Re: 1972 9.5 Which spark Plug
A cooler plug means that the plug itself can transfer more heat away from the plug into the cylinder head (and the water jacket). A cooler plug can prevent pre-ignition when running at or near wide open throttle. Under those conditions combustion is hot enough to burn away deposits.
A hotter plug retains more heat on the plug. It is desireable if you do a lot of trolling since the hotter plug will burn away the deposits that accumulate when putting along, thus preventing plug fouling.
Running too hot a plug can result in pre-ignition, a situation where combustion happens too soon due to high heat in the chamber (it fires like a diesel). Combustion that happens too soon is very destructive to pistons & rods. It can scorch or even melt pistons, or break the ring lands off pistons, trashing the pistons, cylinders; in short the entire powerhead.
I'm trying to remember, but I seem to recall that a smaller insulater on the plug will alow the plug to throw heat into the surrounding water jackets to be carried away. A bigger insulator will inhibit the transfer of heat away from the plug.
