Theo,
You asked for a reply, so I'll do the best I can.
"Smit, Theo" wrote:
> So the current summary of the distributor cap discussion is:
>
> 1. Big cap or small cap is mostly a function of the spark plug gap you run,
> not what you use to drive the coil. Either cap setup will fit early or late
> distributor bodies.
I believe you are only half correct here. Yes, the new cap will fit the old
body, providing, of course, the intermediate adapter plastic body and rotor are
used.
I believe you are incorrect on the need for the cap being a function of the
plug gap size, although it is indirectly correct. The optimum nominal plug gap
will be about double for a high energy ignition system, either Ford's or the
after market units which are much better. It is the high energy Ford system
that required the change.
If you are using a high energy system, the key driver here, you will get cross
firing inside the distributor cap, as the contacts are too close too each
other at those voltages, with a little arc dust build-up inside the cap. Ford
didn't add cost to make it pretty.
I am not that familiar with the later distributors with the built-in coil, but
I'd stay away from any part of them as they were either recalled or the design
discontinued. It was a piece of junk.
> 2. Optimal plug gap is probably in the range 0.040 to 0.050, which still
> allows the use of the small style cap.
With High Energy systems, which may be the unspoken premise of your letter, and
the start of your initial e-mail, but I do not believe you will get by with the
small cap, as above.
> 3. Whatever spark you see if you pull a plug, and ground it with the engine
> running (or just arc from the coil wire to ground), is not representative of
> what happens under high-speed running conditions.
Yes, It is. However, a good comparison to any wire cross fire, or poor high
tension ignition wire issues, is if the same plug, being transferred to
different ignition wires, exhibit different results, that should make you look
further. Looking at your running engine in the dark is another good indicator
of high voltage issues.
> I disassembled three distributors over the weekend and learned some
> interesting things.
> First off, two of the three had worn the upper distributor bushing so badly
> that the mechanical advance drive plate had worn into the distributor body.
> Why does this happen?
Ford's bushings are not the best. Good performance rebuilders use Oilite, I
believe, in a rebuild.
> Doesn't the thrust from the oil pump drive push the
> shaft up? Does the distributor drive get oiled mostly by accident, and the
> buildup of varnish and other cr*p causes it to run dry?
The original Tiger distributor, strangely like some early Lucas models, had a
flip-up cap for external oiling of the bushing sleeve. Later models eliminated
this, but the boss is still there where the flip-up cap used to be.
There is a wick, underneath the rotor, that requires periodic oiling.
> There's a helical
> groove in the distributor shaft but unless I've got my thinking backwards,
> it's there to pump oil back down so it doesn't squirt into the distributor.
> What assembly lube can we use that will alleviate this (bushing end wear) as
> much as possible?
Your thinking is correct. I use engine oil on the outside flip cap and the felt
wick under the rotor. A rebuild to a better grade bearing sleeve is a good
idea, and should cost about $60, with a re-set of your advance curve.
> Second thing: Are the numbers stamped into the advance plates, measurements
> in crankshaft degrees or distributor degrees?
Distributor degrees (1/2 crank degrees)
> Unfortunately I only have one
> plate to choose from that has a reluctor assembly (the other two have points
> cams), but it does have a choice of 21 degrees or 16 degrees advance (in
> whatever units they are measured in). Which should I use, or should I try to
> dig out a different one? The application is a warmed-over 289 (cam, headers,
> 600 CFM carb on an F4B manifold). The distributor will be a mechanical +
> vacuum advance, reluctor style pickup for an MSD6AL amplifier.
The 16 and 20 limit plates are for mechanical advance distributors. The vacuum
advance units are generally stamped 10 (dist. degrees). I use a 10 distributor
degree (stock) limit plate on the mechanical advance, 10 degree (distributor)
on the vacuum adjustable stops on the ACCEL replacement unit with the limit
stop adjustment through the vacuum hose inlet with an allen wrench (supplied).
I don't really count all the 20 degree (crank) vacuum result, as that is under
high vacuum (off the throttle, or low load) and won't cause pre-ignition. To
the 20 degree (crank advance, which I have tailored to be all in by 2500 rpm,
less if your system can take it) plus 9-10 degree static advance on the indexed
vibration damper. This gives a total of 20 + 10 = 30 + about 7 vacuum crank
degrees (guess). About what my engine tune (mild) and today's gas (poor) will
allow me to risk. I believe that 39 is absolute tops, according to others and
the Ford manual.
I hope this individual experience helps, although there are those with the
actual system you are considering, like Bob Palmer, who have "hands-on"
experience. And Bob is futzing with changing his already large gap about 0.05,
so he must be as much into precise optimization as anyone. He may even have
indexed his ignition key, as well - so don't bet on a drag race with him. {9->
If you want another opinion, I am sure you will be receiving more advice than
you ever wanted, some of which might even be correct. {9->
Steve
--
Steve Laifman < Find out what is most >
B9472289 < important in your life >
< and don't let it get away!>
http://www.TigersUnited.com/gallery/SteveLaifman.asp
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