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NGK Iridium IX spark plugs are a high quality performance spark plug featuring an iridium tip for very consistent and stable spark.  Iridium also has a very high melting point which keeps it functional in a high heat, high performance engine.  The Iridium IX spark plug also features special plating to resist corrosion and seizing.  
Commonly used in the Mazdaspeed 3, Mazdaspeed 6, Mazda CX-7 and other engines.  This is a one step colder plug (higher heat range) when using on the MZR DISI engine.