Mopar Muscle Association UK

ok, which heat range for spark plugs is a big debate with older engines. so as an example a standerd 400 b series with cast heads lets say uses a 5 heat range in ngk spec, so we up the engine spec and we go to a colder plug 6 heat range to compansate for heat but if we put ali heads on the uprated engine the heat dissipation is better for the engine
So therefore we could we just leave em at standard 5 as the plug should be running cooler with ali heads.
hope you see where i am coming from or maybe i am just talking garbage
so boffins have your say

Take the car for a run on a fast road and then read the plugs earth strap, that will tell you for sure if the plug is too hot, too cold or just right for your application.

That's a great diagram Blue, more informative than even the Haynes photos.

It's very handy and reading the plug for heat range is the easiest bit of all.