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The NGK TR55IX is an Iridium IX spark plug sold in a box of four. It uses a fine-wire iridium center electrode that NGK rates as six times harder than platinum, paired with a tapered cut ground electrode for sharper ignition and better anti-fouling. It suits performance-minded owners and older GM V6 and V8 engines that call for a TR55 heat range. Against platinum plugs it lasts longer and resists fouling better; against NGK's own Ruthenium HX it is the cheaper step below the top tier.
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Owners of GM V6 and V8 engines commonly fit these as a step up from platinum and report smoother idle and quicker starts. Most criticism centers on cost and on the fragile fine tip during gapping.
The NGK Iridium IX (TR55IX, part number 7164) is a spark plug made for performance enthusiasts. It is sold as a box of 4.
The center electrode is made of fine-wire iridium alloy. Iridium is six times harder than platinum and has a very high melting point. The ground electrode has a tapered cut.
The design is intended to improve acceleration, fuel efficiency and durability. It also offers high ignitability and resistance to fouling. The tapered ground electrode improves starting, idling and throttle response.
