Condenser Capacity

[Bob Bidonde]

While researching V8 service bulletins for info pertaining to the Model "A,"I stumbled upon this chart of condenser capacities.

[alexiskai]

It’s interesting that the B condenser isn’t listed.

[nkaminar]

Thanks, Bob. The coil and condensor work together as a tuned LC circuit. Once the initial spark ionizes the air the spark with jump, changing directions, multiple times. This all happens very fast and you can only see it on an oscilloscope. The condensor has to be matched to the coil. Going outside the specification on either one will compromise the ignition system and lead to burned points.

I have never understood why electronic ignition systems do not have condensors.

[P.S.]

The weird part is, the actual value of the capacitor (condenser) isn't nearly as critical as it's leakage (acting like a resistor). The only time you would have to increase the capacitance is when the coil has to fire more rapidly, like more lobes on the cam for more cylinders and/or more RPMs. Modern ignition systems do it by adding more ignition systems (coils) like each plug having its own coil and trigger mechanism.

[P.S.]

Originally Posted by nkaminar

... I have never understood why electronic ignition systems do not have condensors.

Using FETs to trigger a field breakdown (that's what makes the spark) in a coil, some FETs have capacitance already, so the capacitance needed for the "LC circuit" is minimal. A small surface mount cap is almost certainly on the circuit board somewhere nearby. Unless the FET or switching diode has a great deal of internal capacitance and a really high breakdown voltage.

[alexiskai]

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Yeah, you can use the larger V8 condensers in an A or B car without a problem as long as they don't leak. And an A condenser will work fine when using a B distributor in an A car (although obviously it won't fit like the B condenser would).

[Tom Endy]

Attached is what I have discovered.

https://www.santaanitaas.org/wp-cont...r-rev-2023.pdf

Tom Endy

[P.S.]

Originally Posted by Tom Endy

Attached is what I have discovered.

https://www.santaanitaas.org/wp-cont...r-rev-2023.pdf

Tom Endy

That's a good article, Tom.

One line that might be troublesome for someone who does not yet know the Model A wiring inside the distributor is, "You can easily test a condenser while it is still attached to a distributor installed in a Model A." This is true if NOT using a pop-out style ignition switch (they short the distributor lower plate to ground when off). If using a non-stock ignition switch, like the switches sold by Model A vendors that use a different ignition cable with a wire sticking out of it, then as long as that switch is in the off position, one could test a condenser while still installed in the distributor by following your well written directions.

I agree with you that many condensers that get changed out should have been left in place. I opened a box of Model A condensers acquired from a swap meet, all of them said "bad" in black sharpie. Every single one of them, except the one that had been crushed, tested good on the capacitor checker. When I hi-pot tested them, all held well over 600 volts. A couple showed leakage, those were bad. The rest were perfectly good.

[Rob Doe]

Nice thread guyz. I experienced my first point/condenser failure a couple months ago. The condenser, a modern type upper plate, was over two years old and had about 13000 miles on it. I have a simple capacitor tester like Tom's. At that time, I still had an aftermarket switch but have now upgraded to a rebuilt original pop out.

I first replaced the badly pitted and blackened points and tested for good spark, nope. Replaced the condenser and the ole girl lit off like handgrenade. I tested the old condenser at the bench. It ran the numbers up and stopped at .225 as I recall, showing no leakage. Showed OL when testing the contact and the case. I don't have the real McCoy tester to stress it enough is my theory.

Thanks for the article Mr. Endy and the additional tip Mr. Shinn.

[Bob Bidonde]

These slides may be of help to some of you.

[nkaminar]

Why is the unit of capacitance so large? A one farad capacitor would be two plates 1 mm apart and 10.6 by 10.6 km square. (0.039 inch apart and 6.6 miles square.)

[TomInCologne]

Originally Posted by nkaminar

Why is the unit of capacitance so large? A one farad capacitor would be two plates 1 mm apart and 10.6 by 10.6 km square. (0.039 inch apart and 6.6 miles square.)

'cause 1 F(arad) = 1 C(oulomb)/1V(olt), so this is the standard version in the S.I. system.


and 1Coulomb = 1Ampere * 1 second

[P.S.]

Originally Posted by Bob Bidonde

These slides may be of help to some of you.

That trick about having a spare condenser attached to the wire loom clip on the firewall with an alligator clip is genius! I'm going to make some up right now.

Since the condenser is one of the least failure-prone parts of the Model A ignition system, maybe I will just keep it in the spare parts kit under the seat of the touring cars.

[rotorwrench]

The Mallory design coils used in the early V8 era were a good bit different than the can type coils used in the model A/B and 8BA eras. They used a formula to calculate for capacitor rating for each different design of coil. The Mallory designs also required a ballast resistor to control the current to the coil. It functioned better on a lower current than the average can type coil. This also affected the capacitor rating needed.

[Fullraceflathead]

I had a condenser fail about a year ago. I replaced it with one of the burnout proof ones from Snyder's which I believe is an A&L condenser. I noticed the new condenser measured 19 micro Farads but the car runs perfect!!!