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Originally Posted by Petik
So, the secondary role of spark plugs is to conduct heat from the cylinder into the head (and thus coolant). The "heat" of a spark plug refers to how much heat it conducts away from the cylinder. The colder the spark plug, the more heat it conducts. For stock and slightly modified you most likely want to stay with the stock "heat". For FI and wild cars you'd want to go colder to keep the cylinder temps in check.
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Yep. My 1986 Shelby GLH-S was equipped from the mfr with plugs one heat range colder than standard (for the Dodge 2.2L engine). The owner's manual suggested
two ranges colder for excessively hard driving, if I recall.
Conversely, I run one heat range hotter than standard in my '78 Scout's V8 because it's an old and naturally low-revving truck engine, and the hotter plugs stay cleaner --- they tend to burn off oil deposits, etc.