It seems pretty obvious that a spark plug provides the spark that burns the fuel, but its secondary role as a heat dissipater is equally important. A spark plug's ability to transfer heat to the car's cooling system is based on the length of the insulator nose and the materials used for the center electrode and the insulator.
Standard spark plugs in modern engines have a copper center electrode core surrounded by a nickel alloy, which you can see at the tip of the plug. Inside the plug, the center electrode is encased in porcelain, which helps transfer heat from the engine to the cooling system. Premium spark plugs make use of precious metals, like platinum or iridium, in place of the nickel alloy. These metals have higher melting points -- and higher prices to match.
Speaking of temperatures, spark plugs come in two basic varieties: cold and hot. Cold plugs work best in high-horsepower high-compression engines. They have less insulation, so more heat can be transferred away from the combustion chamber to the outside of the engine. This is no laughing matter: If the plug isn't cold enough for a particular application, it can't get enough heat out of the piston chamber. This can lead to pre-ignition, knocking, and permanent engine damage. If you aren't sure which spark plug heat range to use, err on the side of using a plug that's too cold rather than a plug that's too hot.
Hot plugs have more insulation and are found in most standard engines. The extra insulation keeps the plug's temperature high enough to burn off carbon deposits, which allows for more time between spark plug changes.
As gas prices climb higher, more manufacturers are claiming that swapping out old spark plugs for their premium plugs will boost any car's gas mileage. This is true -- but only to a point. The fact is, dirty, carbon-fouled, misfiring spark plugs will definitely lower a car's fuel economy and replacing them with shiny, new plugs will definitely improve fuel economy. Whether those plugs have exotic metals or nickel-alloy center electrodes doesn't matter quite as much as having the appropriate heat rating and gap between the center electrode and the ground electrode.
Speaking of those gaps, almost any plug you can find at the auto parts store will come pre-gapped for your engine. The days of setting the gap with a gauge are pretty much over, unless you're squeezing every last bit of performance out of your Saturday night hot rod. Engine modifications often mean you must gap new spark plugs, but stock engines with factory-approved, pre-gapped replacement plugs can usually go without adjustment.
Now that we know how a spark plug functions in the engine and the materials they're made of, let's find out what's inside these little guys.