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.
Whatever the resistors actually do, they introduce a failure mechanism into the spark plug.
Out of an article from 2012:
“Completely unrelated to the problem of cracked core nose insulators in Champion fine-wire plugs, there’s another potentially serious issue with all Champion aviation spark plugs, both fine-wire and massive-electrode. It has to do what appears to be a failure-prone design of the internal resistor inside the spark plug.
All aviation spark plugs have such a built-in resistor. Its purpose is to limit the current that passes through the plug when the magneto fires it. The value of the resistance is fairly critical. If the resistance is too low, the spark plug electrodes will erode rapidly. If the resistance is too high, the spark will be weak and ignition performance will be marginal, particularly in demanding situations like engine starting. If the resistance is high enough, the plug will fire erratically and there’s a serious threat of crossfire inside the magneto distributor block, particularly with Slick magnetos."
Indeed; we had some threads on the Champion issue – see “Threads possibly related to this one” below.
However, Champion’s defence, which was partly fair, was that measuring the resistance with a DVM, at maybe 3V or some such, was meaningless. They said their resistor was made of some silicon carbide compound and did not function as a resistor until the voltage across it was very high (1kV or more).
Champion later moved to the same fused construction as Tempest and hoped their reputational issue will go away (maybe it did but at some 20-30% higher price, why use Champion over Tempest?). And now (see the link in my post above) I see Tempest resistors can behave similarly to the old Champion ones, since I have four plugs (was three) all measuring infinity with a DVM but only one of those was actually not igniting. However, two of the infinity ones were showing a sign of flashover on the insulator.
I’ve done more tests on my plugs, first with a 1000V “megger” tester and second with a 0-4500V flash tester we have at work. The one plug which was dead in the engine was also dead with both of these two tests. Even at 4500V (AC RMS) there was no path in it. The others all had conduction at 1000V. For the three plugs showing infinity on the DVM I got the following resistance values at 1000V:
1 300k
2 5M
3 200k
All three did in fact function in the engine. Probably #2 was flashing right over the resistor.
It’s funny to read these threads 10-13 years later. I was telling everybody that they should be checking the resistors, but for some reason stopped doing it myself, and eventually got this hassle. It appears that nobody else is checking the resistors either. It’s like oil analysis; only a few people do it. But checking the resistance is essential because it gives you an early warning of problems.
I check the resistors when I have the bottom plugs out – takes no time at all during compression checks. The top plugs are automotive and cost £3 each so I just replace them.
Interestingly when I got the plane it had the old style resistor Champions which were 20 years and 650 hrs old yet they all tested fine. I swapped them with tempest plugs.
It is very likely that 20 years ago Champion were using a different process. This trouble is much more recent.
My understanding was that the older style had a removable resistor which was problematic. If you look down the hole where the spark plug wire connects, you’ll see that the contact has a slotted head on older Champion plugs. This is actually a grub screw which allows you to remove the little resistor. Newer ones don’t have this screw, like the Tempests.
I’ve disposed of my champions with the removable resistor or I’d have taken photos
