I found a diagram that illustrates the main factor in determining a spark plug heat range that I was trying to describe to you during the chapter get together.
Here it is:
The main factor is the length of the ceramic insulator from the middle of the spark plug to the tip (where the spark happens)
The insulator length of the plug on the left is much longer than the insulator length of the plug on the right.
That insulator length is the path for the heat on the tip of the plug to flow to the cylinder head.
The balance that we are always working on is to make the plug 'hot' enough to burn off the carbon that builds up on the ceramic
(especially during a cold start and warm up when the choke makes the mixture rich) but not so hot that you get pre-ignition
in the cylinder when everything is hot. We have specific tests to determine how susceptible a plug is to getting fouled and a different
test to determine how close it is to pre-ignition. You always design to stay further away from the pre-ignition point because
that can damage an engine much faster than being fouled.