Platinum Vs. Iridium Spark Plugs and Gas Mileageby Richard Rowe
Spark plugs are kind of like axes, in that their apparent simplicity of construction and material belies a great deal of trial-and-error refinement and application specificity. The newest generation of exotic metal spark plug gives a vehicle owner opportunities for optimizing the engine to perform at its greatest efficiency.
Spark Plug Basics
A spark plug is essentially just a really thick wire with a short in it. At the core of any spark plug is an approximately 3-inch long metal rod; one end of that rod plugs into the ignition coil via the plug wire or boot and the other is exposed. Around that rod is wrapped a ceramic insulator, and around that a metal case. Positive energy traveling through the wire from the coil jumps the gap between the tip of the wire and the metal case, which grounds to the engine block. An electrode strap attached to the metal case hangs just over the tip, effectively shortening the distance between the wire tip and the case. In this way, platinum and iridium plugs are the same.
Gasoline will autoignite at somewhere between 250 and 500 degrees, depending upon the conditions and fuel quality. Compressing air causes it to heat up by trapping all of its thermal energy in a smaller space; the more you compress it, the hotter it gets. A diesel engine uses this compression-heating effect to cause its fuel to autoignite by squeezing the air/fuel mixture down to between 1/20 and 1/30 its original size. Gasoline, however, ignites easier and burns faster; so, a gas engine uses lower compression -- around 10-to-1 -- to squeeze the mixture to just below its autoignition point, and a spark to superheat one small pocket of air/fuel. Once that pocket or kernel ignites, pressures around it rise and the flame spreads out in a roughly spherical bubble.
Heat and Copper
The spark plug tip and ground strap exist in one of the harshest and most corrosive environments imaginable. High temperatures in the cylinder can melt the thin metal wire tip and the ground strap, and those temperatures will also drastically increase the rate at which oxygen in the cylinder oxidizes and degrades the metal. Spark plugs rely on excellent electrical conductivity to help the spark maintain its energy and heat as it crosses the gap, and that means using a highly conductive metal like copper. But, while it is fairly inexpensive compared to comparably conductive materials like silver, copper is also extremely susceptible to heat damage and corrosion.
Exotic Coatings and Materials
There are two basic ways to prevent overheating and breakdown of copper tips and ground straps. The most inexpensive and most basic approach is to make them thick, which allows heat to more quickly transfer away from them and into the engine block. The second is to coat the tip and strap with a more corrosion-resistant metal such as platinum and iridium, and that's exactly what many manufacturers do. Such coatings reduce the damage caused by oxidation and enhance plug life, but they don't do anything for performance or fuel economy. The only way to do that is to increase spark intensity, and that comes by using a smaller diameter tip. Platinum and iridium offer comparable levels of protection, but platinum is usually used for coatings because of its cost advantage.
Fine Wire Plugs
A smaller, finer tip reduces the diameter of the spark, thus concentrating its energy into a white-hot stream of plasma instead of dispersing it into a wider and cooler lick of flame. Making the spark plug tip out of nothing but very rugged -- and very expensive -- platinum or iridium allows for a small diameter tip that can still withstand corrosion and high temperatures. The hotter spark will help to enhance combustion efficiency by more quickly spiking chamber temperatures, which gives the flame front a head start on the rising piston. The hotter and more energetic spark will also jump a larger gap without blowing out. Platinum is corrosion resistant, but it doesn't have the extremely high melting point of iridium. As such, iridium tips can be made much thinner than platinum to concentrate the spark energy without melting.
Optimizing the Plugs
To really take advantage of your expensive performance plugs, you may need to adjust ignition timing or spark plug gap. A slightly larger plug gap makes for a larger flame kernel, and that means a faster burn. Some engines will do better with more or less ignition advance, given a larger plug gap and fine-wire plugs. It's something you'll have to play with to optimize the plugs for you specific engine and chassis. There are some minor power gains to be had with iridium vs platinum in very high-output racing applications with powerful ignition systems, but odds are slim that that will manifest at the pump. Fine-wire iridium plugs more than fine-wire platinum plugs -- can expand your tuning envelope in terms of ignition advance and plug gap, but the difference is negligible unless you're tuning solely for huge horsepower.
What it Boils Down to
So, platinum- and iridium-coated plugs with a copper core won't boost horsepower or fuel economy; they'll just last longer and perform more consistently. Performance fine-wire plugs can enhance efficiency by helping the flame front to initiate and spread a bit more quickly, which potentially means more fuel burned per compression event and less fuel wasted. Both platinum and iridium fine-wire plugs will outperform copper-tipped plugs, but the iridium's ability to withstand high temperatures allows for a thinner plug tip. In the real world, though, you're unlikely to see any measurable change in fuel economy using iridium instead of platinum plugs. Platinum will do everything iridium does outside of the most severe racing applications. Iridium can give engine builders a slight edge in very high-compression, turbocharged or supercharged engines, but this edge isn't likely to translate into increased fuel economy in a street engine.
- Racing Engine Builder's Handbook; Tom Monroe
- How to Build High-Performance Ignition Systems; Tom Ryden
- Comstock/Comstock/Getty Images