Iridium Spark Plugs

Kenny had no idea that the solution to this perplexing problem would be a new iridium spark plug technology from Denso. In Denso's research for an OEM spark plug that would provide 200,000 miles of service life and lower vehicle emissions, Denso developed a new iridium alloy electrode spark plug. The progression from nickel alloy plugs to platinum plugs in 1982 was a giant leap forward in technology. Denso's introduction of the iridium alloy spark plug will prove to be even more significant, especially for high-performance and race engines. The major difference in the Denso Iridium Power spark plug and conventional platinum plugs, besides the alloy, is the size of the center electrode. A typical platinum plug has a 1.1mm diameter center electrode. The Denso Iridium Power OEM plugs have a .7mm diameter center electrode and the Denso high-performance plugs have a .4mm center electrode.

Size Matters
What does size have to do with it? Less voltage is required for a smaller center electrode and results in better ignitability. The smaller the electrode, the more centralized the electrical potential is around the electrode tip. The required voltage can be reduced because the level of the electric field is made stronger and local insulation (air gap and electrode surface oxidation) breaks down more easily. The bottom line is that it takes approximately 5,000 volts less to fire a Denso Iridium Power spark plug versus a conventional platinum spark plug.


So why not just make a smaller diameter electrode spark plug out of platinum? It just wouldn't last. The small-diameter center electrode reaches much higher temperatures. Iridium's melting point is 700 degrees C higher than platinum, and laboratory tests have shown that with the same-size electrode iridium, plugs were four to five times as resistant to wear as platinum. Much of Denso's R&D went into finding the perfect iridium alloy (90% iridium, 10% rhodium) that would provide 200,000 miles of service, and working out the manufacturing process to "draw" the electrode into the extremely small .4mm-diameter wire.

The Test
Kenny was contacted by a Denso representative to test a set of Denso Iridium Power high-performance spark plugs under extreme, real-world conditions. So he installed the plugs and ran the turbo boost all the way up to the 40 psi limit. The dyno numbers tell the story: 1,850 repeatable horsepower, test after test. Kenny admits he's still shy 100-hp from the goal of 1,950-hp, but a camshaft change is in the works to make up the deficit.