Condenser capacitance.

[tubman]

The ideal capacitance of the condenser depends on the inductance of the coil and the overall use of the car. Somewhere, I have a page out of an official Ford publication that listed a variety of condensers available from Ford before the war (similar to Terry, above). It also said that the higher speed that an engine is run, the higher the capacitance it needs. (I have the image somewhere on this computer and will try to find it.) This is why the old Mallory "trash cans" and the currently available Vertex magneto condensers are rated at .28 to .36 micro-farads. If you have changed your coil out for a more modern version, you probably need condenser in the lower mid-twenties range)
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In real life, the actual capacitance really isn't that important as long as it's close. When I was developing my "trash can" replacements. we initially used anything we had handy; the engines ran fine on anything from .047 microfarads up to .5 microfarads. The only difference is the effect it has on point wear, and it takes a long time to figure out what that is. As to voltage, condensers don't care; they are however, sensitive to voltage spikes and heat. A generator can raise hell with a condenser that doesn't have a high enough voltage rating.

EDIT : Here's the chart I referenced earlier :