Actually its an interesting question and it takes you into one of those
dangerous areas (that this forum is really good at!!! 
where energy and
physics and things get a little murky.
In a perfect world with a perfect transformer and a perfect primary switch
(and perfect wires) you’d have the instantenous result that people expect.
But I’d expect there to be a finite time required for the magnetic field to
collapse due to the hysteresis of the core (if not then there would be no
such thing as frequency limits for magnetic material and the core wouldn’t
get hot due to the losses) and the simple fact of the inductance of the
secondary (due to masses of windings around the core). Add to that leakage
capacitance and the primary winding current disconnect is not instant. The
controlling transistor/FET has a finite toff (turn off) time, controlled by
the junction capacitance and drive circuits. Points are the same, except you
get arcing while they try to break. You’d be struggling to get it better
than 50us for the “points” break I would think. So I’d be surprised if the
rise time of the HT was better than 100us (0.1ms).
In fact, if the rise time is too fast you will also introduce a nasty
radiated emissions problem. So my guess is they would specifically design
some limit in it. (apparently a forward facing spark plug in a metal bowl is
a great radar jammer)
(OK I just checked online and it looks like there is concensus for inductive
ignition rise times to be around 100us … I’ll take a bow. I assume CDI is
somewhat faster, allbeit with a correspondingly shorter spark)
At 6000 rpm, an engine is doing 100 revs per second. That is 10ms per
revolution. Therefore that equates to 1 deg crank rotation every 27us. So a
100us rise time in your coil is equal to about 4 degrees of crank rotation
at 6000 rpm. At idle, it is only 0.4 degrees.
This is all off the cuff thinking out loud, so I stand to be corrected.
Now in terms of the discussion, so long as all the plug gaps were the same,
they’d all hit their spark over voltage at roughly the same time, (a couple
of degrees after the “points” open) and their wouldn’t be a problem. But if
one plug was 0.050" and one was 0.025" then you’d have to think that the
larger gapped plug could be delayed a couple of degrees at high rpm. It will
also make a shorter spark.
Looking on a scope at the general ignition cycle, you’d be on a longish time
scale, so 0.1ms might not look like much. Also, your scope probes will add
to the problem, because they have rise time issues too.
Then you have mixture and swirl issues to worry about.
Rgds
Mark-----Original Message-----
We’re talking about the
coil being fully charged and suddenly breaking the current flow to
the primary winding so that the magnetic field collapses, generating
a spike in the secondary winding. I think this spike is
instantaneous; in fact, I think the voltage is at max to begin with
and then drops off with time.
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