When you are using a dual point distributor ( or a single point
one for that matter ) with just a coil you can get a fair amount of
current flowing through the points. They arc when they open, and
they arc when they close. That's one benefit of the dual point, one
set arcs on opening/closing but not on closing/opening because
the other set is handling the current at that moment. Only half
as many arcs per point set, so they last twice as long, since
the arcing is what contributes to the pitting and deterioration
of the contact surfaces.
With the addition of an electronic box like Mallory, MSD or
Crane units, the points carry very little current and only provide
an on/off voltage signal to trigger the electronic box, which does
the dwell and saturates the coil and such by itself without any
further help from the points. No arcing to speak of, points last
basically forever ( Well, in race car years, that is )
I'm guessing that the electronic units in conjunction with a
dual point distributor can possibly sense the extra close or
open of the second set of points and get confused, thinking
that the engine is running half as slow or twice as fast, or
is an 8 cylinder instead of a 4, or maybe a 2. Hey, it has been
over a quarter of a century since I took graduate level electrical
engineering courses, I'm just talking out of my, uh, hat here.
Suffice it to say that with an external ignition controller, you
only need one set of points to trigger the timing cycle.
One a sort of related note, I ran a Mallory dual point in
Killer. I needed a new cap for one weekend. Local speed
shops had no 4 cylinder ones, so I bought an 8 cylinder one,
installed it and put the plug wires into every other tower.
I was amused by the folks who would notice that, and claim
that there was no way it work, you can't do that, etc.
mjb.
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