Barring mechanical contact between the spark plug and piston, just about any randomly selected spark plug will ignite the gasoline and air compressed into an engine's combustion chamber. Very few spark plugs, however, will continue to perform well under a wide range of engine temperatures, loads and speeds, not to mention lasting well beyond today's standard replacement interval of 100,000 miles.

SPARK PLUG LIFE


At least four conditions are required for maximum spark plug life. First, the engine management system must mix a chemically perfect or "stoichiometric" ratio of 14.7 units of air to 1 unit of fuel. Second, the combustion event must be perfectly timed to achieve a smooth "burn" in the cylinder. Third, the valve guides, seals, pistons and piston rings must be in good condition to prevent engine oil from entering the combustion chamber. Fourth, the spark plug must be correctly torqued and seated to the cylinder head to help dissipate excess heat from the electrodes. If all of the above conditions are present, the spark plug will have an "out-of-the-box" appearance (See Photo 1).

Most spark plugs are designed to maintain an operating temperature of 900° F and 1,500° F at the center insulator. If the insulator temperature drops below 900° F, accumulated carbon causes a cylinder misfire by shorting the spark to ground. During normal driving, the insulator temperature rarely exceeds either temperature extrem...