Not sure I agree with your numbers or logic there, OA.
The spark event occurs when there is zero current flow in the primary circuit. That’s what causes the collapsing magnetic field in the primary coil and consequential build up of voltage in the secondary coil. The amount of primary current therefore seems irrelevant to how much current flows in the secondary during the spark event.
There is a very useful waveform at this link:
Engine Management - Primary versus Secondary Circuits (see figure 1.0).
The red line is the secondary voltage, and the scale for that is on the right hand side.
The primary voltage and secondary voltage build up quickly when the current stops flowing in the primary (on most steam-driven GA engines, that’s when the mechanical points open).
Then the spark happens. The spark event is happening during the approx 1.1 milliseconds during which the measured secondary output voltage is about 2,500 volts. (The ‘ringing’ happens when the spark stops. Current ceases in the secondary circuit and coils do what coils do when current stops flowing through them.)
During the spark event the ‘sparked gap’ has very low resistance. Therefore the amount of current flow depends on the total resistance in the secondary circuit.
If we assume the plug resistance is 2,500 Ohms (to make the numbers simple), the current flowing during the spark event is
1 Amp.
(E = IR; I = E/R; E = 2,500 volts; R = 2,500 Ohms; I = 1 Amp.)
However, it’s not quite that neat. For example the secondary coil has resistance as well. The tech data suggests that a ‘normal’ coil secondary resistances have a broad normal range that varies between brands, but the very rough average is about 10,000 Ohms. Also, inductive circuits do really weird things ...
However, the spark event is for most intents and purposes a (very short) DC event. The voltage necessary to bridge the spark gap will always be the same but, once it sparks, the amount of current flow in the secondary circuit is determined mostly by the voltage and resistance in the secondary circuit. The higher the plug resistance, the lesser the current flow during the spark event. The higher the plug resistance, the ‘weaker’ and ‘shorter’ the spark.
Of course, these aren’t ‘1’ or ‘0’ things. There is so much ‘slop’ in these systems that it’s a slow deterioration.
But you can feel it in your nether regions and see it on the engine monitor, eventually. And there’s lots of real world experience that shows that if you throw away ‘that brand’s’ plug if the internal resistance measures in excess of 5,000 Ohms on a ‘standard’ low voltage multimeter, the problem goes away (even if the plug was squeaky clean and perfectly gapped, and even if it sparks on the test bench).
Aviater: From the first link in Jabba’s post:
For fine wire iridium plugs, setting the gap is rarely an issue. … If you do try to bend the fine wire ground electrode, you’ll probably break it. …
Ain’t no engineer going anywher near...