We have already had a thread on the dodgy Champion resistors - here.
The Q is what does the resistor actually do?
One theory is that it is there for interference suppression. The spark (any spark) is a powerful source of broadband radio frequency emission. The early radio pioneers (19th and early 20th century) used spark gaps to generate radio waves (they used an LC filter to pick off the frequency they wanted) as this was before even valves were invented.
I think the resistor forms a lowpass filter, together with the capacitance of the ignition lead, to reduce the amount of RF muck radiated by the engine.
If you don't have the resistor, the engine should run just fine but there will be powerful broadband RF muck going back up the ignition lead, all the way back to the magneto, and despite the cable being shielded this will get radiated all over the place and will screw up your VHF comms etc.
Another theory for the resistor is that it acts as a current limiter. The capacitance of the ignition lead acts as a reservoir, with a very low source impedance (i.e. capable of delivering a very high current). This capacitor gets charged to a few kV before the plug fires, and the resulting current (a few thousand amps) is going to cause rapid erosion of the spark gap.
I don't know which of these explanations (or both) is true, but one engine shop has reported that if they run an engine without the resistors in the plugs, their dyno instrument readouts get all screwed up.
The purpose is to reduce EMI, both conducted and radiated.
How they work is another question; I always thought the inductance of the coil and the resistor formed a simple L - R low pass filter
Reading the US AOPA mag on today’s nice day
EGKA 291420Z 18025G35KT 3000 RA SCT005 BKN007 18/18 Q1006
I see they think the resistor acts as a current limiter i.e. the 2nd theory in my initial post above.
I reckon the answer is “both”…
Or perhaps they are two sides of the same phenomenon because RF interference is worse if the radiating antenna current is higher, so by reducing the current you reduce the RFI also.
According to the literature I read, they are supposed to reduce wear on the electrodes and increase the longevity of the spark.
