Because automotive engines differ in their operating loads, compression, engine speeds and cooling, it's impossible to run all engines with a standard spark plug. To ensure peak performance, a spark plug must operate in its optimal temperature range.
A spark plug's heat range is an index of its capacity to dissipate thermal energy. A low heat-range number (for example, 2 to 4) indicates a cold plug that quickly dissipates heat to the engine block and cooling system, whereas a high number (7 to 10) indicates a hot plug that retains heat.
By buying spark plugs with the correct heat range for your vehicle and driving conditions, you'll ensure that the plug will operate between 500 and 900 degrees Celsius (the range in which the spark plug will be self-cleaning yet won't be hot enough to pre-ignite the air/fuel mixture). Check your owner's manual or with your local auto parts store for the appropriate heat range for your type of driving.