Massive or Fine Wire Spark Plugs

Have a look here

This has been all over the internet for a while now.

It appears clear that Champion have had massive QA issues on their fine wire (“iridium”) plugs, and have more or less got away with it only because Bendix mags have so much spare power that they just flash right over the dodgy resistors. Any damage to the mag or the ignition harness happens after some time; too late to blame Champion Slick mags have not fared so well, apparently.

I switched to Tempest plugs (switching from Champion RHS38S to Tempest URHM38S) a couple of years ago and the Tempest plugs have been totally fine. I have been checking the resistor values and they are solid.

The Champion massive electrode plugs appear to be OK, but that may just be a coincidence in the way the batches were made up at the time anybody was looking into it.

I would not touch Champion plugs with a bargepole. Life is too short to take risks with components which are proven to be dodgy.

As to massive v. fine wire (say, both Tempest), I could not say there is a difference in the way the engine runs. But then I’ve been using fine wire since nearly the beginning, 12 years ago. If one was to collect up “internet wisdom” from US forums, the consensus seems to be that fine wire plugs have better performance, possibly, but never worse.

Some claim fine wire are less likely to get shorted by lead deposits and I think that’s true. I have had just two shorts (always a lower plug so hard to get at ), in 2003 and 2005. Most people would have departed with them…. I know a twin pilot who gets shorts all the time and he always departs with it, and it clears immediately during flight.

The fine wire plugs definitely last much longer. At the 1000hr point, there is zero visible wear on them. They look like they will last for ever – maybe 5000hrs? Due to unrelated reasons, mine have never been in for longer than that before getting changed. They may be expensive but given that they last nearly for ever, it doesn’t matter.

I think you got it mixed up Peter. Champion massive electrode plugs had (have?) the resistor problems, not the fine wire.

No – I am certain. See the link and the photos in my post above. RHM38S.

I am not saying the massives didn’t have the problem too, however. But it was relatively much less reported, for some reason.

I didn’t read the link that Peter provided. However, many reports coming from USA users (IO 540 & IO 320) suggested many issues with Champion’s massive electrode plugs. People throw them to the bin and moved to Tempest.
Savvy Aviator wrote an article about it and said the same. Don’t use Champion.

Ben

For balance, maybe a slightly different view is in order.

My shop (which has been servicing a sizeable fleet of big bore Continentals for many years) doesn’t confirm that it’s all so black white. Yes, there may have been some problems with the Champion plugs and yes, possibly the design of the Tempest plugs is somewhat superior. However they don’t see too many problems with Champion plugs in the field. They say the massive attention in the GA scene was partly a focused campaign started by Tempest (who at the time had just bought the business from Unison). US-style marketing. Get a few influential people (Savvy) on board and the thing will spread automatically.

Three years (when the insulator problems with the Champion fine wires came along), I changed from these to Champion massives and the engine smoothness is the same. We occasionally measure their resistance and general state and it looks like I will use them for another year. Next year, I will probbly change them and decide what to do.
I don’t really buy it that fine wires will hold up for more than 1000 hours.

Unfortunately there is/was disingenuous advertising by Tempest. They were comparing the Champion resistors with their own. Unfortunately such a direct comparison is not meaningful because Champion use silicon carbide, which is a “voltage dependent resistor” i.e. a material whose resistance falls rapidly as the voltage across it increases. So if you simply measure the Champion plugs with a standard digital ohm-meter, you get very high and wildly varying (wildly varying from one resistor to another) values because the meter uses a low voltage (a few volts at most). Whereas the Tempest resistors are constant value and always measure the same. In the actual application, where something like 20kV is available, the Champion resistor value shrinks to a much lower figure than the low voltage measurement shows. The disingenuity was compounded by somebody selling a spark plug tester which was advertised as showing most Champion plugs as faulty – because it measured the resistor at a low voltage.

Unfortunately, Champion still do have a serious problem. I spent a lot of time on the measurements. I tested a lot of them (about 20) with a high voltage tester (a modern version of the old “Megger”) which can test them at 1500V RMS AC. Also, at work we have production insulation testers which go to 4500V RMS. Many of the Champion resistors were essentially open circuit even at 4500V. The issue seems to be due to them developing cracks and at 4500V one could see arcing across the cracks. The engine still “runs” because the ~20kV from the magneto just jumps across the cracks, or even across the whole resistor once it is covered with enough burnt carbon-based material.

It all “works” because the resistor is not immediately mission critical. It serves two purposes:

• it limits the arc current going into the plug, which would otherwise be extremely high due to the ignition harness capacitance discharging directly into the gap electrodes, so with the resistor present the plug erodes slower (IMHO Champion plugs will erode faster than the Tempest ones)

• the spark (which like any arc has a negative resistance and thus forms an oscillator when connected to a voltage source) is a massive RF interference generator and the resistor, together with the ignition harness capacitance, forms a lowpass filter, which reduces the amount of RF conducted out of the plug and thus emitted by the ignition system (which may affect comms, avionics, etc); one engine shop bench (dyno) tested an engine with the resistors shorted out and their instrumentation went haywire from the radiated RF interference

So the debate really comes down to how happy one is flying with a system which is only just hanging in there – even if it is hanging in there most of the time. Given that Tempest fine wire plugs are cheaper than the Champion ones, I think life is too short to be doing this.

There was a separate issue with the long reach plugs in the SR22T TIO-550 engines, with cracking insulators. I don’t know much about this but guess that they cracked because their insulators were a lot longer.

Digging up.
I will soon have to consider new plugs.
As I don’t have significant fooling problems, if I lean properly, what could I expect from fine wires on a normally aspirated engine (IO540 ?)

I’ve read somewhere that the engine could eventually run hotter. Is that true ?

Slightly off topic: here at LKBU, we have recently completed certification tests for an STC for Walter M137/M337 engines, under which the stock aviation spark plugs are replaced with automotive ones (plus a different set of wires, of course). The engine passed the tests with flying colours, though we haven’t done any radio interference testing (yet). Once this STC is ready, we may well try doing it with Lycomings/Continentals, given a sufficient interest.

I just bought a complete set of plugs for my Lycoming and it’s annoying to spend so much, although given the expense of aircraft parts buying a new set every decade or so isn’t going to kill me. Automotive plugs are inexpensive and that is a good thing, but REM37BYs work for small Lycomings and are basically the same design. The aircraft style plug wire attachment is better.