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Heat range
The number in the code shows how quickly the plug moves heat into the head. A tuned engine asks for a colder plug than the factory one, usually by one or two steps, otherwise the electrode overheats and lights the mixture before the spark does. Watch which way the scale runs: at NGK a high number means a cold plug, at Denso it is the other way round.
The electrode gap
At high cylinder pressure the spark crosses less easily. That is why motorsport plugs come with a smaller gap than standard ones, and at high boost it is closed further. The gap is checked with a feeler on every plug before fitting, including new ones out of the box.
Copper, iridium or platinum
Copper conducts best and is the racing choice, but it wears fast. Iridium and platinum last tens of thousands of kilometres and are the option for a car driven every day. On a track car, copper plugs changed on time cost less than a set of iridium burnt out mid season.
On fitting
They are torqued to the figure on the data sheet, on a cold engine. Too tight and you damage the thread in the head, too loose and the plug cannot move heat out through the thread and overheats. They are changed as a complete set, not one at a time.