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Re: [T3] CDI question

On 6 Mar 2001, at 19:41, Keith Park wrote:

> I use a home-brew CDI in the Opel, you can use the same plugs but gap them
> to .040.  You MAY have trouble with the wider gap and our squatty little
> distributor caps as the voltage will be higher but my guess is that it will
> be OK.

The CDI manual in 1968 suggested opening up the gap to .040. When I 
tried this I had no end of trouble with poor running until I realized 
that the spark plug cables were my problem, where they ran under the 
air cleaner and were pressed against the cooling tin. New wires 
didn't help.

I finally realized that the voltage was only dependent on the SP gap 
and that I could reduce the peak voltage by simply resetting the gap 
to .028. This completely cured my problems. While it is likely that 
we could tolerate a larger gap, I could never see that there would be 
any advantage to that. By sticking to the stock gap we also get to 
switch back to the normal ignition without regapping. I often do this 
during a tuneup so this was worthwhile to me.

A quick note about spark voltages:

Your ignition system is a simple resonant transformer which, in the 
absence of a spark plug, will put out an electrical pulse which will 
quickly rise to some voltage and then just as quickly die. The peak 
voltage it will reach is the voltage that is advertized (30kV, 40kV, 
50kV, etc)

It is important to understand that in use this voltage is NEVER 
reached. The reason for this is that the spark plug fires and grounds 
this voltage at a much lower value. The actual firing voltage is 
dependent on many things, but the main things are the cylinder 
pressure (load on the engine) and the spark plug tip geometry.

There's nothing you can do about the pressure, but you can certainly 
afffect the geometry. Worn plugs with rounded electrode edges may 
fire at twice the voltage of new plugs, and the electrode gap has an 
effect, too, although it is not linear.

The firing voltage rises slowly as the plugs wear, starting somewhere 
below 20kV. The engine starts to miss when the firing voltage exceeds 
the peak voltage available OR when the firing voltage rises so high 
that the spark finds a secondary path to ground (ie when the plug gap 
is no longer the weakest link in the chain.)

In practice the latter is more common than the former, so buying 
higher voltage coils is usually unproductive. Adding a CDI to a car 
will also sometimes pick out flaws in the other components that a 
standard ignition ignored.

Still, I use and recommend CDIs, but like Silicone Brake Fluid, this 
stuff isn't snake oil, so don't expect it to cure all your ills.  ;-)

Jim Adney, jadney@vwtype3.org
Madison, Wisconsin, USA

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