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Traditional Single Iridium Plugs Denso's Iridium Power and NGK's Iridium IX series have iridium fine-wire center electrodes with tranditional nickel ground electrodes. The ground electrode will wear out first. Typically we have found you can expect 50-80k miles on a traditional iridium plug. But plug manufacturers have to temper their projections as driving conditions and motor modifications differ. Additionally, because gradual wear will "round-off" firing points over time, the concentrated firing power and voltage requirement benefits will be reduced. Therefore, NGK gives them a life expectancy of 40-50k miles and Denso assigns a recommended replacement interval of 30k miles.
OE Iridium Long Life Plugs NGK Laser Iridium and Denso Long-Life Iridium plugs feature an iridium tipped fine wire center electrode, and platinum tipped ground electrode. Bosch Double Iridium includes places iridium in the ground electrode inlay as well. These OEM style plugs typically have special features designed for a specific engine and last 80-120k miles.
Aftermarket Long Life Iridium Plugs Autolite Iridium XP, Champion Iridium, Bosch Iridium and Denso Iridium TT bring the features of OEM iridium long life plugs to the market, with an iridium or iridium enhanced fine wire center electrode and platinum or iridium/platinum tipped ground electrode. Under normal operating conditions, expect a good 100k miles out of these plugs.
Iridium Racing Plugs Denso iridium racing plugs and some NGK iridium racing plugs have iridium center and a platinum ground electrodes. If installed in a regular engine, they would likely last longer than most people keep their car (barring any motor problems that can cause premature plug death). But, these usually come in heat ranges too cold for an unaltered motor and are usually used in racing applications where all bets are off.