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Old 09-18-2020, 07:13 PM   #1
1crosscut
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Default Purpose of dual points?

Okay so I have been messing about with a Mallory distributor that was on one of my industrial Model A engines.
It seems to me that to have it work properly both sets of points will need to open and close at the same exact time. If one were to open earlier it would not cause the high tension spark to occur until the other one opens as they are both tied together. So why have dual points?
Redundancy or am I missing something.
Looks to me that one set of points could be removed and it wouldn't miss a beat.
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Old 09-18-2020, 07:59 PM   #2
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Default Re: Purpose of dual points?

Background reading:

https://mgaguru.com/mgtech/ignition/ig200.htm

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Old 09-18-2020, 09:16 PM   #3
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Default Re: Purpose of dual points?

Quote:
If one were to open earlier it would not cause the high tension spark to occur until the other one opens as they are both tied together. So why have dual points?
It gives longer dwell. Not really needed on a Model A.
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Old 09-18-2020, 09:33 PM   #4
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Default Re: Purpose of dual points?

I believe there are two different types of dual point systems for vehicles.
Bare with me, I've never owned one, but I've seen them worked on.

One type is used on (typically) prewar cars: V-16's, V-12's, and some high-end 8's.
One points set and condenser is used for half the cylinders.
These are tricky and complicated to work on, and need special tools.
You can read about how these systems work if you have a pre-war Dykes Automobile Encyclopedia (the Bible of pre-war vehicles). Bring strong glasses!

The second type is typically from the American muscle car era.
With the two points there is an overlap of point open time (and close time), thereby optimizing spark intensity. The two points sets don't open/close together at the same time,

After-market performance systems for Model A's are probably like the second type, I would guess.

So, remember to bare with me on this. Its been many years since I've seem these systems. In fact, if someone is 'sharp' on how these systems work, I'd love to hear it.
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Old 09-19-2020, 01:47 AM   #5
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Default Re: Purpose of dual points?

Its all about increased dwell and coil saturation.
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Old 09-19-2020, 12:31 PM   #6
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Default Re: Purpose of dual points?

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1930-pickup has it right. Dual points also extend point life a little, but are not really needed on 4 cyl. engines as they have more dwell time, by their nature, than 6 or 8 cylinder cars. (they cycle 4 times per dist. revolution compared to 8 times on an 8 cyl.)
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Old 09-19-2020, 02:01 PM   #7
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Default Re: Purpose of dual points?

I believe that Ford used dual points on the V8 engines through 1948
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Old 09-19-2020, 06:18 PM   #8
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Default Re: Purpose of dual points?

In a dual point system with only one coil, one set of points opens or breaks the circuit and the other set closes the circuit with a very marginal amount of overlap. The Ford/Mallory V8 distributor designs used this feature. This was the only way to get maximum coil saturation for the early design coils on the helmet and crab type distributors. They needed all the help they could get in that respect for efficiency and reliability.

Dual coil, dual point distributors have to have a special cap to function as two 4-cylinder type ignitions in one package. This way they could get the full saturation of a 4-cylinder type ignition for all 8 cylinders. These were the latest in technology for hot rodders back in the day and could work with a lot higher rpm than stock ignitions. It was about the only way to get 12 and 16 cylinder engine ignition systems to work reliably.
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Old 09-19-2020, 06:35 PM   #9
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Default Re: Purpose of dual points?

Quote:
Originally Posted by rotorwrench View Post
In a dual point system with only one coil, one set of points opens or breaks the circuit and the other set closes the circuit with a very marginal amount of overlap. The Ford/Mallory V8 distributor designs used this feature. This was the only way to get maximum coil saturation for the early design coils on the helmet and crab type distributors. They needed all the help they could get in that respect for efficiency and reliability.

Dual coil, dual point distributors have to have a special cap to function as two 4-cylinder type ignitions in one package. This way they could get the full saturation of a 4-cylinder type ignition for all 8 cylinders. These were the latest in technology for hot rodders back in the day and could work with a lot higher rpm than stock ignitions. It was about the only way to get 12 and 16 cylinder engine ignition systems to work reliably.
Lots of good information from you all!

Okay this is starting to make more sense to me. I was trying to figure out why the points weren't opening and closing the same. This is helping.
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Old 09-19-2020, 08:13 PM   #10
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Default Re: Purpose of dual points?

The only dual points system I've worked on is on my 1978 Suzuki Motorcycles. Both are four cylinders. They use a lost spark system. Each time a set of points opens you get spark in two cylinders; one at the top of the compression stroke, which fires; the other at the top of the exhaust stroke...which does nothing.

It is a surprisingly simple and efficient system.
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Old 09-20-2020, 06:32 AM   #11
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Default Re: Purpose of dual points?

Any dual point system I've worked on had the 2 sets of points set about 10 apart/overlap which increased dwell which then increased coil saturation time.
Now this was for older 6 and 8 cylinder engines with distributors and one coil. You know, the old stuff.

British motorbikes were famous for dual points, but, each set of points was for one cylinder. One set was set/adjusted correctly and the timing for its cylinder was then set. Then the other cylinder timing was set by the point opening regardless of its gap setting.
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Old 09-20-2020, 08:16 AM   #12
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Default Re: Purpose of dual points?

Quote:
Originally Posted by 1930-Pickup View Post
I believe there are two different types of dual point systems for vehicles.
Bare with me, I've never owned one, but I've seen them worked on.

One type is used on (typically) prewar cars: V-16's, V-12's, and some high-end 8's.
One points set and condenser is used for half the cylinders.
These are tricky and complicated to work on, and need special tools.
You can read about how these systems work if you have a pre-war Dykes Automobile Encyclopedia (the Bible of pre-war vehicles). Bring strong glasses!

The second type is typically from the American muscle car era.
With the two points there is an overlap of point open time (and close time), thereby optimizing spark intensity. The two points sets don't open/close together at the same time,

After-market performance systems for Model A's are probably like the second type, I would guess.

So, remember to bare with me on this. Its been many years since I've seem these systems. In fact, if someone is 'sharp' on how these systems work, I'd love to hear it.
.
My '31 Chrysler with a six cylinder engine has a distributor with dual points. One set for three cylinders and one for the other three. Nothing complex about them and no special tools needed to set them. Just adjust them like you had two three cylinder engines but make sure the timing is correct for each when you adjust them.

Google "dual point ignition timing" There are several good articles on the subject.

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Old 09-20-2020, 08:59 AM   #13
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Default Re: Purpose of dual points?

what is the dwell on a model a points/
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Old 09-20-2020, 01:36 PM   #14
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Default Re: Purpose of dual points?

Miff, I don't know but I think only about 30*. Winfield, and maybe others (?) made replacement dist. cams to get more dwell out of the original system. Modern 4 cyl. engines have 40-45* dwell.
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Old 09-20-2020, 01:59 PM   #15
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Default Re: Purpose of dual points?

Quote:
Originally Posted by miff View Post
what is the dwell on a model a points/



Someone here actually measured it once and posted it, but, I don't remember what it was. I've never checked it or set Model A points by it. Most other more modern distributors I set the points by dwell rather than feelers.

But, dwell depends on distributor cam being used and number of engine cylinders. A model A cam is different than a model B cam. Both cams will have 90 of separation and dwell is roughly half of that figure. So the dwell for a 4 cylinder engine should be about 45. But, its not usually the case because of the cam profile. It depends on how the cam is ground which determines when the points open and close which determines the dwell angle and therefore coil saturation time.
Our Model B distributor cam [ I don't think anyone sells a Model A cam anymore] allows for more dwell angle and increased coil saturation than the Model A cam because of its different grind profile. I think the Model B cam allows about 40 of dwell.
Maybe I should check just for the heck it.
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Old 09-20-2020, 05:58 PM   #16
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Default Re: Purpose of dual points?

The points cam & breaker gap is what sets the opening and closing of the breaker points. How far open the cam lobe lifts the breaker points off the seat has a lot to do with the dwell time. Lobe separation is 90 degrees but the points are set to open at a certain point on the ramp due to the point gap setting. Set the points farther open and dwell will be higher. Set them closer together and dwell time will be shorter.

Setting points on aircraft magnetos can be a chore. They have to open and close by a certain degree setting of internal timing with the E-gap but getting the breaker point gap within specs can be a b!tch.

To ADD: I add this to correct for a misunderstanding I have created. In the first paragraph I was referring to the amount of time the points are open and not the amount of time they are closed. I should have stated that a wider point gap would give a longer time of points being open but would give a shorter dwell time with points being closed. The smaller gap will shorten the time the points are open but increase the dwell time of them being closed. This is important due to the fact that the time the points are closed are the degrees of rotation while the coil is going through the build up saturation of magnetic gauss in order to create the high tension voltage step up the next time the points open. I'm sorry if this led to any confusion.

Last edited by rotorwrench; 09-21-2020 at 08:57 AM. Reason: Correction.
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Old 09-20-2020, 07:18 PM   #17
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Default Re: Purpose of dual points?

Quote:
Originally Posted by Purdy Swoft View Post
I believe that Ford used dual points on the V8 engines through 1948
Purdy you are right. My 59AB Flattie has dual points.

Biggest problem today is, the absolute crap-garbage-junk (did I mention trash) condensors that are coming in from China for these distributors.

A well known fellow here on FB that works on flathead distributors said some time ago he may go through 12-15 condensors before he finds ONE that is usable. And nobody really cares.

Thank goodness the Model A folks have the A&L quality condensors, wish they had them for early V-8's. I've been stalled out twice while away from home because of these.
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Old 09-20-2020, 08:16 PM   #18
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Default Re: Purpose of dual points?

Quote:
Originally Posted by miff View Post
what is the dwell on a model a points/
See this old thread, post #3:

https://www.fordbarn.com/forum/showt...eferrerid=1126

This is consistent with my measurements, too. I use a Model B cam.

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