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Spark Plugs
An ignition system is made up of three parts, coils, wires, and plugs. The last part of the ignition system is commonly the most misunderstood - spark plugs. Spark plugs are supposed to do two things: (1) ignite the mixture of gasoline and air, and (2) remove heat from the combustion chamber. Most people know about the first job requirement. Many are dumbfounded by the second.
Contrary to a common myth, Spark plugs do not cause engine heat. The burning of fuel causes heat. Spark plugs remove heat by transferring the thermal energy from the combustion chamber, to the metal that holds the spark plug in place (the threads in the cylinder head), where it's eventually absorbed into the engine's cooling system.
Spark plugs have heat ranges and they're typically referred to as hot or cold. The heat range is an indication as to how much heat the plug will dissipate from the combustion chamber. It has nothing to do with the voltage passed through the plug, or how hot the spark is. The main factors in determining the heat range of a spark plug are the length of the ceramic insulator, the materials that make up the insulator, and the center electrode material.
Regardless as to whether a spark plug is considered hot or cold, the ideal temperature for the center electrode of a spark plug is between 930 and 1560 degrees Fahrenheit. Hot plugs are designed to retain more heat at the insulator tip and cold plugs are designed to transmit more heat to the rotor housing or cylinder head, but both are designed to operate in that same temperature range. In that range, the insulator is burned clean and the electrodes don't melt.
If the electrode temperature stays below 930 degrees F, combustion by-products build up on the insulator. This can result in lower spark energy and misfire. The solution to that problem is to use a hotter plug. If the plug is too hot, the electrode can burn away and pre-ignition can occur. This can result in a holed pistons. Given a choice, you always want to err on the cold side with spark plugs.
That begs the question, "Why would I have to use any other plug than the one specified in the owner's manual?" A number of things can influence plug temperature - air/fuel ratios, compression, boost levels (turbo or supercharged engines), ignition timing, engine load, air temperature, air pressure (altitude), and humidity. Basically though, if you have a stock car, the stock plugs are best.