TBatt

Heat range and gap are two totally separate things. The smaller gap is needed because when the engine is under boost the air density in the combustion chamber goes up. Because air is an insulator the gap needs to decrease so that the spark can be generated with the same amount of voltage but with denser air. I hope that explanation makes sense.

Heat range is all about the center electrode temperature. If you look at a hot plug verses a cold plug you will see the hot plug has a longer center electrode ceramic part. This longer heat path to the cylinder head allows the tip to remain at a hotter temperature (less heat loss back to the head). A cold plug is just the opposite, it has a short ceramic part that transfers the heat to the head quicker which keeps it cooler.

A hot plug is typically used on a low compression engine that sees light loading (grocery getter) where the cold plug is for high compression high load use. The cold plug will keep the tip from getting so hot that it will ignite the fuel air mixture before the plug fires. That will cause detonation. At least that is what I have been taught.

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