Spark plug resistance

g zero · #1 03-24-2015, 08:41 PM

Having started another thread about my spark plug dilemma, I'm curious about the 5000 ohm cut off point . Auto plugs recommend 5000 ohm resistance to reduce radio interference ,yet Tempest wants us to trash a aviation plug with the same resistance .
With a magneto sending 20,000 volts (?) to a spark plug how much resistance is too much ?

Bill.Peyton · #2 03-25-2015, 12:28 PM

I just found this I my inbox from the savvy aviator

http://blog.aopa.org/opinionleaders/...to-acceptance/

AndyRV7 · #3 03-25-2015, 12:39 PM

I had my transponder cert done yesterday at a local shop. While I was there I had a nice talk with the mechanic about this. To boil it down, he said that they see MANY Champion plugs over the limit during bomb tests but if they are not causing a problem, or the owner hasn't asked about it specifically, they put them back in service. He also said that they buy Tempest exclusively now. I am paraphrasing from memory but I think I got it pretty close to what he told me.

Doug · #4 03-25-2015, 08:00 PM

G_Zero, I have spent some time trying to figure out what is going on with the resistor. Here is the best I can offer for now, others may wish to elaborate or correct.

A magneto has a primary circuit that consists of a parallel LC oscillator, that is an inductor and a capacitor connected together one side connected to ground. The magneto points short the capacitor (and the inductor since it is in parallel) - current is induced into the inductor when the magnetic flux changes through the core of the coil - when the points open the LC circuit starts oscillating and continues to do so until the energy dissipates in the circuit.

The Inductor also has a secondary high voltage winding that connects via the series resistor to the spark gap. The reason for the resistor is to limit the current to the spark gap reducing the energy rate being bled from the oscillator circuit. Low resistance - very short high energy and less consistent spark, Optimum resistance - appropriate spark duration, lower average energy and consistent spark.

Note the spark is not one single strike but a high frequecy series of strikes at the rate at which the oscillator resonates, continuing until the voltage drops (due to the falling energy in the oscillator) below the threshold required to conduct across the spark gap.

The short duration high energy spark also causes more spark gap electrode erosion.

A high series resistance will result result in a slower discharge and longer duration but more dangerously the high voltage may find another site to discharge other than the plug gap. Since there is no limiting resistance for a secondary breakdown the spark will be high energy thus damaging.

In essance a too high a series resistance allows the HV to look elswhere for a path to ground.

So what determines the optimum resistance? This would be a function of many varibles - inductance, capicitance, LC circuit resistance, isolation gaps, insulation materials and dielectic constant, circuit resistance, spark gap and electrode design.

Incidentally I just realised that the 'p' lead of the magneto shorts the capacitor and inductor primary to ground - so 'p' is probably an abbreviation of 'primary'.

Smilin' Jack · #5 03-27-2015, 04:01 PM

I use Tempest. but I understand from the Mike Bush Last news letter that
Champion is now going to the same resistor deal that Tempest uses.

Jack
It is amazing... so much bashing by Champion and their loyal mechanics.... now they are joining the group

vlittle · #6 03-27-2015, 04:59 PM

Here's a p-lead waveform that shows ringing that provides the multiple spark events. This was taken from an IO-540 in flight.

This also illustrates how it is sometimes difficult to get stable rpm readings using the P-lead signal.

g zero · #7 03-27-2015, 05:32 PM

I cleaned all the plugs , checked them all with an ohm meter . Ohm readings were all over the board ,bomb tested them and they all looked fine . Reinstalled them in the engine and mag tested fine .
Will report back after my next oil change or plug/ mag problem witch ever comes first .

Doug · #8 03-28-2015, 07:06 AM

Quote:

Originally Posted by vlittle

Here's a p-lead waveform that shows ringing that provides the multiple spark events. This was taken from an IO-540 in flight.

This also illustrates how it is sometimes difficult to get stable rpm readings using the P-lead signa.

Vern,
That image is a gem. It also illustrates the decay in the energy during the sequence. Thanks for posting. Any chance you can put some parameters on the grid?

aerhed · #9 03-28-2015, 07:43 AM

This right here gave me $25 worth of thinkin'. Thanks Doug, Vern and the original Doug of course.

maus92 · #10 08-21-2015, 06:56 PM

I'm having trouble checking resistance on my spark plugs using an VOM (Extech MN-35 (China) through Amazon.) The meter works, but none of the plugs seem to have continuity between the center electrode and inside contact (at the base of the "neck" of the plug.) Do I need some super-powered VOM meter to do the test? Is there a trick to get proper contact inside the plug? I've tried bolts, and even bought some especially long-tip probes. To be clear, I'm checking for both continuity and resistance, but the VOM shows an "open" circuit, i.e. no connection. Puzzling.