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Electronic Ignition Comparisons

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While the discussion is on electronic ignitions, does anyone out there know anything about CDI (capacitive discharge ignition)? I have a unit that has been kicking around in my parts bins for years, but which I have never used, not knowing much about it. It is brand new, in the box. According to the enclosed instructions it fits between the coil and the dizzy and has a switch on it that can be switched from standard ignition to the CDI. It looks kind of neat and hi-tech in an old fashioned sort of way. Saw the string on electrics and thought I'd ask if anyone knew anything aboout them!
 
Per my understanding, virtually all electronic ignitions are capacitive discharge (CDI); it's just another name. In a conventional ignition, the coil has full current when the points open, and the capacitor (or condenser, the more common automotive term) is then charged by the coil current to control the buildup of coil primary voltage. In capacitive discharge, the capacitor is first charged and then allowed to discharge through the coil, creating the spark. I'm not sure why it is done this way; probably, it's easier to make reliable electronics at reasonable cost.

Having said that, I confess that I've occasionally seen circuits for non-CDI electronic ignitions, in which the points are simply replaced by an electronic switch, and in all other respects, the ignition operates like a conventional one. So, I guess it can be done, but it's probably not the best--or cheapest--way to do it.
 
CDI is not the same as electronic ignition. Google the topic for more information. Our standard (Kettering) ignition systems are inductive due to the collapse of magnetic fields in the coil. The condenser in a standard system is there to absorb the energy from the primary windings as the coil's magnetic field collapses. By comparison, CDI involves the rapid discharge of capacitors in place of the coil.

As a simple indication about how different the systems are, read the warnings on the various aftermarket tachometers. They will typically advise you that they are not suitable for direct connection to CDI.
 
Sorry if I wasn't clear on this--by "electronic," I don't mean "electric." I was referring to ignitions that use electronic components (transistors, silicon-controlled rectifiers, and so on), as opposed to the conventional ignitions with coil and capacitor.

I realized something else after the last posting. Early CDI ignitions did not use electronic switches to fire the thing. They used the original points. The advantage was primarily that the current in the points was much lower, so they lasted longer--at least until the plastic part that rode on the distributor cam wore down! The first CDI ignition I put in my 66 Triumph TR4A was like this. They often had a switch, so you could switch out the electronic stuff and return to conventional operation. This was largely because people didn't trust all that electronic stuff to be reliable. Probably not without some degree of justification--electronics are much better today, but in 1965, it wasn't clear that they were more reliable than the non-electronic stuff.

Yes, quite true, you also have to be very careful using an electronic tachometer with these, since they just don't work the way the conventional ignition system does.
 
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