[rotorwrench]

A lot boiled down the what materials were used to make ignition coils over the years. If the resistance of the primary was too high then the unit would build up excessive heat during normal use. They used tar as an insulator in a lot of coils. The tar melts and can leak if the case is compromised. The big problem with the old coils would come if a person had an engine stop running for other than ignition off selection. If a person forgets to turn the ignition off when this happens with the added problem of the ignition breaker points being closed then the coil turns into an electro magnet. They can heat up enough to melt stuff inside the coil assembly.

I never refer to a coil as a 6-volt or a 12-volt. Ballast resistors were used to protect the coil. The condenser is in there to extend the high tension pulse but it also protects the breaker points which tend to arc with back flow from the primary. The Mallory design ignition systems of the 30s & early to late 40s used a bit higher capacity condenser than the model A or 8BA designs. Those Mallory coils were a lot different than the basic can type coil.

In the modern era, a particular formula of epoxy is used as an insulator so they can take a primary resistance of 3.0-Ohms with no problems. Now days one coil fires one cylinder electronically so they last fairly well. The modern Motorcraft coils for the modular engines seem to last about 100K miles before they get weak and start to misfire. They're tiny compared to an old can coil.