About Spark Plug Heat Range:

Hot Plugs    

have a longer insulator that prevents heat transfer into the water jackets. It keeps more heat in the tip of the spark plug, which helps burn off combustion chamber deposits. But if there's too much retained heat, the engine will experience preignition.

Colder Plugs    

have a shorter insulator and retains less heat at the tip endare only required when higher boost levels cause increased cylinder head temperatures. If the plug is too "hot" pre ignition may occur. —but if it's too cold, deposits accumulate and eventually the plug will