The "12 volt" coil is designed to run most efficiently at about 9 volts, the voltage that's usually available when the battery is under load while running the starter. As the engine begins running and the charging system kicks in, the voltage can go as high as 14.5 volts. To prevent damage to the points due to excessive current flow, the terminal voltage available at the coil during normal running must be dropped to 9 volts or so by using the resistor. During cranking, the resistors are bypassed by connecting battery voltage directly to the coil, either with a separate circuit from the ignition switch, or a bypass circuit from the starter solenoid.
A "6 volt" coil is designed to run at a maximum voltage of 7.5 or so, so if you plan to run that coil on a 12 volt system, you'll need to drop more voltage with resistors before it gets to the coil. DO NOT HIDE THE RESISTORS UNDER THE DASH! Ballast resistors develop heat, and they must be in an area of good ventilation. I'd suggest putting them near the radiator, maybe on the front side of the core support where they're exposed to ram air as it passes through the radiator.
Jerry
Last edited by Hotrod Lincoln; 03/30/2010 3:45 AM.
