Denso Iridium IQ24 Spark Plug Specifications Whatever your use or engine set-up the correct choice of spark plug is very important. Iridium is very heavy and dense metal ensuring that Denso spark plugs have a longer lifespan than OEM ones withstanding more than 100.000 km in standard use. Which heat range to choose ? If the letters (for example IQ) indicate the spark plug's shape and thread size the numbers (from 20 to 27) indicate the heat range basically meaning how well the spark plug can operate in intense conditions. The higher the number the more the spark plug will be "cold" and more adapted to tuned engines and sporty use. A cold spark plug is designed to operate at higher than average temperatures and disperse the heat inside the cylinders improving combustion performances and engine reliability reducing the risk of detonation. However make sure you only fit cold spark plugs if your car requires them otherwise you could encounter problems such as difficult cold starts and rough idling. Also do not install cold spark plugs if your engine runs on E85 fuel. In short : IQ20 : standard grade for stock engines or for use with E85 ethanol IQ22 : one grade colder than standard for lightly tuned or sporty engines used both on the road or the track IQ24 : two grades colder than standard for tuned and powerful engines used mainly on the track IQ27 : three grades colder than standard for high power and heavily modified engines used in intense conditions Application List Non-exhaustive list. Aston Martin Virage (40HTE) Caterham 21 1.6i Supersport Fiat Panda 4x4 before 2004 Ford Escort 1.6 RS Turbo (80-86) (HL16T) Ford Sierra Cosworth (87-93) (N5B) Honda Accord B20 engines Honda Civic EC EE ED EG EJ EK MB with D13 D14 D15 D16 D17 engines Honda CRX ED D16 engines Honda Legend C25 & C27 engines Honda Prelude B20 engines Mazda MX-5 NA & NB (B6-ZE BP-ZE) Mercedes E-Class A124 (93-98) E36 AMG Mercedes E-Class C124 (93-97) E36 AMG Mercedes E-Class W124 (93-95) E36 AMG Mercedes SL R129 (89-01) 60 AMG Nissan 200SX S13 (CA18DET) Nissan 300ZX Z32 (VG30DE VG30DETT) Nissan Skyline R32 R33 R34 GT-R (RB26DETT) Nissan Skyline R32 GTS-T (RB20DET) Nissan Laurel (RB20DET) Peugeot 205 1.6 GTi 1.9 GTi (XU5J XU9JA) Peugeot 306 2.0 S16 Peugeot 309 1.9 GTI (85-89) Toyota Supra MK3 (7M-GE 7M-GTE) Toyota Corolla GTI AE82 AE86 AE92... (4A-GE) Toyota Levin & Trueno AE101 AE111 (4A-GE 4A-GZE) Toyota MR2 AW11 (4A-GE 4A-GZE)