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Spark plug life

Spark plug life — when to change spark plugs?

Spark plug life depends on more than just the type of metals used in the electrodes. It also depends on the type of ignition system and engine design. For example, iridium spark plugs in a regular port-injected engine with a coil-over-plug ignition system can sometimes last up to 120,000 miles. But those same spark plugs in some turbo engines with direct injection may last only 30,000 miles.

What determines spark plug life?

Spark plug electrode material

A spark always jumps from the sharpest edge of the center electrode to the sharpest edge of the side electrode. As a spark plug wears, the sharp edges of the center and side electrodes erode and round off. How fast that erosion happens depends on the type of metal used, the type of ignition system and the cylinder pressures. A spark plug with a harder metal and a higher melting point will outlast a spark plug made with softer metal.

spark plug melting points

Copper spark plugs

It’s wrong to think that only older “copper spark plugs” use copper spark plugcopper because the vast majority of spark plugs contain a copper-core center electrode; even platinum and iridium plugs. Those precious metal plugs use the same copper-core center but weld on a platinum, iridium or yttrium tip to extend electrode life.

A traditional copper spark plug contains a copper-core center electrode with an outer layer of a nickel-alloy material. That design provides lower electrical resistance that precious metal plugs, so you lose less ignition coil energy. Lower electrical resistance gets you a hotter spark that results in more power. That’s a huge advantage in performance applications where spark plug life isn’t an issue. In addition, traditional copper spark plugs run cooler, further increasing performance.

Unfortunately, the nickel-alloy isn’t as hard as other metals and tends to wear or “erode” faster than precious metals. So the spark plug gap increases, causing misfires and fouling. This is why copper spark plugs must be replaced every 20,000 miles.

Platinum spark plugs

A platinum tip welded to a copper core provides longer spark platinum spark plugplug life simply because platinum is a harder than copper (4.5 versus 3) and has a higher melting point (4,435°F versus 1,984°F). Car makers switched to platinum spark plugs increase spark plug life in distributorless ignition systems (DIS). DIS ignition systems generate higher spark voltages and temperatures that are needed to ignite leaner fuel mixtures.

Single tipped platinum spark plugs use a copper-core center electrode with a platinum tip on the center electrode only. Car makers switched to platinum spark plugs in DIS systems due to the higher voltage and higher temperatures needed to ignite leaner fuel mixtures.

Double platinum-tipped spark plugs contain a small piece of platinum double platinum spark plugon the side electrode to reduce gap erosion in DIS systems where the waste spark jumps from the side electrode to the center electrode.

Yttrium spark plugs

Yttrium spark plugs were offered by Bosch in their earlier +4 spark plugs. Those plugs have four side electrodes. Four electrodes reduce the chance of misfire due to gap erosion. Yttirum is much cheaper than platinum but also has a lower melting point.

Iridium spark plugs

Iridium tipped spark plugs offer maximum spark plug life iridium spark plugdue to iridium’s hardness and higher melting point.

©, 2020 Rick Muscoplat

Posted on by Rick Muscoplat



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