*Pricing is per set of (4) spark plugs. Iridium IX plug, 14mm, 25mm (.984") reach, 5/8" extended shell hex head, tapered seat, resistor, projected core nose, fine wire (0.6mm) iridium center electrode, tapered cut ground electrode .050" gap NGK iridium plugs represent the ultimate evolution of spark plug technology and performance. The iridium center electrode is both stronger and harder than platinum. This allows NGK engineers to design an ultra-fine (0.6mm)center electrode reducing the voltage requirement for spark. This allows for a brighter, stronger spark from your existing ignition system. The ground electrode has a tapered cut at the firing end which reduces quenching for better flame core growth and increased ignitability. The combination of fine wire center electrode and tapered cut ground will increase performance, improve acceleration, and fuel efficiency. If you are running higher boost than stock you will benefit from a colder spark plug as they help combat both pre-ignition and detonation. Don't forget spark plug wires! An upgraded ignition coil is also a good idea!