Jonny, Clewett told me they preferred the Iridium plugs, and below some guidance from the Instructions:
3 Choosing Spark Plugs and Wires
1.3.a Spark Plug Wire Selection
The XDI outputs an extremely high-energy charge for the ignition coils. Resistor (carbon) core wires
work best with this charging method, since they absorb electrical noise generated by the coil firing events. Use
8mm or larger RFI and EMI suppression wire with GM boots. We recommend using a carbon core-style
suppression wire with a resistance of 3,000 to 5,000 ohms per foot. SOLID CORE WIRES SHOULD
NEVER BE USED. Do not be misled by spark plug wire manufacturers claiming to give you a “power
increase” from their wire. The bottom line is that with our charging method, different spark plug wires simply
do not make a difference in terms of spark energy. However, there is a huge difference in noise generated by
different spark plug wire types (solid core wires generate a very high amount of noise with our system).
Quoted from Magnecor’s Website:
“What is not generally understood (or is ignored) is that the potential 45,000 plus volts (with alternating
current characteristics) from the ignition coil does not flow through the entire length of fine wire used for a
spiral conductor like the 1 volt DC voltage from a test ohmmeter, but flows in a magnetic field surrounding the
outermost surface of the spiral windings (skin effect). The same skin effect applies equally to the same
pulsating flow of current passing through carbon and solid metal conductors. A spiral conductor with a low
electrical resistance measured by a 1 volt DC ohmmeter indicates, in reality, nothing other than less of the
expensive fine wire is used for the conductor windings!
Electrical devices, including spark plugs, use only the electrical energy necessary to perform the function for
which such devices are designed. Spark plug wires are nothing more than conductors, and whereas a bad
ignition wire's inefficient conductor can reduce the flow of electricity to the spark plug, an ignition wire that
reportedly generates an "increase" in spark energy will have no effect on the spark jumping across the spark
plug gap, since the energy consumed at the spark plug gap won't be any more than what is needed to jump
the gap. For a more obvious example of this, a 25watt light bulb won't use any more energy or produce any
more light if it's screwed into a socket wired for a 1000 watt bulb.”
Due to the extremely high energy in the XDI coil charging circuit, spark plug wires may wear out faster
than with a standard ignition. As such, it is recommended that the wires be checked periodically for carbon
tracking caused by a breakdown of the internal conductor element. Looking at the plug wires in a dark area and
wetting them with a spray bottle of water will reveal carbon tracking. Pay close attention to the exposed section
of the spark plug (where the rubber boot ends) during the test. To maximize spark plug wire life, keep the
lengths as short as possible (i.e. mount the DFU as close to the engine as possible). Replacement of the wires
on an annual basis is recommended for high-rpm/high-horsepower applications.
For an extremely high-quality wire with excellent noise suppression, we recommend the Magnecor
brand. Specifically, their “Electrosports 80” 8mm wire is very good with our system. Custom wire lengths and
ends are available from them so you will not need to crimp the wires yourself. They can be reached on the web
at:
www.magnecor.com. Taylor Pro-Wire Silicon Resistor wires also work well.
1.3.b Spark Plug Selection
As was previously stated, spark plugs are generally more important to spark quality than spark plug
wires. Most spark plugs exhibit failure when exposed to a large load. Failure usually consists of either
intermittent sparking or arc-over. Arc-over is when the spark occurs between the spark plug wire and the
engine block, instead of at the plug tip. Arc-over is exacerbated by the use of low-quality wires, or wires that
have cuts in the insulation.
The load at which a spark plug fails is different for all spark plugs. With the XDI’s charging circuit, the
more load you put on an engine, the more voltage will be applied to the plug. This is a beneficial situation: for
XDI Manual Version 1.7 Page 8 2015 Electromotive, Inc.
a high compression engine, the voltage at the plug will be inherently higher (since there is more load). The
detriment is that spark plugs and wires are only rated to a certain voltage (30-40,000 volts is typical), and can
begin to “blow out” at around 40,000 volts. If that voltage is exceeded by a large amount for a long enough
length of time, the spark plugs will either blow out, break down or arc to somewhere other than the electrode
(often through the insulator directly to the engine block).
The solution is to run smaller plug gaps on high-compression engines. This is perfectly acceptable with
our ignition charging method, since the high load of the cylinder pressure will allow the voltage to be quite high
at the electrode, but the small gap will keep the plug from seeing an over-voltage situation. Use the
recommendations below as a guideline for spark plug gaps:
Stock Street Engine 0.045”-0.060” (1.1mm-1.5mm)
High Performance Street 0.030”-0.035” (.75mm-.9mm)
Alcohol High Compression 0.025” (0.65mm)
High Power 75 -115 HP per Cylinder 0.025” (0.65mm)
Over 115 HP per Cylinder 0.022” (0.55mm)
Over 12:1 CR or Over 14psi Boost 0.022” (0.55mm)
Use of resistor plugs is highly recommended for optimum noise suppression. If using anything other
than a resistor spark plug wire, a resistor plug MUST be used. The bottom line is this: the XDI system uses an
inductive (long duration charge at battery voltage) charging method for the coils, which is completely different
than the capacitive (short duration charge at higher-than-battery voltage) charging method used by several other
aftermarket manufacturers. What may work well for these systems may not work well for ours. Following our
recommendations about spark plug and wire selections will yield excellent results.
2.0 Engine Conf
