Bosch (7421) HR9LI33U 21 Platinum IR Fusion Spark Plug, (Pack of 1) Fused iridium and platinum center electrode, sintered into an extended insulator nose results in reaching self-cleaning temperature only seconds after starting and provides exceptional wear resistance Revolutionary surface air gap firing technology featuring Bosch's unique four ground electrode design delivers the most powerful spark for the most efficient combustion and optimum horsepower without increasing the voltage requirement Four yttrium enhanced ground electrodes resist spark plug wear and oxidation for exceptional performance life Factory pre-set gap allows for easy installation that never requires adjustment Fusion's voltage requirement increases less over time versus other iridium plugs for longer reliable performance of the vehicle's ignition system