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Ignitionsystems have changed a great deal in recent years, with coil-on-plug(COP) ignition systems being the most common setup on many late modelengines. COP ignition systems have a single coil for each spark plugmounted on top of the plug. This setup eliminates the need for a sparkplug wire and the troubles it can cause. Most other distributorlessignition systems (DIS) still have plug wires, as do older engines withdistributors.

Plug wires are necessary to carry high voltage from the ignition coilto the spark plug. We’re talking firing voltages that can range from5,000 volts to as much as 50,000 volts! That’s a lot of electricalenergy to keep contained inside a wire.

Spark plug wires are vulnerable to heat, vibration, aging, moisture andphysical damage from mishandling during installation or removal. Issuesinclude:

• Plug wires may burn through and short out if they lay against a hotexhaust manifold (depending on the heat resistance of the outer jacketaround the wire).

• Unsupported wires can vibrate and rub against sharp edges, causing the insulation to wear through and short out the wire.

• The carbon conductor that carries voltage inside some types of wirecan degrade after years of use, causing an increase in resistance thatmay cause misfires.

• Boots that seal the ends of the wires can be deformed or pulled loose,allowing moisture to sap away voltage before it reaches the spark plug.

• The metal clips that attach the ends of the wires to the spark plugsand coils or distributor cap can also loosen or be damaged by vibrationor improper handling.

• A different type of misfire called “crossfire” may occur if two plugwires are routed parallel to each other and the two cylinders followeach other in the firing order. The magnetic field around one wireinduces a current in the second wire, causing the other spark plug tofire prematurely.

Misfires are bad news because it causes a loss of performance and fueleconomy, and a big jump in emissions. On late model OBD II vehicles, itwill also turn on the Check Engine light and misfire codes. So if wireproblems are causing misfires, it’s time to replace the wire set.

Most late model original equipment spark plug wire sets now usespiral-wound stainless steel mag wire. This type of construction hasless internal resistance than carbon-core wires (only about 500ohms/foot versus 5,000 ohms/foot with carbon-core wire). It usesinductance rather than resistance to suppress radio frequencyinterference (RFI). The result is a hotter spark with less voltage loadon the ignition system. Mag wire can be recommended as an upgrade forolder carbon core wires.

Some European imports use Fixed Resistor plug wires, which have a steelor copper metallic core with a fixed resistor in the plug boot tocontrol RFI.

Other differences in plug wires include the thickness and type ofinsulation around the wire. Wire sizes may be 7mm, 8mm or larger.Thicker is typically better because it reduces the risk of voltageleaks. Wires with thicker insulation are typically used on high-outputelectronic ignition systems or on engines where the spark plugs havewider electrode gaps.

Premium wire sets typically use silicone or EPDM (Ethylene PropyleneDiene Monomer) insulation around the core. There may be an inner layerof EPDM surrounded by an outer layer of silicone, or a double layer ofsilicone around the core. EPDM has a high dielectric rating thatresists voltage leaks, and silicone can withstand high under-hoodtemperatures and retain its flexibility unlike some other materialsthat can melt or become hard and brittle as they age.

The insulation around the core may also be surrounded by additionalreinforcement such as braided fiberglass, and there may be an outercovering or jacket on the wire such as EVA (Ethylene Vinyl Acetate),EPDM or silicone to provide additional thermal protection and abrasionresistance. Economy wire sets typically use less expensive insulationthat may not be multi-layered or as thick, and does not provide thedurability and reliability of a premium wire set. 

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