http://gty.im/178918705
65 million years ago, iridium crashed to earth in the form of an asteroid nearly 10 miles across. The dust cloud from this impact blocked out the sun, cooled the globe and is widely believed to be the reason for extinction of the dinosaurs. When the dust settled, it created a thin layer of iridium-rich clay that now provides spark plug manufacturers with the raw materials needed to create iridium spark plugs.
Iridium is an excellent raw material for spark plug electrodes because of its unique chemistry. As the densest and least corrosive of all metals, iridium also is temperature-resistant and a reasonably good electrical conductor. It has a very high melting point, is incredibly hard and also has very low electrical resistance. These properties make iridium desirable for use in the scorching environment of the combustion chamber, as well as making it an ideal material for creating a precise, concentrated spark. Its only drawback is its brittleness, so most “iridium” electrodes are alloyed with the softer metal rhodium, which also helps combat wear due to oxidation. The iridium plug’s extreme wear resistance allows for a smaller-diameter center electrode design than platinum plugs, decreasing material costs, and creating a finished product with long life and acceptable retail prices.
Measuring in at around 0.4mm, the small surface area of this tiny sliver of hardened wire provides a more concentrated spark discharge than platinum or copper center electrodes. As a result, less voltage is required from the coil to create a strong spark, saving wear and tear on the ignition coil(s). The coil-on-plug (COP) designs used in many late-model applications rely on compact individual coils, which are capable of multiple spark discharges without overheating. Better ignition reliability is just one benefit of this arrangement. The decreased resistance of the fine iridium-alloy tip also allows for the spark to jump a greater electrode gap, creating a larger flame kernel for better combustion. More consistent spark leads to smoother idle, faster throttle response, and better overall engine performance. Iridium plugs are also able to ignite a leaner air-fuel mixture than other designs, allowing manufacturers to improve fuel efficiency and reduce emissions.
Iridium’s excellent wearability allows these plugs to resist gap erosion better than copper and platinum designs, while providing consistent performance over the lifetime of the plug. As the gap does eventually erode, it does so at a much slower rate, extending the service interval even beyond that of platinum. The extended service life of iridium certainly offsets its initial price, making it comparable in cost-per-mile terms.
Many manufacturers specify an iridium spark plug as original equipment in order to achieve the full benefit of these advantages, and substituting a plug with a different electrode design may reduce the effectiveness of the ignition system. Increased electrical resistance in the system may lead to coil failures, misfires, and even catalytic converter damage as unburned fuel is passed into the exhaust. Replacing an iridium plug with a less expensive conventional design may also cause poor performance, increased plug fouling, and decreased service life.
