By examining "hotter" and "cooler" spark plugs of the same manufacturer side by side, the principle involved can be very clearly seen; the cooler plugs have more substantial ceramic insulators filling the gap between the center electrode and the shell, effectively carrying off the heat, while the hotter plugs have less ...
I'm sorry, but that's a little weird statement. Firstly, I have never seen a plug manufacturer varying the insulator depending the deat range. They stick with the same basic construction and alter the dimensions of coil and the cavity in which the coil sits. If you have seen differences between insulators, it's propably between different brands.
And the insulator thermal conduction plays a homeopathically small role anyway.
Secondly, if you have more insulator, wouldn't that make the plug more hot?🤔
Regarding 4-stroke plugs, technically they are hotter, but instead of thinner filament they have a longer (heavier) coil with higher heat capasity. That helps them stay hot during the exhaust stroke.
Btw, I noticed that the so called Turbo Plugs are available in a huge variety of heat ranges. Does anybody know how their "hot" corresponds with our "hot" plugs? If they are similar, they might be an interesting option, plus you get a more intelligent combustion chamber shape with them. Turbo plugs are like Nelson plugs, but with a metric fine thread. Instead of a soft sealing ring, they have a conical sealing surface in the bottom of plug. L