Most of them are in the 1.5 to 1.8 ohm range. Typical current draw is around 3 - 4 amps, which would translate to something on the order of 20 - 25 watts dissipation. Note that these numbers are with the engine stopped and the points closed; when the engine is running, the points are open a good part of the time and the average current draw is somewhat less.
The resistance and current numbers come from an old Motor's repair manual.
Easiest approach is to go to a good auto parts store and buy a ballast resistor for a 60's or 70's vintage car. NAPA has a universal one, part number SME 185450.
By the way, the reason for cars having external resistors instead of simply using a 12 volt coil was to provide a hotter spark for starting. There would be an extra contact on either the ignition switch or starter selenoid that would bypass the resistor and apply full battery voltage to the coil for starting. Once the engine started, this contact would open, putting the resistor back in line, and reducing current through the points to a value that wouldn't destroy them quickly.
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Today's Featured Article - Oil Bath Air Filters - by Chris Pratt. Some of us grew up thinking that an air filter was a paper thing that allowed air to pass while trapping dirt particles of a particles of a certain size. What a surprise to open up your first old tractor's air filter case and find a can that appears to be filled with the scrap metal swept from around a machine shop metal lathe. To top that off, you have a cup with oil in it ("why would you want to lubricate your carburetor?"). On closer examination (and some reading in a AC D-14 service manual), I found out that this is a pretty ingenious method of cleaning the air in the tractor's intake tract.
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