Re: Err, plug wire resistance?

Legendre /
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Anthony,

Standard practice on motorcycle (and moped) engines is to use a soild copper-core plug wire, with a resistor cap (sometimes called a supressor cap). The resistance of the cap is almost always 5K. I've been doing some reading, and apparently the 1K figure is much more common on engines with magneto ignition.

The resistance is there to supress radio-frequency interference (RFI) generated by the ignition. It also has an effect on spark quality and plug life - it increases the duration of the spark, and reduces erosion of the plug's electrodes. This helps to keep the gap stable over the life of the plug.

In addition, the resistance of the cap works with the internal impedance of the ignition coil, to improve power transfer through the wire - if you view the plug wire as a transmission line (like for RF) the cap acts as a terminating resistance, which improves power transfer to the load (plug).

In high-energy systems (especially CDI systems) the resistance also protects the ignition coil and control module from burn-out, due to excessive current.

However, It does not affect the peak voltage of the ignition - that is a function of the coil's output voltage, and the spark plug gap.

Lastly, the automotive cable you were using really isn't suitable for a moped ignition. It might still spark, but you should notice some power loss, and less MPG of fuel. The core is constructed of graphite-impregnated fiberglass, in a plastic sheath, if I remember.

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