Technical Topics No. 20

The classic spark plug design has a copper centre electrode surrounded by nickel alloy. The spark travels between the centre electrode and the bent piece of steel called the ground electrode. Decades ago resistor plugs were introduced with a resistor in the metal core to prevent voltage surges that cause radio frequency interference (RFI). Resistor plugs usually had an “R” in their part number. The “R” has now nearly disappeared because almost all plugs and ignition systems for late model cars include RFI protection.

A “hot” plug has a ceramic insulator designed to retain heat and keep the centre electrode hotter. Hotter electrodes may be less likely to foul with contaminates like oil and carbon, but if they are too hot then contaminates might melt rather than burning off or the gasoline in the cylinder might ignite too soon causing engine knock. Guessing which hotter plug might make a car run better is iffy with antique cars and a bad idea with modern engines. Spark plugs for newer engines usually have a projected core nose that extends beyond the threaded metal base (see photos). The exposed insulator and electrode makes the plug warm up faster and run hotter at low engine speeds but lets the gasoline and air mixture keep the plug from overheating at high engine speeds.

     

Traditional copper plugs work well. The electrodes wear away relatively quickly, but that might not matter if the spark plugs are replaced every 30,000 miles or so. Longer lasting plugs were needed when transverse mounted V6 engines became common. The spark plugs on the firewall side of a V6 can be hard to reach. Another push for longer lasting plugs came from distributorless ignition designs that fire the spark plug on the piston’s exhaust stroke. This wasted spark means the plug fires twice as often and might last half as long.

Platinum spark plugs came to the rescue and let car manufacturers advertise 100,000-mile spark plug replacement intervals. A platinum plug can last that long but might be fused to the engine. Periodically removing, inspecting and putting an anti-seize compound on the threads of spark plugs is still a good idea. The platinum plug has a tiny bit of platinum alloy on the tip of the centre electrode. The hard, corrosion resistant platinum makes the electrode and the gap between the electrodes last longer.

Platinum plugs breathed new life into the business of spark plugs. Platinum plugs not only lasted longer, but their spark sometimes made cars start easier and run more consistently. Old cars benefited from platinum plugs. I have nothing but good things to say about the platinum plugs I have installed in my old cars.

The push for longer life and better performance led to a spark plugs arms race between ACDelco, Autolite, Bosch, Champion, Denso, NGK, and other plug manufacturers. Longer life meant using more platinum or even more exotic metals. “Double platinum” plugs have platinum on both the centre and ground electrodes. The iridium on “iridium spark plug” electrodes is six times stronger and has a higher melting point than platinum. Only the spark plug manufacturers know the secrets behind some of their metallurgy. The Denso Twin-Tip plug has “titanium enhanced alloy on the ground electrode.” Titanium is strong, extremely corrosion resistant and light. The Bosch Platinum IR Fusion has an “iridium and platinum centre electrode” and “yttrium enhanced ground electrodes.” Yttrium strengthens metal alloys and is used in super conductors.

The quest for plugs that can claim a stronger, faster, hotter, etc. spark is equally interesting. One technique is to add additional ground electrodes. The Bosch IR Fusion has four. Another idea is to change the shape of the surface of the electrodes into a U, a V, a sharp point, etc. NGK Iridium plugs have a fine wire tip that “opens up the area for flame expansion by reducing the mass of the electrodes.”

Late model car engines often have an ignition coil on every spark plug and an engine computer programmed to expect the spark plug to fire only when a specific voltage has been applied to it. The computer also expects the spark flame to be in a certain heat range with a specific duration. A spark plug that fires at lower or higher voltage than what the engine computer expects might not always enhance performance. Multiple electrodes, energy storing capacitors and other modifications should really be designed specifically for your engine by a plug manufacturer you trust.

It is good news that car manufacturers and owners now have so many great spark plug choices. For many engines, great platinum plugs are now sometimes only around $1 more than good copper plugs.