> Need a new distributor. Would someone explain to this mechanically
> challanged individual what the advantages/disadvantages of a Mallory
> Dual Point vs a Ignitor Electronic Ignition. Recommndations on which
> would be better.
Well, shoot. I was doing so well, too. But I've been smelling some mental
farts on this thread so I guess I gotta butt in and clear the air
(pitiful metaphors, all).
BIAS: I have a Mallory Dual point in my '69C/GT and I like it alot.
The standard LBC uses a single set of points and it probably works pretty
good -- when they were new. Now the dizzies can be 20+ years old and
things do wear. The shaft in the distributor is held in place by a really
fat bushing in the lower end of the dizzy -- actually the end that is
stuck into the motor.
The shaft extends through the bottom of the distributor housing, below
the point's plate, and this is where the centrifugal advance weights
live. If you've ever looked in there you might be aghast that some
ignorant DPO must have replaced one of the springs because they are
different sizes. Not so, Toto. The springs are supposed to be a little
different so that the mechanical advance is not linear. That is, you may
get a little mechanical advance at low revs and then that spring maxes
out. The second spring then provides a little more advance at higher
revs. That's the way it works. Yes, if you think about it, the mechanism
naturally unbalances the shaft.
The cavity where the mechanical advance lives is a dirty, grimy place.
Lots of water and grit can get in there and cause wear on the posts that
support the mechanical advance weights and, generally, cause them not
work the same every time. Also, any crap in there can eventually get down
into the bushing itself. Those of you that have your motors out for the
winter project should have the lower end of the distributor soaking in
clean oil. A small baby jar works well.
Above the mechanical advance is the points plate. The cam in the
distributor that rides against the points is a good two or three inches
above the distributor bushing. Guess what a little bit of play at the
bushing becomes when the slop is extend three inches up? Thus, on a worn
distributor, the cam is mostly rotating but also wobbling. And when the
wobble is the right frequency, it isn't so much pushing the points up as
clubbing them open. So, when the wear gets bad enough, you go through
points pretty fast.
Also, the vacuum advance, if so equipped, effects the points plate. That
is, the "points plate" is actually two plates sliding one atop the other.
The vacuum advance can also change the timing based upon engine load and
some other design factors. This is mostly a good thing. But, again, these
are wear items and can eventually bind. Binding causes additional stress
to the rubber diaphragm inside the advance unit so that thing breaks more
often, too.
Finally, as the points plate actually slides, there must be flexible
wires between the distributor body, the lead coming in from the coil, and
the points plate. This is done with two wires. On old distributors the
wire insulation is mostly deteriorated and you can even see the exposed
wire. The wire itself seems to have a lot of very thin metal strands.
Actually it was made in Ireland from thinly-sliced cat scrotums soaked in
boiling mercury and can no longer be purchased. (Okay, to be honest, I
don't know the order of the production process. They may have just dipped
the whole Tom and did the slicing later. But I digress.)
Now we go above the points and we get to the distributor rotor. Once
again any play is exaggerated as we can now be 4 inches above the dizzy
bushing. Play here will change the air gap between the rotor and the
distributor cap. The rotor and cap control the secondary side of the
coil. That is, the points have already opened, the lead from the coil to
the points is now open, the primary field coil winding collapses. That
causes the secondary field of the coil to collapse also, and a big spark
spits out of the top of the coil. This leads to the center of the
distributor cap which connects to the hat atop the rotor, and now the
rotor is looking for someplace to release the voltage.
This happens as soon as the rotor swings around to an electrode inside
the cap. Boom! A sparkplug ignites. However, if the air gap changes due
to shaft wobble, the timing can change a degree or two at different rev
ranges. Also, as welders know, the longer the air gap, the more amps
needed to make the jump. Amps make heat, heats kills electrical stuff.
Okay, so there is a computer religion that says that black-box ignition
is a good thing. "I replaced my points with the black box and the car
runs better and is maintenance free." I hear. But if it is installed in a
worn out distributor only a third of your problems are solved. The black
box often uses an air gap to trigger the spark (nothing touches the cam)
so there isn't any "points" wear. But the advance is still erratic and
the wobbly shaft still changes the timing at the spinning rotor.
And, finally, when the black box dies -- not if, when! -- you're hooters
high looking for a wrecker. I have never seen an ignition failure that
could be attributed to points-type ignition whe a car was properly
maintenanced. I have seen poorly running cars "fixed" at the side of the
road due to poor ignition maintenance. Besides, with our LBCs, you are
going to tune it up at least every spring anyway. I have been stranded
twice in modern iron when the smoke leaked passed the mirrors in the
black box.
So I don't think much of the simple black-box computer miracles.
So, like water pumps, alternators, and wheel bearings, your distributor
is going to wear out. What you do about it is up to you.
Mallory Dual Point:
A Mallory Dual point will not give you one watt of hotter spark. That is
a function of the coil you're running and is marginally related to having
sufficent dwell to charge the coil. But any coil rated for a V8 at
"normal" RPM will work in LBC at twice the RPM -- within adequate dwell
specs.
With dual points, it does halve the electrical load accorss the points.
The points last longer and the dwell is much more stable. With the
typical set of lucas points, the dwell changes as the RPMs increases
because the points are fling longer and bouncing longer. With the dual
points, you will find the dwell is effectively constant.
If you feel horsepower can be made through proper spark, the Mallory
provides an adjustable total advance. You can also order different
springs for the unit. The adjustable advance is through a slot in the
points plate and a bendable tang coming up from the mechanical advance
mechanism. You can bend the tang to get more or less total advance. I
have seen sweeps of 5 degrees adjustment but I have not tried to test the
limits. And adjustable timing light is just the ticket to fiddle with
your total advance timing. For all the gas given to J. C. Whitney, their
adjustable-advance timing light is a SunPro unit that works well.
Finally, the Mallory distributor is a quality unit. It has first class
components and I am completely satisfied in it's service. When you buy
one, you do have to install a new drive dog and pin. It's very difficult.
It took me $15 at a machine shop and an afternoon.
I am always amazed how much money people will spend on carbs, headers,
and cams in search of higher performace but never give a thought to a
steady, consistent, verifiable, and rpm-adjustable timing mechanism.
--
Bob Allen, Kansas City, '69CGT, '75TR6, '61Elva(?)
"If you're thirsty and like beer, Coors Light does the job. If you lack
the palette to knock back a cold beer, then sip all that sissy stuff and
make sure to keep that pinky straight."
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