[rotorwrench]

Coils, by their name, already are a resistor of sorts in relation to the primary winding. The 3-Ohm primary type coils are made to work on 12-volts. The 1.3 to 1.5 Ohm ballast resistor or resistance wire is only necessary on a 1.5 Ohm coil being operated on 12-volts. Manufacturers were slow to go to the 3-Ohm coil. The 1.5 was used clear up into the late 60s & early 70s. Electronic ignition systems finally made the manufacturers change.

External ballasts kept the heat on the outside of the old coils. Many had an oil insulator inside that could leak under extreme conditions. Modern coils use a high temp type epoxy resin inside that can take the heat.