There are no 12v ignition systems prior to 1975. The ballast resistor steps down the ignition voltage to 6 volts and current to approx 4.2 amps. The math all works out perfect using Ohms Law. Since the 6 volt system doesn't require a ballast resistor, the ONLY thing that changed in 1955 was the ballast resistor. Adding it, meant being able to keep the 6 volt ignition system production line working clear into the 70's. Just replace your 12v system's coil with one of your 6 volt systems coil and there will be no difference in performance. Of course I have already done this and it all works out.
It also has to do with the time allowed for a single charge from the coil. Each time the ignition fires, there is only a short amount of time for it to recharge and fire again. Both the 6 volt and "12 volt" coils require the same amount of time and even the same high voltage result. You can take a 1950 6 volt coil and replace it with one that is stamped 12V and it will run exactly the same. Best I can tell, the resulting confusion was why the manufacturers stamped 12v on the coils in the first place. Of course this will be controversial, but once you do the math, it's over. Not that there may have been technology improvements to enhance firing in the 40 years or more, but check it out.. they work exactly the same.
This is why HEI is so popular. It reduces the amount of time it takes for the coil to reach its max firing potential an order of magnitude, so the 45kv coils are used to their max potential and the coil used really is 12 volts with no ballast resistor. Hotter spark in the same amount of time means leaner mixtures and more complete burn.
There is nothing arguable about this, it's just a matter of doing research and finding all of this out for yourself. I had to prove all of this when I helped create the HEI system for our vintage distributors. There just isn't any way to move forward without understanding everything there is to know about the system.
