Ooops, I said you were working on a "Marelli" but I meant to say "Ducelier" - sorry for any confusion.
As far as ignitions go, any idea how the GM modules compare to any of the typical aftermarket CD units? I know some aftermarket units have a "multiple spark" feature, although I've never been fully convinced that is any better than one spark (I seriously doubt the fuel needs to be ignited several times before it burns) - but that's just an opinion. Aside from that, is there much difference between the GM unit and the others? Or for that matter between the standard GM module and the 'high performance' GM type modules?
The GM modules are for a Kettering (inductive) type of system. They basically replace the points with a semiconductor power device. I can't say what the premium equivalent part numbers seem to offer over the basic GM part, but there are other, new designs out there as well. Over on my Bosch schematic post, Dom offered up a couple of interesting links on these modules in general:
http://dtec.net.au/High Energy Ignition Upgrade.htm
http://dtec.net.au/Ignition Coil Dwell Calibration.htm
One of them discusses a Bosch module with built in dwell control, and the other discusses the concept. The GM module is an old design, and it is interesting to see how things have progressed. As far as how they compare with CD units, I would guess that for most applications you would not notice much difference. The main advantage of CD units is at high rpm where the Kettering system starts to run out of bandwidth due to the time it takes for the primary coil current to rise. I don't know how important that really is for a 4 cylinder engine. Even at 8K, the X ignition system sees what a V8 system would see at 4K so I think the GM module has that covered. Another advantage of CD is at cold cranking conditions. Well designed units will provide full spark down to pretty low battery voltages.
On the down side, back when CD ignitions became popular in the 70s, there were a number of designs being sold that were not particularly robust and of low manufacturing quality. In fact, many of them had a built in switch so you could go back to points after the box failed - like they were expecting it (and they were right!). Having been involved with supplying electronics to the large auto manufacturers, I can tell you that back then, none of those CD products would be suitable in an OEM application. Of course, back then, semiconductor power devices were also nowhere near as robust as they are today.
My friend had a 72 Mercedes coupe with the 4.2 V8, and he had an intermittent problem for years of the engine randomly dying and then recovering some time later. They took it to several experts who replaced a number of things but never figured it out. One day, we decided to connect up a number of instruments and pilot lights to various parts of the ignition system and go out for a drive to monitor where the problem might be. It turned out to be the power transistor in the Bosch module (similar to the one in the X) suffering from collector-base channeling due to ionic contamination (common semiconductor issue back then). After operating a few minutes, the internal electric fields would cause positive ions (usually Sodium) to collect above the surface of the base and cause an N type channel to form across P type material, creating current flow where you don't want it. Problem is, once the electric fields are gone, the ions disperse along with the problem making it difficult to detect unless you are watching in real time. We replaced the Bosch module with an MSD CD unit and he never had a problem again.
If I were buying a unit today, I'd probably go with an MSD as they have been around for a long time and seemed to have figured out how to come up with a robust design. Back in the late 70s when they were still pretty new, I bought some of their helically wound ignition wires. They were hyped as having "Magnetic Suppression Discharge" technology (not sure if that is where the name MSD came from). They actually had a set made for X1/9s (which were popular then). I am still using those wires on my X and they still test fine for resistance.
The unit I am going to play with is an old Delta Mark 10B from the 70s. As far as reliability goes, it was probably one of the better ones back then although that is not saying much. This one still works so at least I don't have to worry about infant failures at 40+ years. They later came out with a multiple spark version (Mark 10C??). I really don't know whether or not they are better under certain conditions but it is a relatively cheap circuit feature to add and I doubt it would make anything worse.
