The term spark plug heat range refers to the speed with which the plug can transfer heat from the combustion chamber to the engine head. It has been found the optimum combustion chamber temperature for gasoline engines is between 500C850C. When it is within that range it is cool enough to avoid pre-ignition and plug tip overheating (which can cause engine damage), while still hot enough to burn off combustion deposits which cause fouling.
The spark plug can help maintain the optimum combustion chamber temperature. The primary method used to do this is by altering the internal length of the core nose, in addition, the alloy compositions in the electrodes can be changed. This means you may not be able to visually tell a difference between heat ranges. When a spark plug is referred to as a cold plug, it is one that transfers heat rapidly from the firing tip into the engine head, which keeps the firing tip cooler. A hot plug has a much slower rate of heat transfer, which keeps the firing tip hotter.
An unaltered engine will run within the optimum operating range straight from the manufacturer, but if you make modifications such as a turbo, supercharger, increase compression, timing changes, use of alternate racing fuels, or sustained use of nitrous oxide, these can alter the plug tip temperature and may necessitate a colder plug. ?A rule of thumb is, one heat range colder per modification or one heat range colder for every 75100hp you increase. In identical spark plug types, the difference from one full heat range to the next is the ability to remove 70C to 100C from the combustion chamber.
Remeber that if the plug is to LOW of a certain heat range, it can cause continuous fouling, which can cause detonation. If that happens, why use a colder plug right?[