What is the difference between a hot plug and cold plug?

Answer 1

"Hot" or "cold" in reference to a spark plug indicates how quickly heat is dissipated by means of sinking to the surrounding metal mass into which plug is screwed . . . which, in turn, is dictated by the cathode path distance from the tip of the plug to the threaded base (or ground). If the distance is long (which makes the plug itself longer), then the heat dissipation path is longer and so heat is dissipated slower, thus making for a "hot" plug. Conversely, the shorter heat dissipation path in a (comparatively) shorter plug of same electrical properties would make for a comparatively "cold" plug.

We do not typically speak of cold plugs as those would constitute plugs specified for normal, or average, use.

Hot plugs (of increased length), on the other hand, are used in circumstances, such as extensive idling in park, where plug temperature does not reach sufficient average temperature to prevent carbon fouling (and subsequent, engine damaging knocking) as a result of incomplete fuel burning. The most typical application is in taxi cabs . . . where long period at idle are spent waiting for fares.

Ancillary comment: If you have ever noticed the "cabbie habit" of over pushing the engine with "heavy pedal" when transporting fares, that is just an additional "trick of the trade" (additionally with or or as alternative to hot plugs) that is intended to "burn" off unburned fuel deposition and help maintain "cleanliness" of the plugs' electrodes.