What’s the right spark plug?

The counterwoman at my favourite jobber hung the phone up and shook her head in disbelief. A technician had just blown out one of her eardrums because she sent a Japanese spark plug set for the GM vehicle he was servicing. He had requested AC plugs because they “work better in GM’s.”

I was speechless. I have no doubt there was a time when you matched the nationality of your spark plugs to the vehicle. I used to believe it myself. For you young guys, I used to break it down like this: AC’s in a GM car, Autolites in a Ford and Champions in Chryslers. For “Foreign” cars it was Bosch in European cars (except British, where I used Champions) and the Japanese stuff ran NGK or Nippondenso.

Who made these rules? I don’t know, but every tech, racer and rodder I knew used some form of these spark plug rules and handed them down to succeeding generations. And we were all wrong.

Here’s the reality about spark plugs: the electricity doesn’t know which brand of electrodes it’s jumping across any more than the engine knows what brand of oil is in the crankcase, or fuel in the tank. If it’s the correct reach, heat range, electrode construction and set to the right gap, it will fire the air/fuel mixture, regardless of brand.

The other argument was that by using the brand with which the OEM engineers designed the engine, you’re guaranteed to get good performance. That’s true, but these days spark plug manufacturing is a big business, and every major manufacturer has their own dynos and engineers, so it’s a level playing field. In many cases you can’t even tell who made the OEM plugs.

What about the “these-plugs-work-better-in-older-engines” argument? I’ve seen some plugs work better in old engines (oil burners with worn carbs especially) that used points and condenser ignitions, but how many of those have you seen in the last twenty years? If the ignition system can’t generate enough voltage to jump the factory gap, spark plugs are the least of your problems.

Does that mean that all spark plugs are the same? No, but it does mean that we have to choose based on considerations other than where the plug was manufactured. Electrode material matters. I like hard materials like platinum on both the centre and side electrodes if possible. Heat range is almost a non-consideration these days unless you’re racing, but reach can be an issue, especially if you’ve resurfaced a modern head with a small combustion chamber. I like a sacrificial coating or plating on the threads, mainly because I have better things to do than Helicoil cylinder heads, and a ribbed insulator is a must. Those straight sided specialty plugs look cool, but why invite flashover? Of course, the stuff that really matters, like ceramic composition and the insulator/shell seal, are both important and invisible to the naked eye.

Think it’s easy to keep combustion gasses from blowing by the insulator? I once spent big chunks of a year in a basement University of Toronto lab trying to get ceramics and metals to stick using a technique called transient liquid phase bonding and never got a really good joint. I got the credit, however, and a newfound appreciation for spark plugs. Got a brand you like? Great! Want to try something different? That’s fine too, but don’t try to convince me that there are engines that want a single plug over other quality brands. It’s the same electricity.