Technical Distributor Points & Condenser: Tuning/Diagnosing Condenser

If the capacitance of the condenser is tuned correctly, there will be no metal transfer during the arc.
So:

1. Is there any way to tune the capacitance with a scope? (otherwise "evolve" the value by logging metal transfer on the points.)
2. What type of condenser is used? Ordinary? Electrolytic? back-to-back Electrolytic?

Has anyone used capacitors from electronic supply houses like Digikey to tune?

Thanks
Tom

 

what motor are you working on?

 

To answer your last question, I am making br*** "trash can" external condensers for traditional high performance ignition systems. To get the voltage and temperature tolerances I need, I am using severe duty film capacitors which tend to get quite large. I make two versions, a .22 microfarad unit for single points and a .33 microfarad version for dual point systems. The .22 microfarad version requires a case that is 1/8" longer than the older units, which is not noticeable to the untrained eye. The .33 microfarad version capacitors are of a size that requires a case 3/8" longer than normal which has a tendency to look odd to those with experience with the originals.

I have found that getting exactly the qualities you want, minimum order quan***ies in the four figure range are common.

 

Doesn't matter. All point ignition systems work the same. But Studebaker V8 if you really must know.

 

tubman, it would be nice if you offered a range of capacitances since coil characteristics vary widely.

REGARDING TUNING: Just cranking engine w/cap off, I've seen points arc like a welder and others no discernible arc. I'm betting just that subjective measurement would be good for ball park. - but, I'd bet big arc is to much or too little farads, and that the other end is not obvious since the arc is barely detectable. The best farad maybe the "knee" in the "visual arcing curve."

 

Actually, I have been fooling around with these for quite a while using a bunch of aftermarket caps. What I have found is that these ignitions will tolerate a surprisingly wide variation of capacitance before they start to degrade to any significant amount. Getting that last .002 microfarads of capacitance will probably result in a set of points lasting 40,000 miles instead of 35,000. Given what I said about minimum order quan***ies in the four figure range, chasing the last few thousandths of a microfarad is not worth the effort. It is better to get one that is close in capacitance and can tolerate a few hundred more volts (spikes). Also, high temperature tolerance is important.

I screwed up and made a prototype with a .047 microfard (NOT .47) capacitor and it ran several hundred miles with no ill effects. You have an idea that sounds good on the face of it, but you are actually chasing ghosts.

 

I've had cars that "ate points." Right now I have a 1974 Datsun 4-cyl pickup that won't get 10k miles out of a set. I'm not concerned about 35k or 40k, I'd just like to get 20k instead of 8k.

tubman, when you say degrade, are you referring to the metal transfer on the contacts? I don't think I've ever seen a set of points with even metal transfer. One contact will build up and the other a cavity and when a piece of the buildup breaks off, it can seriously impact the make/break action resulting in an engine that sputters and shuts off. (I've learned you can manually s****e the contacts on one another and literally break off the metal transfer build up - enough to get the car home. This one was a 1969 Olds Delta 88 w/350.)

I would be interesting to try a few different capacitances on an ignition scope and simply observe the waveform. Tuning might obvious.

 

In automotive cl*** in the 1970s I remember the one instructor we had for ignition said "If the negative point is negative material then you're negative capacitance".
So let's say you have a 0.47ufd cap and after a while you check the points on a car that's negative ground and there's a pit on the base contact and "***" on the movable contact...you need maybe a 0.56ufd capacitor (condensor). You could, ***uming the .47ufd is good, parallel another cap, easily on the coil -- or other end of that wire going to distributor a 0.1ufd cap, there and other lead to ground.
I would think a 600 volt Mylar cap would be fine. Of course the opposite is true, pit on movable contact, *** on ground base...that's too much capacitance.

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Another trick a guy showed me years ago was phasing he called it a distributor. He would put on a clear distributor cap, then with motor running shine a timing light on the rotor ****on. He noticed how some although everything was set perfectly the spark occurred with rotor br*** before or after the corresponding distributor cap contact. He would modify that rotor ****on br*** contact so it fired dead on. Modification meant filing, cut and solder, whatever it took to have everything in sync.

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The first part is true, second isn't..."DURING CONTACT: There is a big current rush into the coil. Since the cap is in parallel to the points, for a split second (until the cap is charged), the cap shunts the current away from the points thereby reducing wear."

The heart of the Kettering ignition system is the coil, an autotransformer which has three terminals. Primary has only a hundred or so turns of wire, secondary thousands of turns. Primary the two outside screw terminals (+ and --), secondary exits out of center tower.
So 12 volts across primary...magnetic field builds up, coil energizes...disconnect it, field collapses inducing that energy into secondary and 20-30,000 volts. The points are just a switch causing this buildup/collapse. Without a capacitor (condenser) across points when the points open you would get an arc across them...little or no high voltage spark, points wouldn't last long. A capacitor is essentially two metal plates separated by an insulator...(lots of different types but that's the basic idea)...so points close, coil charges and at this stage since capacitor is across points it is shorted, capacitor is discharged and does nothing. Points open, coil discharges , wham!, high voltage out the tower. At this time when points open the capacitor comes into play...instead of the points arcing the arcing takes place within the capacitor. Too little capacitance and it does very little or nothing...the points arc. Too much capacitance and it does too much, points open and instead of coil field collapsing giving the H V spark there's a time delay because the primary circuit is charging the capacitor. Again...little or no spark. The "sweet spot" is just enough capacitance to prevent arcing, let points do it's job.

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"Lead dress" is important when dealing with capacitance in HF radio and other applications. Early electronic equipment had all the wiring laid out like plumbing, neat and tidy and straight and right angles like a schematic. It was found that this causes huge problems.

So the rats nest of spaghetti wiring was born, that we all know and love.

A longer or shorter condenser wire itself is enough to change the wear characteristic of the contact points from one side or the other.

 

HaHa sevenhills1952: you caught my post before I took it down because it was all FUBAR. But Thanks for the explanation.

Let me try and parrot it back to you:

My error was realizing when the point are OPEN, the cap is Fully Charged.
When they close, big inrush current supplemented by cap charge at start of inrush.
When closed, cap discharged, coil charged and drawing steady state current w/o inductance effects..
When they open, field collapses and hence lots of inductance current, but since cap is discharged, it shunts "outrush" current thereby reducing point arc.

If TOO MUCH capacitance:

When the points make, the coil inductance absorbs a fraction of the cap charge, the bulk of the cap charge now has to go through the points. Hence Metal Transfer.

When the points break, the inductive collapsing field current spike is totally shunted by the big cap and nothing goes though the points. NO arc or metal transfer at all.

IF TOO LITTLE capacitance, reverse the above and Metal Transfer when points break.

TUNING: If one could measure the current through the points, you could clearly balance it. There may also be a way to judge the arc magnitude simply by scoping the voltage during the arc. Bigger arc, would have a different duration and voltage signature.

 

In the early 70s I built a Heathkit capacitor discharge (CD) ignition kit which they prolong point life and gives a hotter spark. They still make them, Accel, MSD and others. I still like points ignition because it's so simple. The older carbureted, points cars imho were the best in that if you know how they work, and do your own repairs, you can always make it home when trouble occurs. It's also to me satisfying knowing proper tune up, maintenance, etc.


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Induction. Electricity flowing through a wire creates a magnetic field and that can induce flow through another close wire. That's how a transformer works.
Like people with these beautiful plug wires all bundled together neatly then getting cross fire/missing.

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A CD ignition might be the best of both worlds. While the points should never burn, beware now that the rubbing block now has an opportunity to wear down due to this extended life. I think I saw somewhere that Nissan had a "transistor module" that was all you needed or a CD system. Studebaker used the Prestolite CD system and I'd buy a case of them if I ever encountered for cheap!

YES about lead dress! I worked for Robertshaw Controls when they were just starting electronic ignition in stoves. As a worst case testing scenario, we would tape the leads onto a big piece of sheet metal to produce the maximum capacitance possible due to lead dress. In some cases, the capacitance was so great, no spark would be created. However if the spark was produced it was much hotter due to energy stored in the lead capacitance. It's like if you pull a plug wire from the distributor and hold it an inch away, you can in many cases fire a fouled plug.

 

I cant really begin to get in this discussion with the capacitance, as I have forgotten most of the electronic theory that I learned many years ago. However, I have had cars that ate points also, and when it came down to it, the bushings were worn in the distributor, and/or the point cam was not consistently opening the points the same for every cylinder that fired. We all set the points/dwell on the number one cylinder, but hardly anybody ever looks at the other cylinders. Just something you might consider checking. Good Luck!

 

Regarding Rubbing Block Wear:

I set he points on the high side of the spec so I can tolerate the most wear w/o coming out of spec.

I was also thinking that as the gap gets smaller, the opening velocity is probably reduced which is Bad for arcing and metal transfer.

 

Here's the practical end of this. The only available modern capacitor in the .33- .40 microfarad range that meets my voltage, temperature, and size requirements requires a minimum purchase from Digi-Key of 4500 units. I tried to source it from the manufacturer in Japan with no luck. This is the capacitance I need to to produce one of my units to match early Ford "Helmet" and "Crab" distributors. These are $5 units. Do the math and you will be able to see the problem. Most of the time, the ideal unit does not even exist.

 

Here's some .47uf 600VDC caps, -55C - 105C temp range, minimum purchase = 1. They've got a lot more capacitance values and voltage ranges.

https://www.mouser.com/P***ive-Comp...apacitors/_/N-9x371?P=1z0wquaZ1ylo5rpZ1z0vhz5

 

.47 micro-farads is too high in my opinion. Most single point condensers are in the low .200 micro-farads, while dual point units are in the low .300 micro-farads. The highest rated one I have in my collection is a stainless steel unit that closely resembles a Mallory "trash can" but is about 20% larger. I have been told it is for a large old bus engine. It appears to be in very good condition and tested out at .45 micro-farads. Since my units are expected to run on older cars with solid plug wires and generators, I like units that can tolerate 1000 volts and 250 degrees centigrade if I can get them.

You can find caps that are close most of the time. What I think the O/P was suggesting is doing a final "fine tune" to get that final couple of nano-farads to make it perfect. Once you come up with that "ideal"number, you usually can't find a capacitor that matches exactly.

 

Not a fine tune, but a match to the coil in most cases. Coil resistances/impedances are all over the place. No single value of capacitance will match all of them. (I recall some posting where someone measured dist capacitors and ended up with values from .05 to .9 - or something like that.) Now I Do Not know what he sensitivity of capacitance is to the rate of metal transfer but I doubt a swing of .05 uf will go from m***ive metal transfer on the moving contact to m***ive transfer on the stationary contact - but I am aware it is possible.

It appears there are 2 "tunings:" 1) tuning for the coil and 2) tuning for metal transfer. These tunings could range from gross to fine. My guess is gross for the coil which is what the factory does and while close enough for good spark, not close enough for near zero metal transfer. So I'm going with gross and fine respectively.

NOTE: RE: IGNITION SCOPES: I saw some ignition scope traces where the Points Close is a step function. One trace was a nice sharp step while another was substantial ringing decay. These are 2 out of 3 examples of Critical Damping (which is easier for me to understand by using the mechanical equivalence.) The ringing is Under Damped and Over Damped would be a long asymptotic curve where the step function. Another example might be the Square Wave where the leading edge corner could be sharp, rounded, or ringing.

 

One thing I wanted to mention is I'm a retired electronic technician, 43 years in troubleshooting,repair, some college (a.a.s. elec. eng. And auto mechanics...1970s).
It's imho commendable what tubman is doing but a few thoughts. Digikey, Mouser, etc. you can tie up a fortune you probably would never recoup.
Years ago the capacitors (condensers) were simple...not like today a myriad of types. Early ones not efficient. If you take automotive one apart I ***ume earliest were paper then later plastic wound caps. For a while I worked for Paktron where they made Mylar caps, wound via air pneumatic machines to a run of capacitances, wires fused on, tested, dipped, stamped...all one machine. I was a mechanic.
My thought is inside distributor things will be cooler & dryer than outside (exhaust manifold, etc.). So parameters different.
Points bounce, especially higher rpm so stronger spring tension, but wear out faster. Bounce makes ringing pulse & misfiring. (CD ignition according to manufacturers cures that by firing on that first opening.).
What I've found here is I've purchased two identical coils...exactly alike...but then check and my measuring resistance, inductance, they're not exactly the same. Which gets back to properly matching the condenser. You can go lower on the main capacitor value then fine tune by paralleling it with another cap.
My thought is if one wants to reproduce the early style cap if you could find a source for the capacitor metal case you could put whatever modern cap inside it.
I did that for example with a 1936 Crosley console radio. Main multisection filter cap was bad, I gutted it, put in three modern higher temp, low esr caps inside then sealed it with original color wax.

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OT Electronic Tech Laments...

Speak of The Devil sevenhills 1952, I've been frustrated over what has happened to tube equipment that was made to run on 110v which is now 125. The filter caps turn into Roman Candles. In particular I have a Sunn Scepter head amp that is basically a Dynaco Mark IV (twin 6550's). And the sad story of that rig is the Quad Cap. Some outfit bought the winding machines from Mallory I believe and are making a higher rated replacement - it's taller. But it is only truly rated for 120v I think. It's sad too because no one makes 600v-ish caps for tube equipment.

I converted the Sunn into series balanced modern caps located at the particular stage only to realize I FUBAR'd the grounds which I never knew were hyper critical. So I'm going to undo all that work and get the Uber QCap.

The power company better start replacing pole transformers with switchig power supplies so they don't have to run 130v into a line to get 110 at the end.

 

Couple things they used to make, maybe still do, isolation power transformers that had taps. So some untapped ones were 1:1, 120vac in , 120vac out. Tapped ones 120vac in, 110 out (or 90, 130, etc.).
Also caps. Parallel two .1ufd @ 200 volt caps and you have .2ufd @ 200 v cap. Series them and you have .05ufd @ 400 volts.

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I'm sorry, but my training always has me skeptical until

I work on tube powered amps all the time and can buy most any cap I need all day long. I have had old caps let the smoke out due to excessive volts from a poorly regulated generator, but a new SS b*** amp failed along with an old tweed Fender Champ at the same time, while 2 newer tube amps were fine. We switched to a different generator to finish the gig. Anyway, just about any value cap you need is available, 600VDC is no problem. I just unpacked and sorted through a stock order today that included several 600V caps.

 

If the rotor br*** is lined up with the cap terminal at idle, what happens to that relationship when ignition advance kicks in?

 

Rotor cap gap is actually a good thing in my book: the gap acts as an intensifier in that you will get a voltage buildup that is higher than that of a mere spark gap. Hypothetically suppose the spark arcs over at 10kv: 10kv will be all the spark plug ever sees. But if you have a larger gap between rotor/dist which needs 20kv to arc, then 20kv appears at the spark plug which can help if he plug is fouled.
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OT: Electronics Caps:
I was doing this quad cap job maybe 10 years ago and there was nothing available. Again the bulky paper Atoms were about it and ONLY from Antique Radio Supply I believe. Now I just saw the Solen (never hard of them) has all these righteous 630v caps in 10's of uF which is just what the doctor ordered. I might undo my series caps at the stages and bring all filtering back to the quad cap. There is a small terminal strip with a series resister ladder for he QC - I may just hang 4 caps on that strip and forget the QC altogether. Or get the upgraded QD, which is $59 which is kind of pricey. Won't get back to this project for at least another few years.... haha

HEY, these Dynaco tweakers are still selling series caps to replace the QC - series because can't get a single cap to do the job. QC values are 30/20/20/20 uF

 

Good question. Phasing would have to change which I ***ume is why rotor br*** is wide instead of narrow. The guy showing me that is a local machinist, well respected engine builder, racer.

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I'm trying to figure that out..."Rotor cap gap is actually a good thing in my book: the gap acts as an intensifier in that you will get a voltage buildup that is higher than that of a mere spark gap."
It seems like the wider the gap then the less voltage at the plug. If you pull coil wire out while motor is running it will arc until at some point motor cuts off. Just trying to understand.

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About rotor gap. Here is a concrete example that I have experienced. I have a 1967 Corvette with the 327/350 engine. The redline on this engine is 6000 RPM. A few years ago, it was becoming apparent that there were problems with the ignition systems on these engines "breaking down" at speeds significantly below the redline. This was eventually traced to the replacement rotors that GM was supplying. About 1973, for emissions reasons, the tip on the rotor was shortened by about .060". Over the years, Corvette owners doing tuneups were installing these "emissions" rotors into their cars, introducing these problems in their cars without being aware of what was happening. Some enterprising souls tracked down this problem by comparing old and new Delco rotors. Eventually, one company started producing rotors that complied with the original specifications. I had this problem, which was immediately solved by installing one of the updated rotors. When I visually compared the two rotors, the difference was obvious. Bottom line? Again, what is good for emissions, is not necessarily good for performance.