I've used an old heathkit condenser checker on several "old" (original?) condensers, either out of a distributor, or off a coil or generator... here's what I've found:
At absolute "best" the condensers I've checked were up as high as .5uF. Some were as low as .1uf but I have to take into consideration that these things lose capacitance as age and heat dictates....
This old condenser checker I use also has a 'load' test ranging from 25 volts up to 450 volts. Hooking up a good original condenser that measures between .4 and .5uF, I can get it to charge and hold on 150v all day. When I bump it up to the next load voltage, 250 volts, it takes noticeably longer to charge completely and sometimes, as indicated by this unit's "magic tuning eye" indicator, it would not charge at all... I've also noticed this same trend on the condensers available from our FLAPS.
Short of looking up data in some original GM engineer's handbook I'd have to wonder does it really matter? The simple RF trap, coupled with the condenser's arc-quenching utility is hardly a 'critical' circuit.
In radios particularly, any given manufacturer will use any number of a dozen different values in areas of the circuit which are engineered to filter out noise.... .47uF, .1uF, .05uf ... the values are all over the scale. So given that tidbit and in understanding some background in electronic engineering, I'd have to say that the value of the filter capacitor in these given 'circuits' largely depends on what frequencies the engineer was specifically trying to filter out.
In an engine, we consider that the frequencies that are being filtered out are constantly changing, and this coincides with the engine's given RPM. At varying RPMs the frequency of the points opening/closing is changing, but the generator as well along with what interference is being produced by that. Keeping things simple, I think it was just easy to slap that .47 value on there and call it done.
Because, not only does can a .47uF capacitor provide adequate RF filtering, but also acts as a good value for arc quenching, which is the primary reason for all the RF noise in the first place.... and even though you can't always see it, the points in voltage regulators and the points controlling the ignition are arcing, and it's that itty bitty arc that produces so much noise!
Just some experience and thoughts.
