Hi Dave and the eminent listers,
>So this is a dual point distributor, right? I don't understand the setting
>tool. If it is driven of the distributor, then both sets of points must
>open and close about the same time. It's hard to understand why Ford would
>create a situation that would require substantial tooling to setup up a
>distributor. Curious.
>
This needs a bit of explaining, bear with me.
There are two reasons for dual point distributors; firstly in the typical
case of post war RR eight cylinder engines it provides eight sparks from a
four lobe cam so the whole assembly including cap can be used on four
(single point) and eight (twin point) engines for supply logistics.
In the case of the early flat-head V8's as you find in Ford CMP's and
Universal (Bren)Carriers it is to provide an adequate spark.
I have the instructions before me for the universal tool Part No. X-12127-D,
made by the Wasco Valve Seat Company dated Jan 31 1944. It would appear
that many American cars of the period used the 8 lobe, twin point
distributor and this tool was made to accommodate various types.
The problem arises because the 6V systems used then could not store enough
energy for an adequate spark in the ignition coil in the short dwell period
available with an eight lobe cam.
It is a physical law that when a voltage is applied to a coil it inherently
opposes any current flow as the magnetic field builds up, the expanding
lines of magnetic force cut the turns of the primary coil and set up an
opposing voltage, a common effect known as back EMF.
The primary coil of the ignition circuit needs to be current saturated to
achieve the maximum magnetic field so that when the points open and the
field rapidly collapses cutting the thousands of turns in the secondary or
ht coil a healthy spark is generated by the many KV generated. There is
also of course a commensurate back EMF again that would cause a spark in the
primary circuit too and this is suppressed by the condenser (capacitor)
across the points.
The 6V supply is inadequate (think of voltage as pressure) to generate the
required field in the time that the points are closed on an eight lobe cam
so a second set, connected in parallel, are provided to extend the dwell
time.
In essence one set of points opens early but the others are still closed
allowing the field to build up, then the second set open causing the spark
and the first set then close again before them extending the dwell time.
According to the trigonometry in this sheet a single set of points would be
closed for only 22.5 degrees, whereas using the twin points a dwell angle of
36 degrees is obtained, this is achieved by the 22.5 degree closure of each
set with a 9 degree overlap where the spark occurs.
It is very important therefore to time the second set of moveable points
very accurately, I'll happily scan the five pages referring to the setting
of both the pre (divers helmet) and post 1942 distributors should anyone
want it.
Richard
(Southampton UK)
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