This is a bit of a mystery which has popped up on a few US forums...
Cirrus (turbo) owners have been reporting a high failure rate (due to cracked insulators) of fine wire plugs, especially. One example is here.
Somehow (I cannot find the reference) people have decided the resistance should be about 3k. Support for that may be e.g. this. The traditional manufacturer of these plugs, Champion, does not publish a value.
A new mfg, Tempest, does, and is making the best of Champion's apparent misfortune.
My fine wire plugs (RHM38S) are shorter thread than the Cirrus ones and I have not had any failures, but I have just measured about a dozen of some 700hr ones and all but 1 read open circuit (>20Mohms) with a normal DVM, with 1 reading a few hundred k. These plugs were installed in 2003 and removed in 2008.
I also had one heavy electrode plug which read a few k.
But I've just re-measured all the fine wire plugs with a 1000V insulation tester and except one (which remains open) they read completely differently. None were over 1M, and this was true at 250V, 500V and 1000V, though the three voltages produced slightly different resistance values.
So clearly the "resistor" is not a resistor as we know them. Whether this is intentional, who knows? I suspect it's a cockup. Maybe there is a tiny air gap at the ends, of the order of 0.01mm, which flashes over easily - the result of expansion and contraction of the plug assembly. But this is clearly not intended.
It could explain all kinds of weird engine behaviour in the field, because all these plugs test OK in a spark plug tester.
The plugs I tested were c. 10 years old. So unless there is some good explanation for this, it appears that Champion were making rubbish spark plugs for perhaps a decade or more, getting away with it only because a magneto puts out enough kilovolts to flash over the resistor, or flash over any air gap at the ends of it. And fine wire plugs are ~ $100 each.
I should add that the engine worked apparently fine, even with the plug which was open circuit at 1000V. No high altitude issues either.
Many years ago I used to work in high voltage (up to 100kV) and one quickly learns that any air gap leads to a rapid degradation of the component. Voltage distributes itself inversely proportionally to the relative permittivity of the material and any air pockets end up with a very high potential gradient across them. Having a "loose" resistor inside a spark plug is a stupid idea.
I need to find some unused Champion fine wire plugs plugs and test those... any volunteers?
I know very little about this subject but talking to the man from Tempest at Sun n Fun recently he said that any massive plug which showed a resistance of more than 5k ohm should be regarded as not fit for purpose. It might produce a spark at sea level but was likely to fail at altitude.
Being naive - does the resistance change with the engine compression; as in the real case? The plug tester I use certainly shows a better spark as the pressure is increased. Which seems contrary to logic, as if it were a vacuum the spark would 'jump' better!!!
The two resistors above are from a) the plug which was totally O/C at 1kV and b) from the plug which was the lowest resistance. One of them shows a little bit of burning at the ends.
BTW the resistor is described as "silicon carbide" here, page 3 here
A google on silicon carbide resistors reveals these are not normal resistors, e.g. here
Resistors which pass a current proportional to the fourth or fifth power of the applied voltage are now widely used in many fields of electrical engineering. The first part of the paper discusses the behaviour of these resistors in terms of the characteristics of single contacts between silicon-carbide crystals. In the second part, the construction and properties of the resistors are described, and some typical examples are given of their uses for protection on transmission lines, in d.c. inductive circuits, in radio, for spark quenching at relay contacts, for voltage regulation, for field control of electrical machines and for various other purposes.
It's no wonder that these don't measure as anything useful at low voltages, but one would expect more consistency...
However it is quite possible that these resistors do basically nothing useful at sub-kilovolt voltages, so measuring them as described may well be a complete waste of time.
And the turbo Cirrus failures have to be a separate issue anyway; there is no way the cracked insulators can be anything to do with the resistors.
The now quite old thread on the Beech owners group is now into 26 pages... but nobody seems any wiser why the Champion fine wire plug resistors have such a high resistance that they should not be working, yet (in most cases) they are working.
Champion are in the meantime keeping silent.
The way this is panning out, nobody should touch Champion fine wire plugs with a bargepole - even though most people find them working fine.
The slightly damaged one "felt" harder to mill. Unfortunately I can't now tell you whether it was the low-Z one or the hi-Z one
But the texture looks identical, and there is no obvious internal damage in either.
So how could they be so very different in their behaviour? Could of course be the composition, or it could be the hi-Z one had microscopic cracks in it.
You can see the thin nonconductive coating on the outside, which explains why one cannot get a resistance measurement along the resistor.
Tomorrow I am going to pull out the top 6 plugs which have been in 2008-present and measure them at both low volts and with the insulation tester. With 6 years between the two purchases, the result will be interesting...
I've pulled out the six top plugs from the engine and measured them. They all measure 15-25k. I have no reason to think the six bottom ones will be any different and since I swap them all round every 40 hrs or so, it would be statistically astonishing if they were. These plugs were fitted in 2008 and have done ~400 hrs. Note the previous set, all ~1M, had done 700hrs.
So we have these possibilities:
1. The resistors in these six were always 15-25k and never changed.
2. The resistors in these six were say 3k and they degrade gradually, taking a jump from a few k to 15-25k before 400hrs, and then a big jump to say 1M between 400hrs and 700hrs
3. Champion changed the resistor mfg process between 2002 and 2008
4. Some combination of the above
My money is on 3. definitely, and 1. possibly.
I numbered the plugs and will recheck them at the next check, in ~20hrs' time. That WILL be interesting.
I didn't bother testing them all with the 250/500/1000V insulation tester because values like 20k just read as close to zero.
It would be really interesting to get a brand new RHM38S and measure it. But anybody who does that is going to blow away ~£95 (Transair price) because they come in sealed packages
As an update, I have ordered Tempest URHM38S and will chuck out the Champions at the next service. The Champions are obviously all defective, with resistances several times bigger than they should be. They still work (have just flown several hours at FL200) but clearly are on their way out.
Tempest did have some manufacturing problems with these but they are now solved. I got a good detailed response from the chief exec there which is very reassuring.
I have just done a 50hr check and took out the Champion RHM38Ss and put in the Tempests.
The resistance on the Champions (15-25k) did not significantly change since they were measured 37 hours ago (airborne time). This to me suggests that they are as Champion made them, and it suggests that the "megohm"-resistance plugs from 2002 were actually made that way, too.
The Tempest plugs were all around 1.5-2k and I have noted the resistances and will keep track of them at services.
Some funny stuff going on here, and clearly Champion have been having massive long term QA issues but got away with them because most magnetos are powerful enough to flash through the faulty resistors anyway.
what makes you believe that the low voltage resistance of your spark plugs would actually degrade their performance?
The Champions have a spring inside for contact and I assume that oxide buildup on the metal is responsible for the resistance. In real life, the high voltage sparks should easily bridge that resistance without any significant loss.
I believe only a spark plug tester should be used to judge the quality of a spark plug.