Second Generation Ascots > Tech Section

Spark Plug Wires... What To Do?

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J6G1Z:
Here is an informative reply that I received on the Yahoo VT Ascot Owners Group


The original purpose of resistance in the secondary circuit was to reduce radio interference. But there are other advantages.

1. “Radio” interference doesn’t only mean “car radio” interference. It really means “radio-frequency” interference, or, even better, “electro-magnetic” interference. This includes cell phones, GPS navigators, engine-control and ABS computers (which we don’t have, of course), pacemakers, etc. In other words, anything electronic that’s close by. The less the interference, the better these things work.

2. By putting some of the resistance in the secondary wiring, there is less power dissipated in the coils. That’s a good thing. On the Ascot, the coil secondary is about 25K ohms. The plug caps are about 5K ohms and the plug itself is about 1K ohms. So the total secondary circuit is 37K ohms. The power dissipated in the coil due to the secondary current is reduced by about 12/37, around 30%. With our coils, every little bit helps.

There’s really no performance penalty to including resistance in the circuit. Before the high voltage jumps across the plug gap, there’s no current flow, therefore no voltage drop across the resistance. So the full voltage appears across the plug gap. Once the plug fires, you don’t need a lot of voltage (because the gas in the cylinder is ionized and therefore conductive), so you don’t care about any voltage drop across the resistance.

The downsides are cost and reliability. But the vast majority of ignitions now use resistance in the secondary circuits, so the cost penalties have been minimized (in fact, it’s hard these days to even find non-resistor plugs). The reliability issue relates to the carbon-core wires used in cars, which won’t stand up to the vibration in motorcycles—that’s why most bikes use solid-core wires with separate resistors built into the plug caps.

HTH,
Neil

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