MSD 6A

[Bruce]

From: "Ron Schroeder" <rjs at bnl.gov>
Subject: Re: MSD 6A
> >Before even looking at a CDI, I'd see if the oem is salvagable.
> >Alternatives are Jacobs, and Crane.

> My question about MSD is even more basic:
> Is the multiple spark really ANY advantage?  Or is it just better than a
> points ignition that has worn out points?

  They have to run multipule sparks to try and light off the chamber, while
the DCI has a higher open voltage, the current is much less, so you need to
run several sparks to get enough spark duration to light of the reaction in
the chamber.  If you look at one running on a high rent oscope you'll see
immediately what I mean.   Also, if you run a single spark CDI you'll see
how really short the firing line is.
  There is a *success* rate that is a whole lot more important, and it
breaks down into ignition types, basically.  Point (95%), HEI (96%), DIS
(98%) and coil near plug runs at 99%.   This isn't a misfire rating, but
rather how often the success rate is of successful, ignition.  (I beleive Dr
J quotes it in his book).  Any one notice that CDI isn't a qualifier in the
stats.
   If you sit down and look at how an auto transformer works, you'll notice
that it takes several rings of the primarie's collapse to generate a high
enough secondary voltage to cross all the airgaps and fire the plug.   With
a CDI instead of several rings, the first pulse thru the coil is 400v, so
the secondary voltage is immediately available.   So the volatge isn't
offered as long as time to leak thur say the center electrodes porclean.
Remember on a cold start all the plugs get a splash of raw fuel to deal
with.  (Feel free to read further on some of the works about what actually
atomises the fuel for ignition).  (Also, be sure to cover the events at
valve overlap).   In a well calibrated EFI car it would be fun to try
advancing the timing to see if that generates similiar cruise results.
   So about now you can see why they all claim, and in practice do generate
easier cold engine starting.   Owwww, but remember for ever advantage there
is a disadvantage.  The opposite of starting is what?.  WOT.  So when you
need the max firing energy (note not voltage or amperage, but a combo of
both), the DCI just can't compete.  Where it does help is on a chamber
design that perfers a short hotter flame kernel.  Now the instant you
mention flame kernel, plug gaps and timing come into play.  Going from .045
to .030 gap will demand a timing change, to maintain the same performance.

I could tell no difference from a Tiger Ignition to a MSD.  On the scope or
in the car.
I've never seen a WOT performance gain using CDI, unless the non-CDI had an
existing problem (often folks take the path of least resistance then cure
the actual problem).

Optomizing an oem igntion is very time consuming.  ie, you'll measure all
the various brands of cap / rotors for finding the min air gap, for
openers.....

Yes, a proper CDI will generally outperform any worn out point set.

> Same type of question about Jacobs:

Looking at the patents will show the differences from Jacobs to any other
CDI.

> Is too much of a spark really a bad thing like Jacobs says?

Personally I haven't seen it.
If you play with some of the GM Smart Coils, you'll see why they are *State
of the Art*.
Hopefully I'll have some further info in real world circumstances with
running CNP stuff
Bruce


> Ron Schroeder


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