I think you have the right idea, but actually there's no close connection between the induced voltage at the coil secondary and the turns ratio. Why? Because the voltage at the spark plug is WHATEVER IT NEEDS TO BE.
How can this be? Well, let me give you a couple of extreme examples: Let's say you short the secondary coil output to ground. What's the induced voltage? Zero! Now let's say you hold the coil wire a quarter inch away from ground and get a nice spark. Now the induced voltage is close to 20 thousand volts. The nominal spark gap of .035 inches is a compromise that gives a nice hot spark reliably under a range of conditions.
The ENERGY discharged by the spark plug is limited to the energy stored in the coil, and it's going to be pretty much constant over a range of conditions. However, the way that energy gets discharged is going to vary. You might short out the electrodes, in which you'll get a high current that's dissipated by the resistance in the coil and plug wires. Or you can a have a wide spark plug gap, which would mean high voltage but weak current: a "cold" spark. Ideally, it's somewhere in between: plenty of current across the plug gap to generate enough heat to ignite the air/fuel mixture.
Sorry, but you can't treat plug gap as a resistance, because its behavior isn't linear. You can MEASURE that resistance during the instant of the spark, because you can capture voltage and current on an oscilloscope. But you'll find that it isn't useful information; resistance is simply a derived value from voltage and current. In this case it varies from infinity to something fairly low back to infinity in a few milliseconds. When the spark jumps the gap, it is because the voltage at the gap exceeds the breakdown voltage of the air/fuel mixture; resistance doesn't come into play at all.
One of the many useful gadgets that J. C. Whitney has given us over the years is the "spark magnifier" (I'm not sure of the name) that went between the spark plug and plug wire. These are just spark gaps that forced the coil voltage to rise higher than normal before discharging. Surprisingly, these might actually work on an engine with fouled spark plugs; you'll sometimes get the same effect when you pull a plug wire loose and all the sudden a missing cylinder starts firing because of the extra spark gap.
My two cents on John's original post:
1. I think that most guys who draw ignition system schematics have no idea how an ignition coil is wired internally. And it really doesn't matter how the coil is tapped; the polarity of the secondary is determined by the physical orientation of the two windings, not by how they're tapped.
2. I've heard that the reason spark polarity matters is because the center electrode is hotter. Makes sense to me. I seem to also recall that spark plug erosion will be greater if the polarity is backwards; that makes sense as well.
The nice think about these sparky discussions is that nobody has ever seen an electron, so you can make up all kinds of stuff!
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