Different lengths of insulator nose result in varied characteristics and different heat ranges. Heat is dissipated from the insulator nose through the center electrode and the inner seal ring to the spark-plug shell. If the insulator nose is long, the heat-transfer point formed by the seal ring is further away from the hottest point on the insulator nose than with a short one. So it follows that spark plugs with a long insulator nose can absorb more heat than plugs with a short insulator nose.
A spark plug's heat range is identified by a code number. Low heat range indicates a cold plug that has low heat absorption due to its short insulator nose; high heat range code indicates a hot plug that has high heat absorption through its long insulator nose. However, as mentioned previously, the plug for a particular application may be a cold plug in one brand and a hot plug in another.
The shunt sensitivity and corrosion resistance of standard spark plugs with center electrodes made of nickel-based alloy have been greatly improved by the development of a compound center electrode with a copper core. Pure metals provide better thermal conductivity than alloys. At the same time, pure metals such as nickel are more sensitive than alloys to aggressive chemical substances in the combustion gases as well as solid deposits. For this reason, the jacket material of the compound electrode consists mainly of nickel, which is alloyed with chromium, manganese, and silicon. Each of the alloy additives has a special task: The addition of manganese and silicon increase the chemical resistance, particularly against very aggressive sulfur dioxide, and nickel-based alloys with silicon, aluminum, and yttrium additives also improve resistance to oxidation and scaling.
A ground electrode, which must be flexible enough to allow gap adjustment, can be made of a nickel-based alloy or a composite material. A copper core with a nickel jacket meets the stringent requirements for high levels of thermal conductivity and corrosion resistance of the ground electrode.
Silver exhibits the highest electrical and thermal conductivity of any substance, but corrodes easily, that is why air-bag contacts are gold instead of silver. It's also extremely resistant to chemical deterioration when unleaded fuel is used. Resistance to thermal stresses can be substantially enhanced by using composite particulate materials with silver as the base. These characteristics are the reason its used for electrodes. When solid silver is used for the center electrode, the diameter can be reduced. Despite the reduced diameter, the silver-center electrode dissipates heat better than a comparable nickel-based electrode.
Because platinum alloys are extremely resistant to corrosion, oxidation, and melting, they are used in the electrodes of long-life spark plugs. Since platinum spark plugs cost more, racers often think they are better, but the opposite is true. Platinum does not react well with leaded race fuel and often allows a coating to form that creates a shunt path and a cylinder misfire. In this application, the standard material or a newer copper-core spark plug outperforms a high-dollar platinum design.
If the same stresses are applied, platinum electrodes can be made smaller than the equivalent nickel-based electrode.
Other Spark Plug Facts
A phenomena that has given many mechanics and enthusiasts a real headache is cold-start coating, or what some refer to as body-shop syndrome. When a vehicle is cold started, especially with unleaded fuel, and run for a very short time (e.g., moving the car a few feet), a clear coating, invisible to the naked eye, forms on the plug electrodes. The coating becomes a shunt for the electricity and causes a misfire on most if not all cylinders. This results from an overly rich cranking air/fuel ratio and warm-up mixture and the plug's inability to reach clean-off temperature. After only three or four cold starts that didn't reach clean-off temperature, a new set of spark plugs can become so coated the engine will hardly run. This is very common for new-car dealers where the vehicle inventory may be moved around a number of times before sold or for the hobbyists who only drive their Pontiac in and out of a trailer.
Over the years, I have fixed this problem on many Pontiacs that had new spark plugs but experienced numerous cold starts. When the plugs become coated, they can't be cleaned and must be discarded. If your Pontiac is rarely used for long trips, switch to a slightly hotter spark plug that cleans-off sooner. This may be needed even on a daily driver that only commutes a few miles.
A number of years back, a neighbor had a 3.0L V-6 Grand Am that was a year or two old. She worked only one mile from home, and it was downhill all the way in with load on the engine. But the car was always in the shop for a diagnostic routine, and the typical procedure was to throw computer sensors at it. Because the vehicle had only a few thousand miles, no one ever considered how its use impacted the spark plugs. Just as she was going to trade it in for a another car, I convinced her to have the spark plugs changed to a set that was three heat ranges hotter. She drove that little Pontiac for 10 more years without a problem!
Apply the reverse logic if your Pontiac has very high compression or uses nitrous or forced induction. The added cylinder pressure and heat often require a colder spark plug to eliminate detonation or high-speed misfiring. And to get the most from your Pontiac, don't forget to tune the spark plugs.
When GM introduced HEI ignition in many applications, the plug gap was increased. A larger gap provides a larger ionization window when the coil has the ability to arc across it. This is very important with lean mixtures since they are harder to light off than a rich mixture. The gap of the spark plug can be altered to achieve the best ionization event, initiating expansion of the flame across the bore.
We hope you enjoyed this primer on the lowly spark plug. Now that you know more about them and how hard their job is, you may feel a little guilty about throwing them in the trash after a tune-up considering how faithfully they have served.