HKS M35i spark plugs Brand HKS (Japan) Website HKS-Power.co.jp Part number 50003-M35I Increasing the boost level will elevate spark plug tip and in-cylinder temperatures. Therefore, a colder grade spark plug can help prevent detonation by removing more heat from the combustion chamber during every ignition. The ST185 uses ISO 14x19 mm (wrench size 16 mm) plugs. Stock plugs are Denso PK20R8 (Toyota part number 90919-01180) which are heat grade 20. The heat grade number is different for each plug manufacturer, for instance: Denso 20 = NGK 6 = HKS 30 I went for one grade colder since I've increased the boost level but I'm not running a massive turbo yet. One grade colder; Denso 22 = NGK 7 = HKS 35. Related links Modifications - Engine ULTRA BP silicone power plug wires