Yes you're right, and I was even thinking of using an old spark gap to illustrate my point.
Perhaps I was a little unclear about my take on the engineers choosing a capacitor; sounding like they just took a shot in the dark, but what my analysis is based on is deciding on one capacitor that would work at clearing up as much noise as conceivably "possible" by only using one simple component... but also based on the premise that the frequencies generated by the spark are in fact changing with engine RPM.... this I don't know.
If the frequencies generated by the spark (not the noise of the "spark" itself) in fact don't change, and resonate at a fixed frequency, then the formula to calculate the filter capacitance for this frequency is rather simple. If it were to constantly change with RPMs, undoubtedly the capacitance required to filter this out varies, and that's where I would have wagered it was one value of capacitor that would have been used as a 'catch all' to prevent the implementation of more complex filtering equipment.
Outfits like Radio Shack carry (or used to carry...) noise suppressors that in themselves are much more complex than a simple condenser... so does this device provide better filtering than a simple cap? I'd say so.... BUT one thing I can think of is that with the advent of transistors also came the availability of the alternator with its silicon rectifiers... I've heard this 'alternator' whine on lots of two-way equipment installed in cars ......
but anyway, getting a little off track, just a few more thoughts.
