Importance of Heat Range
The heat rating is a measure of the amount of heat dissipation.The heat range of a spark plug is the range in which the plug works well thermally. The heat rating of each NGK spark plug is indicated by a number; lower numbers indicate a hotter type, higher numbers indicate a colder type.
Effect of Selecting Wrong Heat Range
When the heat rating is too high
The spark plug temperature remains too low and causes deposits to build up on the firing end; the deposits provide an electrical leakage path that gives rise to a reduction of spark.
When the heat rating is too low
The spark plug temperature rises too high and induces abnormal combustion (pre-ignition) ; this leads to melting of the spark plug electrode that could cause problems such as piston seizure.