Hot and cold plugs have NO EFFECT WHAT SO EVER on the temp te engine runs!

Grab 2 plugs of diff heat ranges and look at the sparking end. the tip gets hot when in use (duh) and the heat is transferred to the threads to the head and son on to the rad and air. The DISTANCE from the tip to the threads is what controls the heat range of the plug - the longer the distance, the hotter the plug tip will be when the car is running, less distance menas more heat transfer, so a colder plug tip.

If the tip is too hot: it may glow and ignite the mixture without sparking, or it may melt.

if the tip is too cold: it may accumulate deposits and foul. Hense the idea in oil burning cars to run hotter plugs - to burn off the crud.

chris