Denso Iridium Spark Plugs Jaguar 4.0L (XJ8/XJR/XK8/XKR/S-Type). Price is for 1 spark plug, you will need 8 Denso Iridium spark plugs for a High Performance Tune-up. Iridium: 8 times stronger than platinum, 6 times harder, and with a melting point 1200 degrees higher. New laser welding techniques make it possible to make a spark plug with an iridium electrode. The result is a plug that requires less voltage to spark, burns more of the fuel in the cylinder, and can spark at LEANER air/fuel mixtures (see inset diagram), resulting in higher horsepower and better gas mileage.

Although the plugs are expensive, you don't have to change them as often as garden variety spark plugs. The iridium electrode is extremely resistant to erosion, so it can last MUCH longer than a normal plug. (Longevity will vary by usage, fuel mixture, and other factors.)