Yep! Hib is correct about "hotter vs. colder" spark plug
rating. But, far as the arc itself goes, it is true too that the amount of GAP will affect the voltage necessary before current will jump the gap, and the resistance combination of the plug wires and plug's internal resistor (if any) will affect the follow-on current (heat) of the arc itself.
The
diagram* is a little confusing. Heat range of the
chamber and
heat (type) of the corresponding spark plug are opposite.
Chamber temperature range increases with engine load and RPM. So, low speeds and low power demands = low chamber temps which require a hotter plug to burn off fouling,
and visa versa.
See attached from
Denso site:
*Just about any of us could have selected better (less confusing) descriptions, me thinks. Just remember low speed, low performance demand = need for HOT plugs to keep from fouling, and visa versa!