These come in a set of 4 plugs.  These spark plugs come with a heat range of 8 unless otherwise specified.

HKS M-Series Super Fire Racing spark plugs are high performance Iridium plugs designed to handle advanced levels of tuning and provide Improved ignition performance, durability and anti-carbon build-up

M-Series Super Fire Spark Plugs Feature:

• 0.6mm Iridium Alloy Center Electrode - Iridium Alloy Center Electrode reduces voltage requirement as well as improving ignition performance and durability.
• Outer Electrode with Platinum Chip - The Outer Electrode incorporates a square platinum chip which improves durability, and when combined with the iridium center electrode boosts ignition performance.
• Short Type Outer Electrode - (Anti-Vibration) The outer electrode has been shortened and engineered with the ideal shape to decrease weight for resistance against vibration. This anti-vibration design improves longevity of the outer electrode.
• Thermal Edge - After long idling or various combustion conditions carbon buildup can occur causing misfires. The Thermal Edge of the ceramic insulator will discharge to help decrease carbon deposits.
• Spark Support Gap - Discharges will occur in the Spark Support Gap (space between the tip of insulator and the base of the outer electrode) to help prevent carbon buildup.

*M Series is recommended for reliable ignition, durability and anti-carbon buildup under high load.

The Iridium alloy is used because of its durability and reliability. On an average plug, a narrow core typically shortens life span of plug but with the intense durability of iridium there is no sacrifice in any longevity. With a high melting point of 2454 degrees C, opposed to the 1769 degrees C of white gold, the electrode can tolerate the high levels of compression and minor knocking without any deterioration. A nickel coating on the housing also prevents the spark plug from premature deterioration.