the spark plug in the combustion chamber depends on the specific engine type in each case. 
The heat range is important because on the one hand, a certain minimum temperature (self-cleaning temperature) must be reached to prevent "sooting" and associated ignition misses, but on the other hand, a certain maximum temperature (auto-ignition range) must not be exceeded if auto-ignitions are to be prevented. For these reasons, the working temperature of the spark plug (operating temperature range) that is set depending on the engine power specified, must be maintained by design within the above set limits.
The heat range of a spark plug is determined by the heat-range code number and must be adjusted to suit the specific engine characteristics. The heat-range code number is a component of the Bosch spark plug type designation. 
Low code numbers (e.g. 2 to 4) signify "cold" spark plugs, in other words, low heat conduction in hot engines (e.g. Ferrari). 
High code numbers (e.g. 7 to 10) signify "hot" spark plugs, that is, high heat conduction for cold engines (e.g. VW Beetle). 
Bosch spark plugs are individually adjusted without compromise to the specific engine type in each case. 
Temperature characteristics of spark plugs with different heat-range code numbers: 

                      Temperature
                                    |

Engine power
Auto-ignition range
Safety range
Operating temperature range
Self-cleaning temperature
Spark plug with high heat-range code number ("hot spark plug").    Spark plug with medium heat-range code number.  Spark plug with low heat-range code number ("cold spark plug").
Large insulation nose surface absorbs much heat. Low heat dissipation. Insulation nose surface less than with "hot spark plug". Reduced heat absorption. Improved heat dissipation.  Small insulation nose surface absorbs little heat. Extremely good heat dissipation.
Heat-absorbent surface
Heat conduction

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