Fellow listers:
The fog has lifted, and a picture is beginning to form! Thanks to all the
input from all you experts, I now know how, and I think I know why, dual point
distributors function. Let me run this by you all, and see if I am on the
right track.
To produce maximum spark energy, a coil needs time for the primary field to
build.
This time required is constant, regardless of rpm.
The time allowed for this is determined by the point dwell angle.
Dwell angle is constant with respect to rpm.
Since dwell angle is constant with respect to rpm, the time allowed for the
primary field to build is reduced as rpm increases.
Dwell angle is determined by the point gap.
To maximize dwell angle (and to increase the time allocated for charging the
coil), point gap is minimized.
If dwell is maximized by making the gap too small, reliable operation is
jeopardized, and erratic triggering occurs. Also, the time between gap
resetting due to wear becomes short.
How do you increase the dwell angle, without increasing the point gap? One
way would be to open the points to a wide gap, but use a very steep cam
profile on the distributor rotor to snap them open quickly, and use a very
heavy spring to snap them back together very quickly. This might work, but it
would be at the expense of excessive wear on the point rubbing block, and
still would give problems at very high rpm. Enter dual points!
As several of you have pointed out, the dual points are wired in parallel, and
operate a little out of phase - point set #1 opening and closing just a little
before point set #2. What a clever idea! Since nothing happens till both sets
of points open, this first set can be opened as gently and as wide as need be
for reliabilty/accuracy. When the second set opens, the coil will fire. This
second set can also be opened gently and wide, because the first set can be
timed to close immediately after the second opens. Even if the first set
closes gently, if the timing is adjusted correctly, they can be closed in
plenty of time to start the coil charging process, maximizing the dwell angle.
Does all that sound correct?
In my original post, I said that you could not get a hotter spark from a
single coil no matter how many points you use. Theoretically, that is true,
but, as I now understand, from a practical standpoint you can indeed get a
hotter spark with a single coil by using dual points. I sit corrected - and
content, knowing I have learned something.
Thanks to everyone for setting me straight.
A few other observations, if I may. Maximum dwell angle on a V8 is 45 degrees.
Maximum dwell angle on a four cylinder engine is 90 degrees. Thus the charging
time for a coil on a V8 at 5500 rpm is the same as 11,000 rpm on a four
cylinder. So, unless you run your MGB at 11,000 plus rpm, any coil that would
work on a V8 would be more than adequate for use on an MGB. Any increased
performance gained from the hotter spark from a dual point set-up would have
to be from the fact that the dual point set-up is just a better distributor,
mechanically speaking, then the worn out stock distributor it replaced
Dan Masters,
Alcoa, TN
'71 TR6---------3000mile/year driver, fully restored
'71 TR6---------undergoing full restoration and Ford 5.0 V8 insertion - see:
http://www.sky.net/~boballen/mg/Masters/
'74 MGBGT---3000mile/year driver, original condition
'68 MGBGT---organ donor for the '74
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