MB Dude, yes, you are right about the full secondary voltage appearing across the plug gap until the spark occurs.

About the plug points. Electrons will collect and jump between the closest sharp points on the plug electrodes. As the center plug electrode erodes though use, it becomes rounded and the spark can be less strong or sometimes no spark can jump ( a miss). We used to pull the plugs, file the plug points flat, and reinstall.

The platinum plugs have a very small center electrode. This should provide a good 'jumping off place' for the spark. Because the platinum is more resistant to spark erosion than the old copper points, the center electrode should provide a good spark longer. This also lengthens the life of the spark plug, a good thing with plug replacement so hard on many engines. The smaller point may also allow the spark to occur with the same secondary coil voltage but a larger plug gap. As I understand it, a longer spark has advantages in igniting the air/fuel mixture. All of this begs the question of why platinum plugs do not work well in some engines.

As I mentioned before, the HR9D plugs I was sent seem to have copper electrodes which are about the same size as the H9D plugs my 103 engine requires. No platinum. I haven't tried the resistor plugs since I was able to buy a set of non-resistor locally. Maybe these resistor plugs will work well, I don't know.

What I'd like to try is to rig up an inductive pick-up for my oscilloscope which would allow me to compare the spark waveforms on the plugs. Then I wold replace one of the regular plugs with a resistor plug and try to see the difference. I would not have a voltage calibration but I should be able to do a rough comparison the the two waveforms.

I don't think that the added plug resistance would produce any ringing in the coil. The resistance actually would serve to dampen oscillations. This is why resistor plugs reduce radio interference.

This all has my curiosity heated up. I imagine somewhere on the web there will be a good description of all of this stuff. There must be a reason why some engines, and their ignition systems, work well with platinum-resistor plugs and others do not.

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1986 300E
1993 Volvo 940 Wagon
1984 Volvo 240 (daughter's)
1976 Dodge 1/2T pickup, gas hog