> When he tested an older MSD unit he found that the
> advertised voltage, energy storage, duration, etc. did not stack up with
> his test results. He found the spark energy weak, presumably a negative
> effect from putting emphasis on multiple sparks.
With respect, Duncan, I think there must have been something wrong with the
unit he tested.
When I first tried a MSD 6A on my street TR3A back in the 80s; it would
erode the center electrode of the spark plugs until the electrode ended
inside the ceramic, and keep on running just fine. Since mixture, timing,
etc were unchanged, it had to be higher spark current causing the erosion
and obviously much higher voltage to keep firing the plug. I switched to
much colder plugs to reduce the erosion.
It also burned out all the carbon in the resistor in the distributor cap.
Once the resistor was non-conductive, it kept throwing a spark around the
resistor, eroding the tower in the cap. I was still driving to work every
day, there was no change in how easily the engine started or how well it
ran.
When it did finally start misfiring, I saw a fat blue spark, climbing up the
coil tower and running over to a side terminal! That was when I discovered
the damage inside the cap.
Tried to limp home that way (obviously switching back to points wasn't going
to work), but the coil tower couldn't take the heat and within just a few
miles the spark was going directly through the tower to the side terminal
and the engine wouldn't run at all. I was able to get it running again,
with the ruined cap & coil, by wrapping a paper clip around the resistor and
bending the tip to touch the rotor.
However, every MSD unit I've owned has failed within a few years of
installation, always in some strange way. One of them would simply stop
working for a minute or two, always during initial warmup on a cold day,
then work fine for the rest of the day. For some reason, always at idle
(usually in rush hour traffic waiting for the light to turn green). One of
them would work fine until underhood temps got over about 150F then abruptly
stop working. Open the hood to troubleshoot, and it would be working again.
And one of them became weak, hard to start and obvious low spark energy.
-- Randall
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