Ok. Here we go. It has to wit the heat range of our engines. This is the speed at which the spark plug can transfer heat from the firing tip to the cylinder head water jacket into the cooling system. Choosing the right heat range is critical to high performance engines. The sp 542 is a "colder" plug with a shorter insulator nose length(the distance from the tip to the spark plug shell), and transfers heat rapidly from its firing tip to the water jacket. Cold plugs are ideal for high rpm engines, FORCED INDUCTION applications, and other instances where engine temps operate at a high rate. The bottom line is that these plugs help our engines with heat transfer to help keep the engine from detonating or pre ignition. Colder plugs transfer heat faster. Hotter plugs(like the stock ones) can hold heat longer and transfer it at the wrong time and pre-ignite the air/fuel mixture. This can cause broken parts. Hope that helps.