Exhaust temps - fine tuning carbs

[rphillips]

I dun'no, if a hotter plug gets hotter by holding heat in longer, I'd expect everything inside the cyl, including plug tip, ex. gasses, and valves would be hotter.

[Grimly]

I dun'no, if a hotter plug gets hotter by holding heat in longer, I'd expect everything inside the cyl, including plug tip, ex. gasses, and valves would be hotter.

A colder or hotter plug will make damn all difference to the cylinder temperature.
It's only relevant to the plug itself.

[rphillips]

I guess I'm thinking too simple. If I have 2 - 1/2 pt jars, put a spark plug heated to 600 degree in one and a spark plug heated to 400 degree in the other, if there's a thermometer inside both jars, I can't imagine both thermometers will read the same?... well maybe.

[zed1015]

The heat rating only applies to how quickly its center electrode sheds heat not its running temperature.
A plug needs to run at the lowest temperature as possible to prevent pre ignition but hot enough to prevent fouling.
In general a high performance engine producing more heat requires a colder plug that sheds electrode heat faster to prevent pre ignition and a lower performance engine that runs cooler requires a hotter plug to retain electrode heat to prevent fouling.
A hotter plug doesn't run hotter than a cold plug and vice versa in their correct applications it's just how quickly they shed the heat to maintain the electrode temperature needed for correct running.
Across all engine types running the correct plug for the application the actual electrodes would all run in the same constant temperature range.

[rphillips]

Pretty dang good explanation zed... Even I can kind'f understand that. No matter how hot or cold the cyl. and ex. gasses are, the different plugs maintain near their optimum operating temp.