Spark Plug Wire Types

• When replacing spark plug wires, follow the manufacturer's specifications. Jupiterimages/Photos.com/Getty Images

  Each basic spark plug wire type has a different core, jacketing materials and method of preventing radio frequency interference (RFI) or electro-magnetic interference (EMI). Solid copper wires excel at conducting high voltage but are the worst for generating RFI. Silicone wires generate little interference but may wear out more quickly and may be unsuitable for non-high-performance engines. Graphite impregnated fiberglass core wires naturally suppress RFI with some emissions and durability issues. Do your best to follow your car manufacturer's specifications when replacing spark plug wires.

Distributed Resistance Wire

• Distributed resistance wire consists of a fiberglass core impregnated with latex graphite to suppress radio frequency interference. Also called carbon suppression wires, these wires are often used as original equipment by manufacturers. High-resistance carbon core wire is inexpensive to make and, with a fiberglass, Kevlar or combination substrate layer and silicone latex outer covering, suppresses RFI and EMI. This keeps on-board electronic systems from picking up interference from the spark plug wires. The tradeoff is that the high resistance reduces spark current and spark plug life. Distributed resistance wire also has some problems with long-term reliability and excessive emissions.

Inductance (Mag) Wire

• Inductance wire has a core of spiral wound copper/nickel alloy wire. The loops of wire at the core suppress RFI rather than using wire resistance to act as the suppressor. Because Mag wire doesn't have the resistance of distributed resistance wire, it doesn't need as much current to fire the spark plugs, which makes the spark plug wires last longer. Ironically, this "high performance" wire may actually, in most cases, reduce performance.

Fixed Resistor Wire

• Fixed resistor wire has a solid steel or copper metallic core. A fixed resistor is built into the plug boot to control radio frequency interference. Don't confuse fixed resistor wire with so-called high-performance capacitor effect wire.

High-Performance Wire

• High-performance wires are low resistance wires designed to deliver high voltage to fire the pistons. While solid core copper or aluminum and alloy wires may deliver powerful fire to race car engines, they don't suppress RFI well and can seriously interfere with a normal vehicle's under-the-hood electronics. Capacitor effect wire is surrounded by a grounded metal braid acting as one plate of a capacitor. The wire insulation acts as the other part of the capacitor dielectric. It is often sold as a high-performance spark plug wire, but like many other high-performance wires, it does not suppress RFI or EMI and can damage on-board electronics or computers in passenger automobiles.

Insulation

• Premium spark plug wires may use silicone or ethylene propylene diene monomer (EPDM) insulation. Other premium wires are jacketed in ethylene vinyl acetate (EVA), which improves strength and temperature resistance. Beneath the outside cover is a braided fiberglass layer to make the wiring stronger and more flexible. Another layer of EPDM insulation below the fiberglass helps prevent arcing and shorts. With Mag wires, the core is usually wrapped in silicone to improve the stability of the wire connections. Carbon core wires tend to be wrapped in Kevlar and/or silicone with a silicone latex outer layer.