Spark Plug

If the plug is too hot for the application, it can become a source of pre-ignition. If the plug is too cold, it can experience fouling problems.

The operating temperature of a spark plug depends on a number of variables. The two most influential are cylinder head temperature and the relative richness or leanness of the fuel mixture. Given such variables, it is impossible to have a single spark plug that would work well in every application, even if thread sizes and reach were standardized.

Heat range is determined by several design features, one of which is the distance heat must travel from center electrode tip to the plug’s shell. A plug with a short ceramic insulator between electrode tip and shell runs cooler than one with a long nose insulator.

A cold plug is good for high speed, high load operation because it sheds heat quickly and is less likely to overheat and cause pre-ignition. Colder heat ranges are used most often in high performance and turbocharged engines.

For short-trip, stop-and-go driving, a cold plug may not run hot enough to keep itself clean. A hotter heat range plug may be needed to resist fouling.

For sustained high speed or high load running, a hotter plug may become too hot and cause preignition. The trick is to use a plug hot enough to prevent fouling yet cold enough so there is no danger of pre-ignition.

One way to extend or broaden the heat range of a spark plug is to extend the tip of the plug further into the combustion chamber. The longer insulator makes the tip run hotter for better self-cleaning at low speeds and light loads. It also exposes the tip to more of the incoming air/fuel mixture, keeping it from overheating at high speeds and loads. An extended tip spark plug typically has a much broader heat range than a standard spark plug.

Another way to increase heat range is to use a center electrode with a copper core. Copper is an excellent conductor of both heat and electricity. With a copper core center electrode, heat is carried away from the plug tip through the electrode during high speed, high load operation. This allows the plug to dissipate heat more quickly like a colder plug, yet stay hot enough to burn off fouling deposits.

Because of the increased heat range copper core plugs offer, one plug can be used in applications formerly requiring several different plugs with narrower heat ranges.

The use of a platinum or gold palladium center electrode is another design innovation that improves fouling resistance while greatly extending plug life. The special alloy at the tip of the center electrode is more wear and corrosion resistant than standard electrode metal. It allows the use of a longer insulator, helping plugs reach a self-cleaning temperature of 750 degrees F in only a few seconds.

Spark plug manufacturers avoid making specific mileage claims for such premium plugs, but many experts say the plugs will often last up to 60,000 miles. Other benefits include better cold starting, less cold fouling, and improved operation during both stop-and-go and highway driving. These plugs are considerably more expensive than standard plugs.