Dual point distributor

Discussion in 'General Automotive Questions' started by Clark, Jun 20, 2020.

  1. Clark

    Clark In Maximum Overdrive

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    I just happen to remember back in high school when the Johnson boy (new Army draftee) visited home on leave with a brand new red 1962 2-door hardtop Ford Galaxie. It was in my Dad's shop for something and Dad was showing me, I think a 390, the internals of the dual point distributor.

    So I'm trying to think how that works with two point sets and the purpose. I could understand if the distributor were built with an internal gear reduction so the the point cam ran at half speed to stop point flutter at high rpm.

    Can one of you gear heads explain?
     
    Last edited: Jun 20, 2020
  2. tony o

    tony o In Third Gear

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    V8 dual point distributers have 2 sets of points out of phase with each other by 9 degrees thus increasing the dwell by 9 degrees over a single point distributer. They also distribute the current load over 2 sets of points. The leading set handles the current when the points close, the trailing set handles the current load when it opens. This reduces the pitting and allows the use of a higher current coil while still maintains reasonable point life. Spring tension on the points is what controls the point at which the points will bounce or flutter. More tension won’t start bouncing until a higher RPM. This is analogous to valve spring pressure and valve float..

    I have never seen or heard of a gear reduction distributer. That would be an strange beast. If you used an 2:1 gear reduction on top of the 2:1 reduction of the timing chain you would need a 16 lobe distributer cam and as the rotor is now moving at 1/4 speed you av1y hole cap and 2 wires to each plug. I suppose you could add a second gear drive above the points cam and bring the rotor up to its normal 1/2 speed.

    the only drive that I have ever seen that is not 1:1 is an eccentric drive that speeds up and slows down the distributer 22.5 degrees per revelution. My buddy made it so he could use MSD v8 mag on a Harley. The mag Has 90 degree poles and a Harley is a 45 degree v twin.
     
  3. Clark

    Clark In Maximum Overdrive

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    I was thinking an additional 2:1 reduction only for the point cam while the rotor turned at the usual half crank RPM.

    But thanks for the explanation.
     
  4. 5.0 Chero

    5.0 Chero Bahumbug Staff Member

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    The main purpose was to be able to run a Flatter distributor cam lobe causing less point bounce at higher RPM . With the 2 sets of points being out of phase you can set each set with less dwell 26° (smaller gap) thus decreasing point bounce and still have total dwell at 32°. The secondary benefit is increased point life with higher primary coil voltage.
     
  5. handy_andy_cv64

    handy_andy_cv64 In Maximum Overdrive SILVER MEMBER

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    "higher primary coil voltage" gives you higher secondary coil saturation, a hotter, longer, spark. Why they were used in racing engines most of the time. Had to get through that 10.5-1 compression pressure.
     
  6. tony o

    tony o In Third Gear

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    Where are you getting the higher primary voltage from? The number of point sets won’t change the input voltage and that’s all the coil will see. Most vintage Fords with an alternator only have around 14v max and the ballast resister is going to drop the voltage going to the coil.
     
  7. handy_andy_cv64

    handy_andy_cv64 In Maximum Overdrive SILVER MEMBER

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    Actually, you're right; what he and I both meant is winding field strength. The more strength built up in the windings, the more amperage it will impart to the secondary winding, so as to build a stronger field in that secondary winding.
     
  8. 5.0 Chero

    5.0 Chero Bahumbug Staff Member

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    The higher voltage comes from less resistance in the primary feed wire. with the key in the run position you are only getting abort 8 or 9 volts to the coil. Ford dose not use a "balance resistor" it uses a resistance wire in the primary feed. Shorter wire more voltage.
     
  9. tony o

    tony o In Third Gear

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    Sorry about the confusion Caused by incorrect use of terminology. To me 3’ of resistance wire at .5 ohms per foot and a 1.5 ohm ballast resister are equivocal. Functionally the same only a different form factor. Back to my original question. All else remaining equal, how does installing a dual point distributer in place of a single point yield a higher primary voltage? I understand that reducing the resistance in the primary side coil circuit will increase the potential voltage at the spark plug. The actual voltage the spark plug will is the minimum voltage necessary to ionize the spark gap. At that point the coil will discharge.
     
  10. Hillbilly

    Hillbilly In Maximum Overdrive

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    Inquiring minds and all that crap want to know = Does the coil unload to the spark gaps when the contact points are closed or when they are open ? Or looking at the process of "Dwell" , are we measuring the time the points are closed or when they are open ? Reminds me, I need to charge up a few condensers to strategically place on my shop benches for the boys to find tomorrow. Yeah, this is a test.
     
  11. tony o

    tony o In Third Gear

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    Coil is discharged when the points open. The field in the primary collapses and induces voltage in the secondary. Dwell is the saturation time in degrees on the primary side of the coil when the points are closed.
     
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  12. handy_andy_cv64

    handy_andy_cv64 In Maximum Overdrive SILVER MEMBER

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    Couldn't've said it better.
     
  13. tony o

    tony o In Third Gear

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    Oh good, then I passed. Hillbilly do you give letter grades or just pass/fail on your tests?
     
  14. Hillbilly

    Hillbilly In Maximum Overdrive

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    Pass / Fail , nothing incriminating goes to the wife or mother to use to beat on you later. Pretty sure everyone on this site has a good handle on the basic old school engine needs. Kicking and screaming whilst' being drug into the modern tech world is the new normal. I don't remember signing on for computer programing but it seems to be required if you plan on do it yourselfing today's rides. Too much of a cheapskate here so this old fart has to learn new tricks on components I wouldn't even dream of fifty years ago.
     
  15. tony o

    tony o In Third Gear

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    I have to admit on today’s cars it convenient to plug in a scan tool and let the computer tell you what’s wrong. Order the parts, whack a few you tube videos on how to get it apart to fix it. It’s amazing how cars snap together. When you own old stuff like our Ranchero’s you have to be able to diagnose and work on it yourself. Although it might be good fun to take it to a shop and watch the repair tech trying to find the pug for the scan tool.
     
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  16. handy_andy_cv64

    handy_andy_cv64 In Maximum Overdrive SILVER MEMBER

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    Most of us are gearheads, so we usually know which side our bread's buttered on. What's really fun is when you have a foreign car that's computer-controlled, but only a factory scan tool can plug in (especially Euro cars), so you have to use a cheat to blink the dash light, or you have a Ford EEC III, where you have to have a vacuum pump to pull 20" on the BMAP sensor's barometric port, to get it to spit out codes. Man, those were the days.
     

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