Clive,
Honestly I don't know what difference in delay or required
voltage would be between .025 and .030". It would all depend
I'm sure on the particular coil, and also on the pressure in the
combustion chamber at the time of firing.
Ignition coils have quite a lot of inductance and are pretty
slow devices as far as transformers go. I've played with a few
high voltage projects using ign coils to generate HV and they
don't work well at switching frequencies over about 300-400 Hz.
It wouldn't surprise me if a change in plug gap of .010 could
change the firing delay by 10's of microseconds.

I agree that the change in timing may be minimal, and that it
may be swamped by other factors (sloppy distributors), but
my point is it does make a measurable difference.
And every little bit helps, eh?

If someone had some time with an O-scope and a high voltage
probe, it would be easy to put some real numbers to the whole thing.

Steve

At 01:38 PM 2/20/2003, you wrote:
>Steve,
>Great, and thanks for some serious input. A breath of fresh air.
>Inductance has a lot to answer for...
>
> >At 3600 RPM one degree of crank rotation takes about 46 microseconds...
>Changes in spark gap can definately make small but easily measurable
>changes in timing.
>
>a) So what is the difference between a 0.025" and a 0.030" gap (typically,
>assuming the ign. system will indeed fire across 0.030" gap
>Unless its of the same order as 46 micro secs then the timing effect is
>negligible as few can set timing to within 1 degree, even today and not long
>ago that same oscilloscope would show the centrifugal and vacuum advance
>mechanisms splattering the timing across a huge range, maybe +/- 5 degrees
>of the mean (design) curve, in a bad dizzy.
>b) So this effect effect would have no relevance to a capacitor discharge
>system?
>c)... AND, everyone, this effect is BENEFICIAL, as far as timing goes (not
>spark quality). Because, as the charge volume and BMEP increase the spark is
>retarded (guessing at 1/20th degree!) and as we lift off, the spark
>advances - - exactly what the vacuum advance system does, 'cept it moves the
>timing 100 or more tiomes as much.
>
>
>Clive