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EFV-8 Club Forum / General Ford Discussion / Crab-type distributor help

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Discussion Topic: Crab-type distributor help

41PU

01-02-2014 @ 3:14 PM

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Can someone point me to an explanation of how the dual points work in a '42-'48 stock distributor? I don't understand the function of the 2nd set of points. In my distributor, the 8-lobe distributor cam appears to open one set a few degrees before the other. It would seem to me that would always leave one set grounded and the distributor should never generate a spark (my current problem).

ford38v8

01-02-2014 @ 6:05 PM

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The eight lobe cam opens and closes too quickly to provide enough dwell for the coil to build a good spark. Ford had a better idea: With two sets of points, dwell increases by the degree of overlap of the two sets of points. One set makes, the second set breaks. Hope you can read minds, I'm not getting my words out right today, sorry!

Alan

supereal

01-03-2014 @ 4:33 AM

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Good explanation, Alan. When we set up a distributor on the machine, we adjust the point sets individually to achieve the correct "dwell", or the total time the points provide ground to the coil so it can saturate. During development of the 8 cylinder engine, it was discovered that spark intensity fell as engine speed increased with a single point set, as Alan explains. Without it, the V8 would not have reached the market. At the time, the system was hailed as an "elegant" solution to a perplexing problem.

TomO

01-03-2014 @ 10:09 AM

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By using the make and break points, the Ford distributor has a combined total dwell of 36 degrees allowing full coil saturation. Each set of points is closed for 22 1/2 degrees with a 9 degree overlap.

The right hand set closes and current starts flowing from the ignition switch through the coil and points to ground, then the left hand set closes, next the right hand set opens but current is still flowing in the coil until the left hand set opens causing the field collapse and firing of a plug.

There are a couple of reprinted books that help explain the ignition system and other mechanical part in the V-8 flathead era. Here is a link to C&G catalog page for the CD and printed versions.

http://cgfordparts.com/ufolder/cgcat.php?searchtype=Search&year_choice=46&searchtextdesc=shop+manual&sd=Search&searchnumber=%3F

Tom

41PU

01-03-2014 @ 2:23 PM

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Great explanations gang, thanks very much. I'll put the second set back in my distributor. I've only recently returned to the flathead fold after a 50 year absence and am having to learn some things all over again.

Mel

ford38v8

01-03-2014 @ 3:19 PM

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I like TomO's explanation best. Tom's words always come out of his fingertips better than mine.

Alan

Stroker

01-03-2014 @ 4:37 PM

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Alan:

I concur with your platitudes regarding Tom's explanation. As you recall, I did a "diatribe" a few weeks ago on this subject. It is not an easy concept to explain, especially when many Baby Boomers equate dual points with dual coil high performance aftermarket ignitions, which of course are simply two isolated 4 cylinder set-ups. I came up with a stupid, but I believe relevant analogy some years ago that likened the Ford dual points to walking into a room with 3-way light switches, switching the light on, and when exiting the room out a far door, switching the light off. Of course this doesn't address the issue as to "why", and that's what I was trying to explain in my previous rambling expansive effort. Thank's Tom, you said all that needs to be said.

TomO

01-04-2014 @ 8:21 AM

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Thank you for the compliments. This is much easier to explain than the things that I had to explain when I was working.

Tom

FrankM-RG5

01-04-2014 @ 9:12 AM

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The way I understand it the points need to open a certain amount before the circuit is broken. From that distance they need to close. The dual points allow the circuit to break and eliminate time lost closing the "wide" gap.

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