The ignition wire replacement market isn’t what it used to be, but it still an important segment. Distributors disappeared from engines many years ago and Coil-On-Plug (COP) ignition systems are used on most late-model engines. Even so, there are still a lot of older 1960s, ‘70s and ‘80s vintage cars and pickup trucks on the road with distributor ignitions and spark plug wires. The biggest segment of the market is the huge number of 1990s and 2000s era vehicles with Distributorless Ignition Systems (DIS) that have coil packs with replaceable ignition wires. As these vehicles continue to age, many will need to have their original equipment ignition wires replaced.
Most late-model original equipment ignition wire sets are designed to last 100,000 miles or more. Some do and some don’t. The wires should be inspected when the plugs are changed, or when there’s a misfire problem.
As plug wires age, resistance can increase as the conductive core becomes less conductive. The voltage needed to fire the spark plugs goes up and eventually the engine may start to misfire. The protective insulation around plug wires also can burn, chafe and crack, causing shorts and misfires. The same thing can happen to the plug boots. Loose end terminals also can make for poor connections and misfires.
On 1996 and newer vehicles, the OBD II system will usually detect ignition misfire, set one or more trouble codes and turn on the Check Engine light. An engine with a misfire problem will not pass an emissions test, so the problem has to be fixed to pass the test.
Aftermarket spark plug wire sets are available in a range of styles and prices. The Society of Automotive Engineers (SAE) has standards (J2031) that establish minimum performance criteria for different classes of ignition wires. Class C wires only have to withstand temperatures of 311 degrees F. Class D wire sets must be rated to 356 degrees F, while Class E carries the highest temperature resistance of 428 degrees F. Higher temperature ratings require better (and more expensive) insulation, so most premium wire sets are made with better, more heat-resistant materials, such as silicone. Pure silicone insulation can withstand up to 600 degrees F.
The design and quality of ignition wires also varies. Many late-model engines use “mag” wire, where stainless steel wire is wound around a conductive layer and a Kevlar reinforced core. Mag wire has less resistance than carbon-core suppression wires (500 to 750 ohms versus 5,000 ohms), which reduces the voltage required to fire the spark plugs. Many original equipment and premium aftermarket mag wire sets have 125 turns of wire per inch versus as few as 25 turns of wire per inch for some “economy” wire sets. The cheaper wires many not provide the same Radio Frequency Interference (RFI) suppression as better wire sets, which may lead to radio static and possibly interference with some sensor signals.
Some customers may not want to buy a higher-priced premium wire set if their engine already has more than six digits on the odometer. Even so, if they are installing 100,000 mile platinum or iridium spark plugs in their engine, they also should be installing plug wires that will last as long as the spark plugs.