Monday, November 20, 2006

VW - CDI Debated

In a message dated 96-02-27 12:30:40 EST, Michael A. Radtke provided an accurate, well-reasoned summary of capacitance discharge ignition modules and inductive discharge ignition modules (other than capacitance charged), in relation to my several articles lauding the advantages of an after-market CDI module over the stock VW ignition system. His well-written dissertation concluded with the following:

THE QUESTION: Is CDI alive and well in current automotive design? What was done about the high voltage issue? >

Mike, Until you raised the question, I had not realized my lack of verbal precision. In my ‘sermons’ I’m guilty of mixing the terms ‘electronic’ and ‘CDI’ when I start waving my arms to convince people to update their Kettering ignition systems.

All of the electronic ignition systems I’ve worked on in recent years (Ford, GM and Toyota) are relatively simple inductive discharge systems that differ from the classic Kettering system only in the manner of switching the coil current. Out of curiosity, I’ve broken open several defunct ignition modules . . . ‘ignitors’ in Toyotaese . . . and found only components consistent with signal amplification and DC switching, no indication of HV inverters or storage capacitors. In so far as I know . . . which is not as far as many think. CDI modules are not used by any of the major auto makers. I assumed it was due to the lower cost of IDI systems but the background you provide in your message casts a new light on that conclusion.

Even so, I feel the factors explored in my articles on electronic ignition, and on which I based my recommendation for the installation of the CDI module mentioned in my article, remain valid. And perhaps I should add that I’ve no association of any kind with that company other than as a satisfied customer.

As to the high voltage issue, it may be a non-problem. Parked down in the field below my house is a ‘67 VW sitting with the engine exposed to the elements, as it has been since it was created more than 15 years ago. The original vehicle suffered extensive rear-end collision damage. The damaged sheet metal was cut away and a sturdy crash-cage of chrome-moly tubing fabricated to protect the engine. This is a typical ‘Baja’ conversion. The engine is fitted with a CDI module and non-metallic silicon/fiberglas spark plug leads. The leads, distributor and module are exposed to the weather. The vehicle has never failed to start, and runs reliably in all weather and temperatures ranging from zero degrees (two occasions) to more than 120 (many times). Such reliability was not always the case, but has been true for at least the last eight years, from when I installed the silicone ignition wires.

As to plug gap and the effect of high voltage, with the CDI modules I use, I gap the plugs to about .045". I arrived at this figure by widening the gap until ignition became unreliable at high rpm, then narrowed the gap by .010". I’ve noticed some variation here between engines of different compression ratio or chamber design but in general a plug gap of between .040 and .050 appears to work quite well with this particular assemblage.

I have not posted your entire message to the list because I believe a private message is exactly that, but I will make this reply a general posting because of my failure to speak clearly on what I consider a very important factor in Volkswagen maintenance. I’m also making this a general post in recognition of your successful effort to clarify the matter.

I believe your insight and experience are assets that make this list of value and would like to hear any other comments you may have on the ‘sermon’ files. I hope you will share them with the list at large.

-Bob Hoover

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