Re: [AMC-list] sheesh! f'!@!@!@ing

[tom jennings <tomj@xxxxxxx>]

It's a subtlety of the physics of spark coil design, trying to deal with the
limits of the old points type Kettering spark system...

Coils store energy in them in a magnetic field. The catch is, the current
flowing INTO the coil starts to make a magnetic field inside the coil, but
paradoxically, that same magnetic field actually opposes the incoming
current. THe end result is, it takes quite a few milliseconds to get the
spark coil magnetic field fully "saturated" (what it's actually called).
Once it's charged up, the current simply maintains the magnetic field. The
instant the current turns OFF (eg. the points OPEN) the magnetic field
"collapses", and all that magnetic energy causes a huge wallop of current in
the coil. All those amperes of current at only 12 volts that took many
milliseconds to charge up, now collapse into a huge high voltage spike. THe
catch is, for only some microseconds (no free lunch). It trades charge TIME
in the primary side of the coil, for VOLTAGE in the secondary.

(Distributor "dwell" is the time BEFORE the spark that is used to charge the
coil. You basically want as much dwell as you can dial in, consistent with
enough time to discharge through the plugs, bounce, etc.)

OK, why the ballast resistor? It's part of a design tradeoff -- decrease
coil inductance to decrease charge time (faster charging for higher engine
RPMs of modern engines) but at low speeds, where the RPM gives plenty of
time for the coil to charge, the current into the coil after the coil has
charged up, turns into heat, eats points, and makes the coil very hot.

So the compromise is, a low-inductance coil that charges up faster, but
needs a resistor to limit total current. The other downside to this
arrangement is it consumes a LOT more energy that simply turns into heat.
It's wasteful.

The real solution is capacitive-discharge and other schemes beyond the
simple Kettering system.

The bottom line is, the COIL is what determines whether it needs a resistor
or not. The epoxy coil from Pertronix wants one.

Confounding this is, some very smart electronic ignitions actually monitor
the current flowing into the coil, and control all this crap in software.


Personally I'm switching my cars to Ford EDIS wasted spark. I'm tired of
worn out, sloppy, never-quite-right distributors. I just ordered the last
part I need, the Megajolt Like Junior kit. I got all the other parts now and
the 36-1 wheel is mounted on the 195.6 OHV already. I can't believe there's
not a kit for the 232/258/4.0 yet.
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