The Stovebolt Site Forums The Shop Area Engine Shop What for, those dual points?

While I’m thinking about distributors, I see where some of our members have put dual points conversions in their AD’s. Could some one explain the advantage of dual points for a 6 cylinder 216 Chevy?
Denny Graham
Sandwich, IL

Denny Graham,

I had a Mallory dual point setup in my '54 coupe when I was a kid...this is how I understand it. The dual points create a overlap that allows the coil more saturation time. I have thought about how it works several times in the last couple of years and can't picture it in my pea brain.

If you drew a little cartoon and understood how the points were wired to each other it might become evident. Any-hoo, it's all about coil saturation..I think!

Stuart

Denny, The point sets are wired so that if either set is open, the coil is being energized. The coil's ground is completed only when both sets are closed, that's when the coil discharges, creating a spark. So, with more 'build-up' time, you should get a hotter spark when it discharges. Higher voltage sparks are required to ignite leaner mixtures & in higher compression ratio conditions. At slower RPM's the coil has enough saturation time to reach its maximum output voltage using only one point set. At somewhere around 4,000 to 6,000 RPM the 'dual-point' setup becomes a definite asset. Note-there were other, some OEM, & some aftermarket, distributors made with dual point sets & these used a dist. cam with 1/2 the number of lobes as there were cylinders. They used two coils, ea. point set triggering its own seperate coil. With only 1/2 the number of cam lobes, it could be shaped to allow more sat. time. Doug

Denny - I am pretty much an idiot in regards to engines but common sense tells me that if desertdog is correct in stating that you need 4000 to 6000 rpms to get any benefit from dual points then I personally would not see a whole lot of advantage to putting them on a stock 216 or am I missing something here? two sets of points to mess with? etc

Stu I think I see your point. The dwell time while the points are open is a very small percentage of the time between lobes, even on a stock engine. Off hand I would say that the points are closed more than 90% of the time. Which gives the coil more than enough time saturate at low rpm, making the full voltage available in the secondary. In the dual point systems the points are obviously wired in parallel and it appears that by timing the two sets of points to break with an over lap you could theoretically reduce the dwell time down to zero. If the primary circuit were energized for 99.99% of the time that would be about as close to the maximum time that you could give a coil to saturate before both points were open and the secondary collapsed.

Sorry Doug, but the first part of your logic doesn’t seem right to me. The primary circuit is completed when the points are closed and the field builds up during this period. When the points open the field collapses and induces a current flow in the secondary creating the high voltage.

I would however agree with you on the point of coil saturation time when it comes to a high rpm engine. I have no idea in what time period the coil would saturate, I’m sure, that figure would cover a pretty broad spectrum depending on what coil you were testing. It would be interesting to see dyno tests, I’m sure that Mallory did a lot of that back in the day, unfortunately it’s probably all been trashed by now.

I think what your saying is, that at 3,000 rpm with a 6.7:1 compression ratio there doesn’t seem to be any need for dual points nor can I, see any reason for an increase in performance from the inliners that we are generally talking about here. I also can’t see where the compression ratio would be a factor in the lower rpm range. Obviously you always want the hottest spark possible and coil design, rpm, and compression ratio would all be factors that become increasingly more relevant as the rpm increases.
And Pop, yep, logic also dictates to me, that it’s pretty much a useless add-on for a 1950, 216 or just about any inliner!
Thanks for the comments, keeps the old brain exercised.
Denny Graham
Sandwich, IL

I've done some pretty extensive distributor tester work and dyno testing of dual-point systems if anyone's interested in my observations.
Jerry

Denny, Glad you caught that. An obvious error on my part. Doug

I have a dual point setup in my 261. I mainly did it because I can go a lot longer between adjustments- since on set only sparks when it opens, and one when it closes, the contacts run a lot cooler and don't pit out as fast. The dwell angle is a lot more stable. Plus if one point set burns you can just yank it out and run on the other one.

With all that said, I just bought a plate, that is supposed to replace the 1933-56 chevy plate.
Might just play around with it some, when I have nuttin' else to do.
Denny Graham
Sandwich, IL

HRL - YES we want to see your observations.

I have an opinion, but would like to see/hear your real data.

In the mid-1950's, my father had a very successful sideline to the family shop going, modifying Lincoln Zephyr 12-cylinder distributors to run on the alcohol-burning Ford flathead V-8s most of the local dirt track racers ran. The Lincoln had a much bigger coil than the Ford, and it made a hotter spark. It would also survive being hooked up to an 8-volt battery, one of the few good uses I've ever found for that battery. All the Ford distributors, 8 or 12 cylinder, had dual points. I was about 12 years old when I got my first lesson in setting up dual point distributors.

Practically all d/p distributors have a "leading" and a "lagging" point set. One point closes a few degrees of rotation ahead of the other, and the lagging point opens a few degrees late for the same reason. This allows the "dwell" (the number of degrees of rotation the points are closed) to be increased. A normal single-point distributor for a 6-cylinder Chevy engine will be somewhere in the 38-45 degree range up through 1954, and 28-35 degrees for later stovebolt engines. Since the total dwell possible on a 6-cyl. dizzy is 60 degrees, and the ideal coil saturation point would be 66%, or 40 degrees, the early dwell setting of 38-45 would seem to be the best choice. In reality, the necessity to set the point gap near .016" to get that much dwell would cause points to arc and burn as the condenser tried to discharge. Dropping the dwell to 28-35 allowed for a .019" gap, and increased point life, but gave up some coil saturation and spark power in the process. This was somewhat offset in 1955 by the switch to a 12 volt ignition system.

Dual points, with their overlapping opening/closing cycle allowed the engineers to take the dwell angle back to the ideal 66% figure, without narrowing the gap sufficiently to cause point burning. For instance, the leading point could be set for 30 degrees of dwell, and the lagging poing set the same, but the opening/closing of the points could be set 10 degrees apart. This would result in a total dwell angle of 40 degrees. The increased dwell time resulted in better magnetic saturation of the coil, and a hotter spark.

It's possible to get dual-point performance from single points, but only for a short time, and at lower speeds. As speed increases, the slapping action of the rubbing block on the distributor cam sets up a harmonic vibration that makes the distributor shaft wobble, even on a dizzy with good, tight shaft bushings. A little bushing wear, and high speed "spark scatter", the tendency of the spark to happen at slightly irregular intervals, becomes quite common. Adding a second set of points approximately halfway around the distributor cam tends to stabilize shaft wobble, and reduces spark scatter at high speed. Sharing the primary current between two sets of contacts also makes a dual point system more reliable and longer-lasting between tuneups.

It takes a distributor test bench and an experienced tech to set up dual points properly, but there's a few tricks a shadetree mechanic can pull to get a good point setting. Set the points one at a time with a dwell meter by inserting a piece of business card stock between one set, and checking the dwell on the other point set with the engine running. The idea is to adjust each one to the same dwell reading, and allow the overlap to get the total dwell reading correct. If you're going to do lots of high speed work, cut the spring off an old set of points, and double-spring the ones in the distributor. Be aware that trick will make for VERY rapid rubbing block wear, so use a good brand of high-temp wheel bearing grease (just a little) to lube the distributor cam. We would turn the old flatheads over 5,000 RPM with double-spring points, but the dizzy needed new points about every 5th. race or so. (less than 50 miles, but at wide-open throttle most of the the time).

Dyno tests confirmed better HP figures with a race-prepared dual point distributor, even on the 350 Chevy engines we ran in the mid-70's, just before electronics took over. I think it was mostly due to more consistent timing and less spark scatter. Hope this helps some of you understand the concept!
Jerry

HotrodLincoln,,,,,Youre a wizard. Im beginning to understand it now.
Thanks for sharing your knowledge.
Marty

When I put the Mallory setup in my '54 car when I was in high school, I also purchased a giant Blue Streak coil with a finned aluminum case. The Mallory kit came with an external condenser the size of a Pringle's can. After reading the post above, I can see the rationale behind the mega-coil and king size condenser.

Stuart

There's a factory set-up sequence for Chev V8 (oh,
perish the thought) in the '56 Chev manual that can be linked to under Tech Tips. It's pretty much like Hotrod described. You set each set of points individually to 29 deg. and the combined dwell angle is 34 deg. Pretty cool set-up for it's day. Had the advantage of much longer point life too.

Well there ya go Jerry, that's the kind of answer I was looking for. Pretty much confirms what I suspected about the overlap. Not much you can do to change the ramp on the cam so they had to find another way to keep the primary closed a little longer.
Ya got me thinkin bout the time period for coil saturation. You say the ideal coil saturation point is 66%, but wouldn't that vary according to the rpm? 66% at 500 revs is going to be a lot longer time period than 66% at 5000 rpm, isn't it?
I’m also wondering about the point gap. We were talking about a Chevy 6 banger here correct? Closing the point gap increases the dwell angle of course, so theoretically speaking if you could gap the points at .001" (or less) and trip things off right at the peak of the lobe, you would have the most dwell time possible for the coil to reach saturation. If I understand you correctly, you are saying that the problem comes in with a gap of less than .016" with this setup and you start burning points, is that right? Couldn't the capacitor value be selected to match up with the gap setting? Know anywhere that there might be more info that, I'd like to understand that relationship a little better?
What you say about balancing the loads on the shaft with the dual point system makes complete sense. It’s been 40 years since I’ve seen a distributor machine but I'm sure that problem would show up clear as a bell with a scope on the secondary. What comes to mind is the way the timing marks bounces around when you bring the revs up with a light on it. All of the slop in the system causes the timing to bounce around quite a bit so anything you could do to lessen it is good.
Thanks
Denny Graham
Sandwich, IL

Denny, it sounds like you've got a good basic understanding of ignition principles, so let's get a bit more technical. Coil saturation is a two-edged sword. Leaving the points closed longer allows for more coil saturation, the buildup of a strong magnetic field that's able to induce a higher voltage as the field collapses across the secondary winding. That same strong field also cuts across the primary winding. A primary scope pattern will show a spike of 100 volts or more induced in the primary, which is supposed to carry 6 or 12 volts. That surge is what burns points, as it hits the winding as soon as the points break, even .001" or less. Enter the condenser- - - -which appears to be a complete circuit to the voltage surge. While the condenser is charging, it allows the surge voltage to shunt away from the points. In the fraction of a second the condenser takes to charge up and stop flowing current, the points open sufficiently to avoid arcing over.

Now comes the second phase- - - -getting rid of the stored voltage in the condenser. It's like a base runner trapped between first and second base. Once the condenser charges up fully, the voltage tries to escape by running backward through the coil, the ignition switch, the battery, and charges the grounded side of the condenser. It loses a little voltage on the way. This flywheeling action repeats for several cycles until the voltage dissipates. The descending-amplitude sine wave evident right after the "points open" signal on a scope pattern is visual evidence of the condenser discharge cycle. This must happen while the points are open, or any leftover condenser voltage discharges right across the points when they close again. The condenser must not only absorb the points-open surge; it must discharge while the points are open, or they will burn on the points-close cycle. If metal transfers one direction on a set of points, the capacitance of the condenser is too high. Metal transfer the other way indicates too low a capacitance.

I hope this increases your understanding of the condenser's role in the conventional ignition system. It's not necessary on a solid-state system, because the diodes and transistors take its place.
Jerry

O.K., great stuff. I have a question lingering in the dark recesses of my mind going way back to Electronics 101. You say the longer the points stay closed the "more coil saturation" you get?
Isn't it true that once saturation is reached, you can let primary current flow until doomsday and you get no further magnetic field build-up?
So I guess the second part of my question is - is
coil saturation the right term to use when the points are closed and primary current is flowing?

Good point, Waldo. Yes, there is an optimum saturation point where the magnetic field won't get any stronger. In the General Motors HEI ignition system, for instance, there is a built in "current limiter" which attenuates the primary current when the theoretical saturation point is reached. It can be observed on the primary waveform of an oscilloscope as a drop in the raster pattern line voltage about 2/3 of the way to the transistor cutoff point to trigger the next spark in the sequence. I can tell if an original-equipment GM ignition module is in use, or one of the cheaper aftermarket models, simply by looking at the primary scope pattern.

Now, jump back about 40 years. None of the solid-state wizardry of the 70's and 80's and later times was available to the guys building high-performance ignition systems just after WW II. They had to make do with what they had, and dual points were one of the ways available to improve ignition performance. Since primary current flow is opposed by both the DC resistance of the coil, and the inductive reactance of the coil winding, plus the action of the points' contact resistance, adding a second set of points to share the current load was a logical improvement.

At 4,000 RPM, the points on a stovebolt six are creating 12,000 sparks per minute. That's 200 sparks per second, and each one has to happen at precisely the right time. Given the somewhat primitive operation of a set of mechanical points, a centrifugal advance system, and a vacuum advance, I think the engineers of the day did a pretty remarkable job with the options they had at their disposal!
Jerry

Yeah, I second that!! I had a factory dual-point ignition on my 270 h.p. '57 Chev so the technology goes back at least 50 years, prolly 60 or more. Sheesh, I'm getting old!

Back up a few more years- - - - -the 1932 Ford Model B 85 HP flathead V-8 had dual points!
Jerry

Ah yes, back EMF, I forgot about that. Also with one set of points breaking sooner then the other that seems like only the break set should have a problem with arc over, no? I understand how to read the points to tell how the cap is responding. That’s why I was asking what you thought about matching the cap up with the point gap. So your saying that when the condenser discharges back through the system while the points are open, that you have a basic oscillator circuit, right? You should be able to read the scope trace and adjust the capacitance so it degrades fully for any given time period.
Fun stuff Jerry, thanks, when I was a teenager I use to stand in awe of the guys that really under stood ignition. Oh I understood the basics but I really didn’t understand what all those meters and scopes were telling them about performance. Now that I have been through electronics school in the service and a bunch of NRI course and been collecting and fixing up old radios and test equipment for 15 years the automotive stuff is much less of a mystery.
Waldo, you mean that ’57 Chevy I was drooling all over in high school really is 50 years old??? Where did the last half century go. Hope the next 50 don’t go that fast!!!
Denny Graham
Sandwich, IL

Denny, those huge condensers Mallory used to be famous for weren't always the cure for point-burning problems. The lagging point would be the one that might burn on the circuit break, since it will be conducting after the lead point opens. If the condenser is over-capacity, the leading point would arc as it closes, completing the discharge. It's pretty tricky, matching condenser capacity to a specific application. Automotive-type condensers generally aren't available in very many different capacitance values. If you chose to find a suitable capacitor from an electronic-parts supply source, you could probably custom-tailor a capacitor to a particular coil/point combination.
Jerry

Right, didn't think that one out, of course the lagging piont set would be the one having the problems. This discussion cleared up a bunch of things this time round, thanks for the edu. Gonne move on now.
Denny Graham
Sandwich, IL