Switching to Iridium Spark Plugs

Eliminate misfiring by upgrading to Iridium spark plugs

Article updated October 2011

Iridium spark plugs can cure your high-performance car's misfiring issues. It's that simple. As they say, the devil's in the details and your spark plugs are one little detail that can lead to higher horsepower ratings and more efficient ignition. The truth is, spark plugs get no respect! In fact, a lot of performance enthusiasts don't give them a second thought. They spend thousands of dollars building the ultimate performance engine then simply screw in whatever plugs the local auto parts store has in stock. The perception is that spark plugs are the lowest-tech components in the modern internal-combustion engine.

Change is Good

It's no wonder. Contemporary electronic ignition systems have made spark plug replacement in everyday passenger vehicles a thing of the past. And when a racer or high-performance engine builder has a misfire problem, the first thing he does is to add a more powerful ignition system. That's about to change. When the word gets out about the new Denso Iridium Power spark plugs that added 750-hp to Kenny Duttweiler's 450-cid twin-turbocharged Ford, engine builders and tuners will gain more respect for the lowly spark plug.

Kenny Duttweiler of Duttweiler Performance in Saticoy, California is no stranger to making horsepower, especially with turbochargers. After years of building little turbocharged V-6 Buicks that produce in excess of 1,500-hp, he found a lucrative market in NMCA's "World Fastest Street Car" classes building 1,700-hp small-block Chevy V-8s for winning racers such as Bob Rieger and Rod Saboury.

So when he bolted a customer's NHRA AA/Altered Turbo Ford engine on the dyno and had problems making the requisite 1,950-hp (out of 450 cubic inches on gasoline), he left no stone unturned. The engine had state-of-the-art everything-Motech engine management system, MSD Digital 7 ignition system and everything else you could think of. Kenny had isolated the problem to inadequate ignition performance. There was no audible misfire, but the engine made 1,700-hp at 17-psi of turbo boost and only 1,100-hp at 24-psi, indicating that the increased cylinder pressure was causing an undetected intermittent misfire. Reasoning that it was an engine-management or ignition-system problem, he replaced both. However, his Stuska dyno yielded the same results. Kenny replaced spark plugs several times with the best racing and platinum plugs he could find; still no improvement.

Kenny had correctly diagnosed the problem, but as far as he knew, there was no solution. He was already using the most powerful engine management and ignition systems on the planet, and he'd tried most of the "state of the art" spark plugs on the market. Kenny was running out of options and stated prophetically, "Some engines are spark-plug sensitive, especially Hemi-style engines. That's why Chrysler designed dual-plug cylinder heads for their Pro Stock motors in the early '70s. A turbocharged race engine is a variable-compression engine. At 25-30 psi of boost, the cylinder reaches an incredible 2,800-3,000 psi. The higher the cylinder pressure, the harder it is to fire the spark plug. This Ford engine we're developing is the worst of all circumstances. It has a hemispherical-shaped combustion chamber and a 4.670 cylinder bore that is a large area to light off at high rpm."

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.

So what does this mean to the average performance enthusiast? Your spark plugs may be the source of intermittent misfiring if you're running any engine with high cylinder pressures generated by high-compression pistons, nitrous oxide, superchargers or turbochargers. Or, if you have less than the latest, greatest ignition system, you can essentially gain another 5,000 volts of ignition performance by just changing your spark plugs to Iridium power.


Denso, www.denso.co.jp/index-e.html

Duttweiler Performance, 1563 Los Angeles Ave., Saticoy, CA 93004, (805) 659-3648

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