Why are There Many Different Types of Spark Plugs?

Spark plugs need 5,000, to 40,000 volts from the ignition coil before a spark will jump across its electrode gap.  It takes a lot of volts to push the spark across the gap because air doesn't conduct electricity unless it is ionized first.  The spark jumps from center electrode to side ground electrode.

The reason why a plug fires from center electrode to side ground electrode, instead of vice versa, is because it's easier for spark to originate at a hot electrode than a cooler one.

The center electrode runs much hotter than the side electrode because the center electrode is encased in ceramic (a good insulator of heat as well as electricity).  This slows down heat transfer from center electrode to cylinder head.

If ignition polarity is reversed, it can take up to 40% more firing voltage to send the spark from ground electrode to center electrode.  The results can be misfiring under load and poor engine performance.  Keeping the center electrode hot also helps burn off fuel and oil deposits that form on the insulator tip.  Deposits can conduct voltage away from the gap causing the plug to misfire, so keeping the center electrode hot helps prevent fouling.  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 designed features, on 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.  Cooler 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 to hot and cause pre-ignition.  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 increase heat range copper plugs offer, one plug can be used in applications formerly requiring several different plugs with narrowing 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 Fahrenheit 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.