Dave,
It's probably not the dual points that Duchess likes. It is more likely that
the new distributor is in better shape. That's why she idles better.
The real purpose of dual points is to increase the dwell time. This is the
time that both points are closed and the ignition coil is being charged. The
dwell angle for single points is typically 30 degrees and for dual points is
36 to 39 degrees. At an idle or low RPM, the coil reaches saturation
(maximum charge) before the dwell time is over and the points open, even
with single points. At higher RPM, the dwell time shortens and becomes less
than the dwell time needed to saturate the coil. When this happens, the
spark voltage starts to fall. With dual points, the dwell time is longer at
higher RPM and the voltage is maintained to a higher RPM. Dual points only
have an advantage at higher RPMs.
A distributor is realy more complex than just having good points. It has
several interacting systems that affect performance. If any one of these is
not up to par, performance may suffer.
When I first got my rebuilt 56 D500 running, I used a known good single
point distributor from a 56 Dodge 315 Poly engine, while I refurbished the
original dual point D500 distributor. When I installed the original D500
distributor later, there was a significant drop in performance. I asked
myself "why would a performance engine's distributor give less performance?"
Then I researched the specifications for these two. It turns out that the
D500 has a very conservative advance curve. I think that the factory
engineers were concerned that the D500's higher compression ratio and cam
would have problems with the gasoline of the day. The Chrysler 300B uses the
same distributor. Then I looked into the specs for other 55 to 57 Mopar
distributors. Their specs are all over the map. EVERY engine was different!
Some of the "low" performance distributors actually have more agressive
advance curves, because their engines have lower compression and can
tolerate more advance. You can mix and match parts to adjust your advance
curve to almost anything you want.
The systems that can be varied are:
1. Total mechanical advance. This is controlled by the length of the slots
in the "point cam and plate".
2 Rate of mechanical advance. This is controlled by the strength of the
advance weight springs. Typically one heavy and one light spring.
3. Total vacuum advance. This is controlled by the amount of travel in the
vacuum advance chamber.
4. Rate of vacuum advance. This is controlled by the stiffness of the
removable spring in the advance chamber.
5. Start of vacuum advance. This is controlled by the length of the spring
in the advance chamber.
6. Initial timing at idle. This is set according to the tune up specs for
your engine.
The selection of parameters from all of these variables by the factory
engineers is a compromise between performance, fuel economy, available fuel
quality, expected driving conditions, stress on the engine, breathing
capability of the carburetion, camshaft profile, compression ratio, etc. The
mechanical advance curve is mostly responsible for performance during
acceleration at full throttle. The vacuum advance affects fuel economy and
partial throttle performance. Too much of either or both can be deadly.
For my distributor, I made several changes:
1. Increased the total mechanical advance from 19 degrees to 30 degrees.
2. Decreased the total vacuum advance from 23 to 16 degrees.
3. Changed the heavy mechanical advance spring to a light spring.
4. Set the initial timing at 5 degrees.
5. Changed the vacuum advance spring to a softer one.
These are closer to the recommendations provided in the "Mopar Performance
Engines" book, P/N 4452790, for performance engines. Works much better!
Well, I did it again and got carried away. Enough for now,
Dave Homstad
56 Dodge D500
---------------------------------------------------------------
At 09:55 PM 3/30/99 -0500, you wrote:
>Well,
>
>Since I gave up smoking in February, I've been finding myself always
>cleaning and cleaning to pass the time. Tonight was "clean the kitchen"
>night, and strangely enough, I found a dual point distributor (IBP-4003)
>in my crock pot.
>
>How it got there is anybody's guess.
>
>I had rebuilt it back in '97, but somehow I forgot all about it. I
>decided that since it was a nice night, I'd throw it in the Duchess and
>see how it worked. To my amazement, the Duchess is simply
>purrrrrrrrrrrrrring now. Dual points, she likes! I never realized how
>bad my other distributor was until now, but with the new one, many of my
>engine problems are now gone. The idle has smoothed out to a
>perrrrrfect purrrrrrrr, the power has increased about fivefold, and she
>starts with just a flick of the key.
>
>My first question is, is this a "dual-point" thing, or simply a
>"distrubutor that works" thing?
>
>Question number two: She always has to warm up for about five minutes
>in reverse before the Torqueflite will pop into any forward gear. A few
>of us on the list here have the same problem, but none of us have been
>able to track down the cause, let alone the solution. As a
>possibly-related sideline, the trans won't kick down when I stomp the
>pedal, either. Anyone have any thoughts on a fix before I have the
>whole thing rebuilt?
>
>-Dave
>
>PS - It was a nice night for cruising in Western Pennsylvania. I'm
>loving the summer again already!
>
Dave
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