If you drive a late model vehicle that has a distributorless ignition system (DIS), engineers at Autolite Spark Plugs say you should NOT use ordinary copper core spark plugs or "single" platinum spark plugs on these applications. NOTE: This applies only to engines that have a "waste spark" DIS ignition system, not engines that have a separate ignition coil for each cylinder and spark plugs, or engines that have a distributor.
A waste spark DIS ignition uses one ignition coil for every two cylinders. Each coil fires once every revolution of the crankshaft, and sends a spark to two opposite cylinders simultaneously. It is called a "waste spark" system because only one spark ignites the air/fuel mixture (in the cylinder that is on its compression stroke). The other spark occurs in a cylinder that is opposite the first cylinder in its firing order, and does nothing because the spark occurs on that cylinder's exhaust stroke.
You can tell if your engine has a waste spark DIS ignition system by counting the number of ignition coils, then comparing the number of coils to the number of cylinders in the engine. For example, if your engine has 3 coils and is a V6, the coils are shared and it is a waste spark system. If there are six coils, it is not a waste spark system.
Though many technicians and do-it-yourselfers believe that copper core plugs can be used on waste spark DIS ignition systems, Autolite says that is not true. Ordinary copper spark plugs may function well initially, but the electrodes will wear prematurely and eventually cause ignition misfire.
Unlike conventional ignition systems, waste spark DIS ignition systems use half the spark to fire the opposite spark plug unconventionally from the side electrode to the center electrode. Copper core standard plugs and copper core single platinum spark plugs are not designed to withstand this reverse polarity firing and will suffer premature gap growth due to center electrode erosion. Gap growth will stress ignition system components by requiring more voltage to fire, eventually leading to misfire, loss of performance and fuel economy. This degradation can be noticeable as soon as 20,000 miles after the plug is installed. NOTE: Ignition misfire can cause the Check Engine light to come on.
Many motorists do not realize that very few new vehicles still use copper core spark plugs, and those that do such as the Chrysler 5.7 Hemi V8, specify a 30,000-mile spark plug change interval.
That is why installing a premium spark plug with platinum on both electrodes or an iridium electrode is recommended for waste spark DIS engine applications. Double Platinum plugs and iridium plugs are specifically designed for engines with DIS systems and are 30 percent more durable than the average spark plug. The platinum-to-platinum firing offers virtually no gap erosion and performance that lasts for up to 100,000 miles.