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How does a series resistor inside a spark plug rduce EMF noise?
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Beamin:
Wouldnt a capacitor work better? Plus you want the spark as hot as possible and a resistor would lower that. I have no idea what the ohms are if its like 10 ohm or 100k, I dont have one to test. I know putting a ceramic cap across your alternator will reduce noise but a resistor in the plug? Plus its going to get super hot and change value.
Mazo:
Reduces the edge rates(think R-C),and dampens oscillations,in some cases might make the burn time longer at the expense of weaker spark(will that reduce sputtering of the spark plug electrodes is something I don't know) .
Just for reference BP7ES and BPR7ES are essentialy the same plug,the difference being the R version has a 5K resistor inside.
Circlotron:
Depending on in-cylinder conditions on that particular cycle, the plug gap might take say 20kV to jump the gap. Once the arc occurs the voltage across the gap drops to maybe 100 volts in several nanoseconds for a duration of several nanoseconds while a couple of hundred amps flow (true arc) as the stray capacitance is discharged. After this, depending on the ignition coil parameters, a glow discharge will occur for 1mS or so with a current of 50-100mA running linearly down to zero, with a voltage across the gap of about 1500V. The sudden drop in voltage from 20kV down to 100V in nanoseconds makes the plug wires radiate noise, along with the sudden current pulse. The resistor reduces this rate of fall of voltage and also reduces the peak current.
amyk:
A popular performance upgrade when points ignition was still the norm was copper-cored spark plug wires instead of the OEM carbon-cored ones.
gnuarm:

--- Quote from: Beamin on February 08, 2021, 09:33:30 pm ---Wouldnt a capacitor work better? Plus you want the spark as hot as possible and a resistor would lower that. I have no idea what the ohms are if its like 10 ohm or 100k, I dont have one to test. I know putting a ceramic cap across your alternator will reduce noise but a resistor in the plug? Plus its going to get super hot and change value.

--- End quote ---

The resistance is high, but not seriously high.  Remember, the voltage on the plug is high, but the current is very low.  The spark is actually an oscillation with many cycles of current, not unlike a bell ringing.  The spark only needs to fire once and the first arc is the hottest spark.  The cycles die away after that.  But not fast enough to reduce the RFI adequately. 

The resistor takes a bit of energy from each cycle to make the oscillations die away more quickly without significantly impacting the initial spark.  This hugely reduces the RFI from the spark.  The only other way to mitigate the RFI is to use braid to enclose all the high voltage wires from the coil to the spark plug.  But these days, aren't most coils on top of the spark plugs?  I guess they still use one wire per coil to reach the other plug the coil fires. 

The capacitor on the points is on the low voltage circuit.  It was originally added not to reduce RFI, but to preserve the life of the points.  The oscillations in the primary circuit are a much higher current and erode the points.  With the cap in place the back EMF from the coil is reduced and so the arcing is much reduced as well. 
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