BEST SPARK PLUG WIRES

[Fred Winterburn]

I realized I misread the article. The author was not comparing the distributor air gap to the plug gap in the sense that I thought when I first read it. And he is quite right that if the plug gap is doubled in size, the voltage requirement is also doubled. However, there is a crucial piece of information missing, and that is the voltage overshoot. All ignition systems have a voltage overshoot beyond the actual breakdown voltage requirement of the spark gap. This is due to the fast rising voltage and the finite time it takes to sufficiently ionize the gap (corona discharge) prior to the actual breakdown. So, if an inductive ignition has a peak voltage that is 1.5 times (a realistic number by my testing) the gap threshold voltage at a 30 thou gap it will also have a peak voltage 1.5 times the gap voltage at 60 thou. So if it takes 20 kV nominally to break down the gap (supposing the voltage and current is brought up slowly) and the peak voltage is actually 1.5 times that, the actual voltage stress on the wires and other HT insulation will be 30kV. At a 60 thou gap due to the voltage overshoot, the voltage stress will be 60kV, not 40 kV as expected. Also, a high energy inductive ignition has a lower voltage overshoot than does a weak inductive ignition, so a high energy ignition is kinder to HT insulation at least as far as peak voltage is concerned. Fred

-More erroneous information in the post is that the distributor gap cannot be compared directly with the spark plug gap as far as voltage is concerned. The plug gap requires a much higher voltage due to compression and other factors while the distributor gap requires a much lower voltage unless the rotor is trimmed (as some used to do way back with weak ignitions and narrow plug gaps to help with fouling). In addition the statement that spiral wound suppression wire only adds extra resistance due to the length of the wire without contributing anything is also false. It is an inductive suppressor. A method that works and has been proven to work, including spark plugs with inductive suppressors. Champion Q series for example mostly used in outboard motors with CDI. I actually run marine inductive suppressor NGK plugs in my Volvo 1800 because they fit and work well with the CDI. Very robust.
-Another point of contention with the post I have is the statement that HT wire resistance can be as high as 15 kohm because the spark gap is in the order of Mohms and therefore 15 kohms is low by comparison. That comparison is as incorrect as apples and oranges. The spark gap, before breakdown is essentially a leaky capacitor which behaves more and more like a resistor as the gap breaks down, to the point where there is almost no resistance across the gap when fully ionized and the spark well underway. Once that happens (and to a certain extent even before breakdown) the series resistance certainly does matter and limits ignition energy proportionately to the resistance. Fred

Not sure where this write-up below comes from, but secondary side resistance matters just as much with an inductive ignition as it does with CDI. The physics of energy transfer remain the same in that regard. I recommend keeping the total secondary side resistance less than 10 kohms ( wire resistance, plug resistance, rotor resistance etc) but excluding coil secondary winding resistance (which is designed in). Any more than 10 kohms more than the secondary winding resistance, even with a powerful ignition system, inductive or CDI, is detrimental to spark energy and duration. Fred

Spark Plug Interference
Control begins with the elimination of the interference sources whenever
possible. Lacking prevention, there are several methods of interference suppression:
ohmic resistance, capacitive reactance, inductive reactance and screening.

Interference suppression using Resistors
Suppression resistors are installed in series, as near as possible to the source of
interference, in order to dampen oscillating voltages and reduce interference energy.
Their secondary purpose is to prevent spark plug wires from becoming antennas
which broadcast interference frequencies.

Interference suppression resistors can be used in circuits with high voltage (and
amperage) such as spark plugs, ignition wires, coils and distributors. They cannot
be used in low voltage systems since their use would cause an excessive voltage
drop and power loss.

Spark plugs
The arc produced when a spark plug fires is the primary source of interference signals.
As the spark arc terminates, unused electrical energy resonates or oscillates at a high
voltage and frequency. These high voltage oscillations backfeed through the wires and
distributor into the coil, broadcasting unwanted noise signals as they go. Spark plugs
are available with internal resistance to help dampen undesirable voltage oscillations.

Distributor caps and rotors
The air gap between the rotor and distributor cap post produces an electrical arc (and
voltage fluctuations) similar to a spark plug. High frequency oscillations will backfeed
into the coil wire and into the coil. This unwanted voltage ringing can be dampened
by use of a rotor with built-in resistance or a resistor type distributor cap.

Spark plug connectors
Similar to spark plugs, spark plug connectors are available with internal resistance,
typically 1K ohm or 5K ohm, to help dampen high voltage oscillations or ringing.
They are particularly useful when resistor type spark plugs are not available in the
correct type and heat range.

Some spark plug connectors are available with metal jackets to act as shields that,
when grounded, dampen the broadcasting of interference frequencies.

Spark plug wires
Since a spark plug wire (or coil wire) will act as a broadcast antenna, it is necessary to
minimize voltage oscillations in the wires which propagate circuit interference. Various
wire designs have been used to dampen voltage fluctuations, including spiral wire
wound conductors and distributed resistance graphite impregnated wires.

Spiral windings do very little more than increase the effective length of the wire and
therefore its resistance value.

Distributed resistance wires also increase resistance by use of a conductor material
with higher resistance. They are often prone to internal breakage due to heat and
vibration, causing multiple arcs and producing their own interference signals. These
small breaks are often intermittent in nature and very difficult to find.

Neither solution is satisfactory for racing and should be removed or not installed.
Multi-stranded copper wire conductors with high dielectric resistance insulation
should be used instead. They are less likely to break internally and total resistance
values can be controlled more accurately.

For inductive type ignitions (either breaker or transistor triggered) total ignition
path resistance should not exceed 15K ohm to prevent loss of spark intensity.

The resistance value of air gaps in the spark plug and distributor cap is on the order
of megaohms, so while resistor suppression of 15K ohm sounds like a high value,
it is of little consequence to firing voltage.

Not so for capacitive discharge ignitions. For CDI type ignitions, the ignition path
resistance should be as low as possible to prevent intolerable current reduction
and loss of spark energy. Therefore, noise suppression by use of resistors is not
possible and any attempt to do so will result in a loss of performance.

Shielded cables
Shielded cables are used to reduce the interference from electical noise. Some electrical
data connections require the use of shielded cables to reduce the noise to signal ratio.
Failure to use the proper type of shielded cable can result in erratic data readings from
the sensor instrumentation.

There are various types of shielded cable available for different applications.

Foil shield cables consist of aluminum foil laminated to a polyester or polypropylene
film. The poly film provides mechanical strength and some additional insulation. The
foil wraps the entire signal wire with no air spaces and provides 100% cable coverage
for electrostatic protection. Foil shields are often used for protection against capacitive
coupling where shielded coverage is more important than low DC resistance.

Braided shield cables consist of groups of tinned or bare, copper or aluminum strands.
One set is woven in a clockwise direction, and interwoven with another set in the opposite
direction.The air spaces between the braided wires allow some penetration of noise
signals. Braided shields provide superior performance against diffusion coupling where
low DC resistance is important, and to a lesser extent, capacitive and inductive coupling.

Spiral shield cables consist of copper wire (usually) wrapped in a spiral around the
inner cable core. The spiral shield is used for functional shielding against diffusion
and capacitive coupling at audio frequencies only.

Combination shield cables consist of more than one layer of shielding. The combination
shield is used to shield against high frequency radiated emissions coupling and
electrostatic discharge. It combines the low resistance of braid with 100% coverage of
foil shields and is one of the more commonly used types of shielded cable.

To be effective, the shielded material must be grounded properly in order to transmit
noise signals to ground and prevent interference with the measured signal.

So what size spark plug wire should be used, 7mm, 8mm, 8.5mm?
The advertised wire dimension refers to the total outside diameter of the wire
but does not indicate the size of the electrical conductor wire inside, nor does it indicate
wire ohmic resistance.

Two methods to determine ohmic resistance are; direct measurement of the wire
without connectors using an ohm meter, and measurement of wire strand size and
number of strands. Once the actual conductor equivalent diameter is known, along
with the material, we can calculate the resistance per foot of wire.

For example, a 19 strand copper wire of .011" diameter is equal to a conductor of
0.0364" and will result in a 30" wire having a resistance of 0.020 ohms. If we add
5 kOhm resistor connectors at each end, we will have a wire with 10,000.020 ohms
resistance. The value to the right of the decimal representing the wire resistance
and to the left, the connector resistor contribution.

Now consider that the air gap at the distributor cap and spark plug gap have a
resistance of a few megaohms. The distributor air gap is usually not adjustable,
but the spark plug gap certainly is. If we consider a spark plug gap of .028"
compared to a gap of 0.060", we see that the required firing voltage will
double; 13 kV to 26 kV, depending on gap air temperature and pressure.

Years ago, every gas station had a scope to examine ignition systems;
now few shops have one. But, a USB scope is both cheap and easy to use and
reveals an incredible amount of very useful information.