systems have changed a great deal in recent years, with coil-on-plug
(COP) ignition systems being the most common setup on many late model
engines. COP ignition systems have a single coil for each spark plug
mounted on top of the plug. This setup eliminates the need for a spark
plug wire and the troubles it can cause. Most other distributorless
ignition systems (DIS) still have plug wires, as do older vehicles with
Plug wires are necessary to carry high voltage from
the ignition coil to the spark plug. We’re talking firing voltages that
can range from 5,000 volts to as much as 50,000 volts! That’s a lot of
electrical energy to keep contained inside a wire.
wires are vulnerable to heat, vibration, aging, moisture and physical
damage from mishandling during installation or removal. Issues include:
wires may burn through and short out if they lay against a hot exhaust
manifold (depending on the heat resistance of the outer jacket around
Unsupported wires can vibrate and rub against sharp edges, causing the insulation to wear through and short out the wire.
carbon conductor that carries voltage inside some types of wire can
degrade after years of use, causing an increase in resistance that may
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 plugs and coils or distributor cap can also
loosen or be damaged by vibration or improper handling.
different type of misfire called “crossfire” may occur if two plug
wires are routed parallel to each other and the two cylinders follow
each other in the firing order. The magnetic field around one wire
induces a current in the second wire, causing the other spark plug to
Misfires are bad news because it causes a loss
of performance and fuel economy, and a big jump in emissions. On late
model OBD II vehicles, it will also turn on the Check Engine light and
misfire codes. So if wire problems are causing misfires, it’s time to
replace the wire set.
Most late model original equipment spark
plug wire sets now use spiral-wound stainless steel mag wire. This type
of construction has less internal resistance than carbon-core wires
(only about 500 ohms/foot versus 5,000 ohms/foot with carbon-core
wire). It uses inductance rather than resistance to suppress radio
frequency interference (RFI). The result is a hotter spark with less
voltage load on the ignition system. Mag wire can be recommended as an
upgrade for older carbon core wires.
Some European imports use
Fixed Resistor plug wires, which have a steel or copper metallic core
with a fixed resistor in the plug boot to control RFI.
differences in plug wires include the thickness and type of insulation
around the wire. Wire sizes may be 7mm, 8mm or larger. Thicker is
typically better because it reduces the risk of voltage leaks. Wires
with thicker insulation are typically used on high-output electronic
ignition systems or on engines where the spark plugs have wider
Premium wire sets typically use silicone or EPDM
(Ethylene Propylene Diene Monomer) insulation around the core.
may be an inner layer of EPDM surrounded by an outer layer of silicone,
or a double layer of silicone around the core. EPDM has a high
dielectric rating that resists voltage leaks, and silicone can
withstand high under-hood temperatures and retain its flexibility
unlike some other materials that can melt or become hard and brittle as
The insulation around the core may also be surrounded by
additional reinforcement such as braided fiberglass, and there may be
an outer covering or jacket on the wire such as EVA (Ethylene Vinyl
Acetate), EPDM or silicone to provide additional thermal protection and
abrasion resistance. Economy wire sets typically use less expensive
insulation that may not be multi-layered or as thick, and does not
provide the durability and reliability of a premium wire set.