Capacitor Confusion

[ford41]

To start with, I want to mention that I've read every thread I could find on the Barn concerning the subject of which capacitor is correct, yet I'm still not clear.

For a fully rebuilt, stock 11A helmet distributor with NOS points and a Skip coil, what is the correct capacitance value that should be used? I'm currently using one purchased from Bubba way-back-when, with a capacitance value of .36 mfd.

Is this correct and if not what are the downsides to running it?

[marko39]

That should be fine, that’s what mine measures and so far it’s good.

[tubman]

Theoretically, it depends on the coil. Modern coils usually use a condenser with a capacitance in the low to mid .20 microfarad range. The older coils used a condenser of a higher value (usually in the mid .30's). It also depends on engine speed;higher engine speeds work best with higher capacitance.

Actually, they will work over a wide range of capacitances; the variable will be point life. Over or under capacitance will cause gradual material transfer from one point surface to the other. The only practical way to tell if there is a problem is to check the condition of the points after several thousand miles. In the final analysis, the car will run fine over a wide range of capacitances and you shouldn't worry about it except in the case of extremely rapid point wear. When I was developing my "Trash Can" replacements, we tested capacitances as low as .07 microfarads. I have some Mallory "Bus Capacitors" (They look like over-sized "Trash Cans" except they have stainless steel cases) rated at .45 microfarads. They all work just fine. The problem these days is the questionable manufacturing processes that cause many units to fail "right out of the box".

My "Trash Can" units are rated at .22 microfarads because just about all coils used these days are current technology.

[34fordy]

I noticed that ford41 is using a Skip Haney rebuilt coil. I have always wondered if his coils would lean toward the original mid 30 mfd condensor or the later lower 20 mfd condensors. I also wonder if each rebuilt coil of his is the same or if there are slight variances. I am using Skips coil also but have a Mallory condensor rated in the 30s. At my age and less than 1000 miles per year I am not fearful of much point deterioration. I have a "Tubman Trashcan" as a backup but it is so beautiful I have it hanging next to my desk so that I can enjoy it! LOL

[Pete]

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The subject of actual proper capacity for a given coil seems to come up quite often.
This would show that there are people that are perfectionists and would ideally have the capacity exactly right for all rpm's.
Since the resonance of the circuit is basically what we are talking about and the resonance curve for an iron core ignition coil is fairly broad, it is quite simple to construct a variable tuning circuit that will either follow rpm variances or frequency variations in the primary circuit.

An electro-mechanical circuit would have a variable capacitor driven by a servo motor that is controlled by an rpm pulse sensing circuit.

A full electronic control circuit would involve a differential amplifier and sense off resonance tuning of the circuit caused by rpm or heat variables and correct the amount of capacity electronically.
Similar circuits have been in use for many years to keep precision radio receivers on frequency to the extent of 1 part per million.
That should satisfy the most finicky of drivers.