How to thread drift in 720 posts!!!

[Jabawocky]

Progressive, no worries I figured you had misread.

I have never had a problem with massive electrode plugs gapped properly every 100 hours, although few do this properly.

You might be surprised, usually around 75-80 hours you can tell the difference. I have proven this with many a set, and let them go past 100 and it is dead easy to spot. Most pilots would never know.

Silicone carbide resistors are generally NON-LINEAR in resistance and optimized for resistance at operating voltages so they would need to be tested at operating voltage/conditions.

OK this argument has been made before in the USA, however a few things come to mind. Confuscious say do not tell man something impossible when he is already doing it. Now besides the funny aspect, when you take a set of Champion fine wire plugs that the pilot says is running not as smooth as he thinks they should and you measure them, with the Tempest method, and they are anywhere from 100,000 ohms to 1,000,000+ ohms and you replace them, and yet the gaps were still fine, because they hardly wear at all, with a new set and it runs baby bottom smooth. What does that tell you?
It tells me that the non linear tests even though it is not at 20000 volts it is still detecting a sub optimal plug, so the end result is what matters.

Now I could say I do not have the test equipment here to go testing plugs at very high voltages, although I do have the gear here at work, I am not doing it. But what if i said I know a bloke who has?

Perhaps one of the founders/directors of the TASER company, who has or had a C421, might also have a 441, anyway after being semi stranded due poor plugs on the ground with family etc loaded aboard, he decided to do some research.

I think this guy is somewhat qualified, here is what he had to say; Max Nerheim (edited to remove irrelevant stuff for brevity)

I decided to take the Autolite UHRB32E (same as Tempest?) spark plugs I had from before I put in the Champion Fine Wire plugs, as well all the Champion fine wire plugs and test the continuity in them using a Digital Volt Meter (DVM). The Autolite plugs read from 1115 to 1351 ohms. The Champion plugs read from 1330 ohms (the one new plug) to open/infinity. So not only did the fine wire plugs crack, they also had resistive readings that were seemingly totally random!

I suspected the open readings were due to slight corrosion buildup between the elements of the Champion spark plug: The Champion spark plug is assembled with a resistive “slug” that is held in place by a spring and a screw, which (at least in theory) can both develop some surface corrosion. I thought, if there is just some slight surface corrosion, this would be no big deal as the magneto would “burn” a path through the bad contact areas, and the resulting “high voltage impedance” should be close to nominal. To test this theory I utilized a high voltage source (a TASER X26 stun gun), an oscilloscope, a non-contact current probe, and a 25kV high voltage/high frequency probe. The oscilloscope captured the current and voltage waveforms across the plug from the spark plug wire terminal to the front round electrode (not including the spark plug gap) during the high voltage discharge. The voltage and current readings were captured after all the “bad” circuit elements had been "burned" through. By using the captured voltage and current, the plug resistance during the firing of the plug could be calculated.
I tested all the plugs using this setup. The X26 is capable of generating up to around 50,000 volts, and had no problem jumping across the spark plugs (from the back spark plug wire connection to the round electrode on the front).

Using the High Voltage oscilloscope approach, all of the Autolite plugs measured within 11% of their corresponding DVM readings. Very nice. This demonstrated an acceptable consistency and also validated the approach.

The 16 Champion plugs (4 that were replaced initially, and the remaining 12) measured from 194 to 11,000 ohms using the oscilloscope method. A big surprise was that 6 Champion plugs measuring from 135 to 534 ohms. These plugs had all measured “open” on the DVM. When discharging the TASER across these plugs I noticed there was arcing noise within each spark plug. The arcing noise is caused by the TASER having to arc across an air gap. This caught my attention - why would the TASER have to arc across INSIDE the plug (the spark plug gap was not part of the test).

To investigate I opened up the Champion spark plugs by unscrewing the screw where the spark plug wire is normally inserted. The spring and the resistive element was then removed from the plug. With good access to the element I used a DVM with sharp test clips attached to try and measure the resistance of the element. OPEN!!! Well, it must be "surface corrosion" I still thought. I then attached the TASER X26 to both sides of the resistive element, and discharged the X26, hoping to observe the current going through the resistor, so I could measure the resistance. Instead, the X26 formed an arc on the outside of the resistor: Champion Fine Wire Spark Plug RHB32S resistive element testing - YouTube Each time the X26 discharged, a blue arc of ionized air formed across the surface of the resistor. The resistor element itself was completely OPEN. It did not conduct any electricity at all! Normally in a test setup like this, the current would flow through the body of the resistor, not arc across it. In this case, the resistor element acted as an insulator. Hence, spark plugs with “resistive” elements like this one would present the magneto with roughly a 0.5” extra spark gap in series with the approximately 0.02” spark plug gap! With this much extra gap to jump, there is not much point in arguing if a “proper” spark plug gap should be 0.02” or 0.03”!!! With my limited understanding of how a magneto works, I am also concerned regarding the possibility of the high voltage required to arc across a plug like this, could instead jump from the rotor to the next cylinder contacts (instead of to the current one). The probability of this happening would increase if the adjacent magneto contacts had normally working spark plugs. Hence, if one plug is bad and requires a much higher voltage, it is better if they are all bad!

The arcing across the resistive element also explains the very low resistive oscilloscope readings during my previous experiment: The X26 generates enough voltage to arc and ionize the air across up to around 2 inches through air or even longer across a partially contaminated surface. Once the air has been ionized the resistance of the air (gap) drops from “open” to close to zero (that is how a TASER device can across up to 2 inches of clothing.) The resistive element that was open also read “open” on the DVM TEST. I weighted the resistive element on a laboratory grade scale. It came in at 0.23 grams. The TASER X26 discharging 19 pulses per second for 5 seconds (compared to a magneto at 28 pulses per second continuously) through a working resistor made it too hot to touch. Is it a possibility that the resistor is too small to dissipate the electric energy from the magneto without degrading..? How much energy does a magneto put out? It sure seems like a good idea to have the plug resistor thermally and mechanically coupled to the spark plug body to help dissipate the electric heat (like the Autolite plugs).

Arcing Spark Plug Voltage:
I also measured the peak voltage necessary to arc across the various spark plugs. This was a slightly different test setup, as now the spark plug gap is part of the circuit. The voltage was measured from the spark plug wire attachment point to the body of the plug. In order to fire the plug, the magneto would have to put out this voltage in addition to the extra voltage needed to jump through the compressed air mixture present in the cylinders during compression – more cylinder pressures at higher manifold pressures requires higher magneto voltages. At ambient pressures the old Autolite URHB32E plugs measured somewhat consistent (nothing in High Voltage is entirely consistent) peak voltages from 6200 to 9300 volts. With the Champion RHB32S the voltages measured from 6300 to 17000 volts. With some of these I had intermittent arcing at the back connection of the spark plug – from the screw connection, across the ceramic to the metal body. In this case there would be no arc across the spark plug gap. Operating the spark plugs in a running engine with cylinders compressing the air would require a higher break-down voltage across the spark plug gap, and thus would make this condition more likely to happen.

Summary/discussion:
I think there are two separate problems with the Fine Wire Champions:
-Roughly half the plugs I removed had ceramic missing (1 plug) or cracked ceramic.
-Only one Champion plug read around 1300 ohms (this was the one new plug from Provo). The other 15 plugs, including 3 that were only “slightly used” (installed in Provo) displayed readings on the DVM from 4000 ohms to “open”. 9 of the plugs read “open”. I had only one plug out of 16 that read close to 1300 ohms. I would not use a plug that read higher that 1500 ohms.

FOLLOW-UP April 11

The TASER brand stun guns do have a current limit, which is why the output of these devices do not wary much depending on if they are discharged into a "short" or a high impedance load (fat person). Most of the cheap contact type stun devices use only a capacitor discharge across the front, and hence they are loud, and not current limited. Different technology.

Luckily I have access to equipment to measure high voltage and current at the same time, calculate the resistance of the spark plug resistor WHILE the current is flowing through it, and also measure the peak arcing voltage across the plug and across the resistive element. These measurements raised my concern. The engine will run fine as long as he magneto can jump across the burned out resistor AND the spark plug gap - without the rotor inside the magneto arcing to the next contact point. That scenario is my largest concern. I fly a pressurized twin to 28,000 feet, and am concerned about keeping the ignition system fault-free. As we all know, the lower air pressure at high altitude makes it more likely for an arc-over inside the magneto (to the next cylinder with a "good" spark plug requiring lower arc-over voltage).

I also thought there were inherent low-voltage effects that prevented the fine wire plug resistance from being measured by an ohm meter. I proved myself wrong: What makes the plug read a higher resistance (with an ohm meter) is not surface corrosion or effects from the type of resistor used, but rather that the resistive element is burned out. Notice that the only Champion plug I had that tested close to resistance specification was the ONE new plug I had. All the other ones had various resistances, up to infinity/open. The ones that read "open" were loud when I arced across the plug from back to front electrode. The plugs with low resistance reading, including all the old Autolites were "quiet" when I passed the current through them. After taking them apart, I discovered that the resistive elements were completely open. The loud arcing was the current having to arc across the resistor to make a connection.

I think the parts should work according to specification/theory. I am afraid that if my plugs malfunction at altitude I will be out a new engine.....
The mag check is done at lower manifold (cylinder pressure) and therefore require less spark-over voltage across the plug gap. At altitude I run higher manifold pressure, resulting in higher cylinder compression/pressures and therefore higher spark plug arc-over voltages. In addition, there is less air pressure in the magnetos, making the possibility of an internal magneto arc-over higher....