Conducting ignition voltage from the coil to the spark plug

An ignition cable is actually a very simple part, one would think. This was certainly true in the first half of the 20th century. A metal core of twisted copper wires with thick insulation around it, done. Either soldered to the ignition coil or connected with a mandrel or a self-tapping screw, attached to the spark plug with a knurled nut on the central M4 thread. Strange, actually, that spark plug manufacturers agreed on a uniform thread pretty early on ... However, it didn't stop there. And who was to blame? The radios of the neighbors.

What is electromagnetic compatibility? Why are there ignition cables made of carbon fiber?

Ignition coils are transformers in which magnetic fields build up and dissipate. High voltage is "transported" in the ignition cables. This also generates magnetic fields. Strong magnetic fields can affect electrical and electronic devices in the immediate vicinity. There is e.g. the story of our colleague Stefko, who crashed the computer at the performance test bench of a dealer with five unprotected ignition cables (yes, double ignition at the Shovelhead and House-of-Horsepower racing ignition distributor, that makes a total of five sparking ignition cables). Electromagnetic compatibility, or EMC for short, ensures that this doesn't happen all the time when a Harley engine is roaring somewhere.

EMC is achieved by shielding the ignition cables or by using special materials that replace the simple ignition cable core. For example, carbon fiber. These ignition cables then have a specific electrical resistance. In addition, spark plugs with built-in resistors are used.

You have so many ignition cables to choose from! How do I find the right one?

You go by the model allocation, which is indicated in the store. This is always a reliable guide. The restorer and hobbyist goes by the requirements of the ignition. Tip: for electronic ignitions, take ignition cables with resistance and shielding: carbon fiber core (graphite), Spiro-Pro, metal jacket, etc. For old style contact ignitions both graphite or copper core cables will work. If you have a magneto on your bike, choose copper cable in any case. With the neighbor you can then schedule a clarifying conversation about the radio with a well cooled Wrecking Brew.