Denso Iridium Power Spark Plug. It was no secret that Iridium is 6X harder then Platinum, 8X stronger and resists melting in temperatures exceeding 4,000 degree Fahrenheit. Denso Iridium spark plugs feature the world_s smallest center electrode, measuring 0.4mm in diameter. Because the majority of sparks is exposed to the air / fuel instead of the metal surface, the fuel burn is greater (0.5 increase over platinum), which ultimately improves gas mileage.