ignition coil questions

As 67Dart273 suggests coils are more complicated than resistance. They are a transformer, and have about 150 more turns on the secondary than the primary. While most think of a transformer with four wires, a coil has three, since two are internally connected. Since transformers are made with a magnetic core and windings they are also inductors. Inductors store energy, this is what happens in the dwell period. The total energy stored, E = 1/2 L * I * I. In this equation, E is energy in watts, L in inductance in Henry, and I is the current in Ampere. I * I is current squared. Not all the energy goes to the plug, some is lost due to the resistance losses in the coil, and plug wires. Some energy is lost in the distributor gap, and some energy is not delivered when the spark is unable to maintain the spark due to gap distances.

So energy is increased by L and I squared. L increases with turns of magnet wire on the core, however more turns increases resistance. More resistance decreases the maximum current, following Ohms law, I = V/R or the battery voltage divided by the coil primary resistance. With a mopar box the ballast resistor is in series with the coil, so is added to the coil resistance. Without the ballast, current builds to excessive levels, and may damage the coil and mopar box.

One property of inductance is to oppose change in current. When coil is first energized by points or mopar box, current increases in time, to the maximum amount limited by resistance. The time to reach maximum maximum current is typically between a few milliseconds to several milliseconds. This time is about right at 6,000 RPM and firing 8 cylinders. At 600 RPM with points, or mopar box the dwell time is much too great, the ballast limits the current to a safe value. A HEI module avoids the excessive current by limiting the dwell time to a few milliseconds, prior to spark. There is more to the story, too much current can saturate the coil, when that happens, inductance drops as if there is no magnetic core and spark will be non-existant.

Modern ignitions are a step beyond the HEI, they better control the dwell with a micro-controller and are direct fire.