[TR] electronic ignition vs CEI, HEI vs CDI - a little technical

[Randall]

FWIW, I ran a point-triggered multiple spark CDI system for many years on my
TR3A.  There were some teething problems, but overall it worked very well.
I eventually tried some other systems, mostly because the bushings in the
distributor were getting pretty badly worn and I was seeing a lot of spark
scatter from the points.

I can't say there was a significant change in max power, but with the MSD
the engine always started a lot easier and throttle response seemed crisper
at low and medium rpm.  When (and if) I revisit the topic, I may try a
Pertronix pickup combined with a MSD module in hopes of getting the best of
both worlds.

As I see it, the multiple coil systems are mostly to eliminate the spark
plug wires and distributor (cap & rotor), all of which are problematic from
an emissions point of view.  There has been a lot of pressure from the EPA
to build engines that will continue to meet emissions targets without having
a tune-up for 100,000 miles or more; as well as eliminating "user"
adjustments (like distributor position).  CDI systems are always more
complicated (and hence expensive) than Kettering, which is why most car
makers stick with Kettering.  And with precision mixture control, the
ability of CDI to fire wet or fouled plugs is much less of an advantage.

The teething problems I mentioned seemed to all be related to the MSD
delivering much more current and voltage to the plugs.  I found that plug
life was shortened unless I ran colder plugs (or platinum); and the high
current was apparently resposible for the suppression resistor in the
distributor cap burning out.  Apparently, even with all of the carbon burned
out of the resistor (it was literally white), the spark continued jumping
around the resistor until it burned away the tower in the cap.  The first
time I noticed anything wrong was when the spark started jumping from the
center tower of the coil to one of the side terminals!  I was able to limp
home (actually it ran fine) by fabricating a replacement button from a paper
clip.  On the new cap, I removed the original resistor button and replaced
it with a chunk of relatively pure carbon (carved from the center post of a
flashlight battery), which was still working fine many years later when I
decided to try a Crane XR3000 (optical triggered CDI).

I could be wrong about this, but I think you will find that a GM HEI module
will work just fine with a 1.5 ohm coil; at least on a 4 cylinder engine
turning no more than 5000 rpm.  Due to the current-limiting nature of the
HEI, it will still saturate the 1.5 ohm coil, it just takes longer than with
the lower inductance 0.7 ohm coil.  The longer time to saturation means the
spark energy will fall off at higher rpm, but since you only have 4
cylinders to fire, that will be twice the rpm where it would fall off with a
V8.  And still higher than with the stock points & 3 ohm coil.

-- Randall