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

From: [email-masked]
Date: Mon, 26 Nov 2012 13:19:35 +0000 (UTC)

I have been thinking about adding transistorized ignition to my TR4A. I had thought about adding a transistor switching unit. I have a Pertronix right now, but I am not at all sure it really adds any real benefits over a good condition points system, other than not having points. But with a failure on the road it,s a pain in the neck to reinstall the points parts. Plus the push-on magnet ring does not fit well on my distributor. I was thinking about going back to using points and add a transistor switching unit like the Boyer-Brandsen Kit0069. That way if there were a failure, it would be real simple to go back to points alone. With the transistor switching unit, the points will not arc and the contacts will last essentially forever (maybe not the rubbing block, though) 

I have the Delco HEI sysem on my TR7 and I have done a bit of research on it. It is a really nice system but requires a special low resistance (and lower inductance) than the normal 3 ohm or 1.5 ohm coils. It limits current to 5.5 amps for the stock HEI unit and adjusts the dwell to JUST get to the current limiting state before the ignition fires. This minimizes coil heating so it won't burn out at these high currents. 

I thought I might apply the HEI system to the points triggered TR4A distributor. But if that is done, then dwell control is lost unless you use some fancy electronics to convert points into a reluctor system. But then I saw that the Lucas CEI ignition has the GM HEI module at its heart! What coil does the CEI system use? I have not found reference to any special HEI-style coil being used with the CEI. 

Does the CEI system have a special HEI module that limits current to something less than 5.5 amps? Or does it have a special HEI type coil? Or does it lose the ability to adjust dwell and current and become a plain transistor unit like the Boyer? 

Here, I am going to get a little technical. I was thinking I could make my own pseudo-HEI system by making a regular transistor switching unit, but then add a current limiter. This would be a "12 volt" system, but would use a 6 volt (1.5 ohm) coil without a ballast resistor. The current limiter would replace the ballast resistor. The current would be limited to that which the coil would get at idle RPM with a ballast resistor.This allows the coil to saturate more quickly (or at higher revs, just be closer to saturation) and therefore would maintain spark energy to higher revs somewhat like what the HEI system does. Also, this design would allow more current to get to the coil when battery voltage is reduced during cranking, similar to the ballasted systems. From my calculations, I think I should be able to have double the spark energy at 4000-5500 RPM. 

And then there is Capacitive Discharge. I can see that CDI would be a good design for very high RPM, like well over 5500. But I am skeptical that it adds anything compared to a well maintained Kettering (points or transistor) system at the RPMs we would have on the street with a maximum of 5500 RPM. I suspect that the very brief spark(s) of the CDI system may not ignite the mixture as well as a nice long 2-3 millisecond burn of an inductive system. The other reason that I am skeptical is that auto manufacturers continue to use inductive systems. They have gone to great lengths to do so, resorting to things like one coil per plug, coil-on-plug, etc. If CDI were superior, it seems to me that it would have been a LOT cheaper to use CDI than make these complicated inductive systems. 

I am looking for people's thoughts on these ignition systems. 

-Tony 

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From: "Randall" <[email-masked]>
Date: Mon, 26 Nov 2012 08:03:49 -0800

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