Spark plug resistors bad
- Thread starter alfybill
- Start date
Larry Lowenkron
New member
Yes, there is quite a bit about this if you google aircraft spark plug resistance. For brief summary, see:
http://www.chanceaviation.com/resources/ChanceAviation-Tempest-SparkPlugCard.pdf
http://www.chanceaviation.com/resources/ChanceAviation-Tempest-SparkPlugCard.pdf
Dan Cullman
New member
Larry,
I attended the ignition seminar at the N.W. Aviation Trade Show in February. It was pointed out that recent production Champion plugs were showing excessive resistance, new out of the box. Excessive resistance will cause magneto and harness problems, causing rough running engines. Plugs with more than 5000 ohms should be junked. New plugs should test 500-1200 ohms.
Dan
I attended the ignition seminar at the N.W. Aviation Trade Show in February. It was pointed out that recent production Champion plugs were showing excessive resistance, new out of the box. Excessive resistance will cause magneto and harness problems, causing rough running engines. Plugs with more than 5000 ohms should be junked. New plugs should test 500-1200 ohms.
Dan
Dan Cullman
New member
Bill,
I had '414 as a project years ago. Check the chain of ownership. I never did around to working on it.
Dan
I had '414 as a project years ago. Check the chain of ownership. I never did around to working on it.
Dan
I'm suspicious of the 5K Ohm limit on plugs. EVERYTHING I see I can trace to Tempest, who uses a carbon resistor. I believe those plugs, like Champion, who use the spring and slug, are using ceramic (silicone carbide based) resistors to stand up to the vibration in that style. I seem to remember from EE school that silicone carbide is a semiconductor and that it's resistance is not constant over the voltage range of sparkplugs...but that was a long time ago.
In any case, measuring a spark plug with an ohmmeter is like doing the same to the high tension leads. They should at least be measured with an insulation tester using about 2500 volts, not a VOM using 3 volts.
Mechanically, a cast in place carbon resistor I would expect to read more consistantly with a VOM. The contact surface is large between the resistor and the rest of the plug.
Clearly a very high resistance will be a problem. But if I assume the 'distributor' and plug gaps combined (0.025"?) need about 10kV to fire in the running engine, and that a magneto can reliably put out at least 20kV, then there is 10kV available to push the required current thru the resistor. I read somewhere recently that the most any plug would need in current to properly fire the cylinder is 0.1Amps. 10kV will push 0.1 amps thru a 100kOhm resistor (including the resistance of the various contacts in the plug). If the plugs are fouled at all, the fouling will look like a big resistance, and I don't know how big.
I am interested because as I look at switching from the old C10S resistor-less plugs to the 707SR-10s I just bought, I find that the first three 707s I opened measure about 13kOhms with my Ohmmeter. Has anyone using 707s measured them???
In any case, measuring a spark plug with an ohmmeter is like doing the same to the high tension leads. They should at least be measured with an insulation tester using about 2500 volts, not a VOM using 3 volts.
Mechanically, a cast in place carbon resistor I would expect to read more consistantly with a VOM. The contact surface is large between the resistor and the rest of the plug.
Clearly a very high resistance will be a problem. But if I assume the 'distributor' and plug gaps combined (0.025"?) need about 10kV to fire in the running engine, and that a magneto can reliably put out at least 20kV, then there is 10kV available to push the required current thru the resistor. I read somewhere recently that the most any plug would need in current to properly fire the cylinder is 0.1Amps. 10kV will push 0.1 amps thru a 100kOhm resistor (including the resistance of the various contacts in the plug). If the plugs are fouled at all, the fouling will look like a big resistance, and I don't know how big.
I am interested because as I look at switching from the old C10S resistor-less plugs to the 707SR-10s I just bought, I find that the first three 707s I opened measure about 13kOhms with my Ohmmeter. Has anyone using 707s measured them???
I concur, Bill, that the 5K ohm limit might be too arbitrary to apply to various plugs.
You mentioned C10S plugs. I know they were approved, but very poor at lasting very long. I vaguely remember that they may have been removed from the approved list for the Franklin, or maybe de-listed by Champion. If not they should be. With some mags - ~20 hours and the C10S's are gapped out too far for the engine to start. I had very good luck with BG707's though.
If one is testing the through resistance of the plug, an ordinary ohm meter should be fine. For testing for clinker or carbon fouling, a megger(~2500 V) might work. For insulator testing, 15K+ volts and air pressure is the normal spark plug tester test as you know. Both of these tests are across tests.
More thought on the subject about 13K ohm on your BG707's. If they have been in storage for a long time, much of that 13K ohm could be contact surface related and should not matter since it would break down (a good thing) under operation leaving the series resistor only at maybe 2 or 3 K. I did measure some old BG's and I think they were under 5K ohms.
Ron Marks
You mentioned C10S plugs. I know they were approved, but very poor at lasting very long. I vaguely remember that they may have been removed from the approved list for the Franklin, or maybe de-listed by Champion. If not they should be. With some mags - ~20 hours and the C10S's are gapped out too far for the engine to start. I had very good luck with BG707's though.
If one is testing the through resistance of the plug, an ordinary ohm meter should be fine. For testing for clinker or carbon fouling, a megger(~2500 V) might work. For insulator testing, 15K+ volts and air pressure is the normal spark plug tester test as you know. Both of these tests are across tests.
More thought on the subject about 13K ohm on your BG707's. If they have been in storage for a long time, much of that 13K ohm could be contact surface related and should not matter since it would break down (a good thing) under operation leaving the series resistor only at maybe 2 or 3 K. I did measure some old BG's and I think they were under 5K ohms.
Ron Marks
The contact surface is why I think a higher voltage should be used to test the resistors in non-Tempest plugs. The older plugs have at least two mechanical connections (at each end of the resistor slug). There is quite a buzz in the electronics world today with simple switch contacts and the very low currents and voltages in today's electronics. Sometimes the switch, even tho a snap action switch, may not break thru the minor oxidation that shows up on the contacts in as little as an hour. There is a big changeover to wiping contacts or gold in those cases.
I bought my Cruiseair with c10s and a bunch of new spares, so have been using them. But you are right in that they are only good for about 25 hours before regaping, and about 75 hours total, so I was really happy to see the references to the 707's...
I will try some of the the 13kOhm plugs soon and report back.
I bought my Cruiseair with c10s and a bunch of new spares, so have been using them. But you are right in that they are only good for about 25 hours before regaping, and about 75 hours total, so I was really happy to see the references to the 707's...
I will try some of the the 13kOhm plugs soon and report back.