two comments.

first, the resistance of a fouled plug is SMALLER, because of the parallel
path across the gap provided by the contaminants. the voltage drop across
the gap of a badly fouled plug may never rise high enough for a spark, and
this causes missing.

second, the spark current is initially limited by the resistance across
the plug gap, but at peak spark current is ultimately limited by the
resistance of the secondary circuit (coil secondary, plug and coil wires,
series resistance built into plug, etc.). unfouled plug resistance goes
from essentially infinity at zero spark current down to maybe 1,000 ohms
at peak spark current. for comparrison, a typical secondary circuit
resistance is about of 20,000 ohms.

this is a non-trivial problem invilving electrical transients, including
reactive effects in addition to a nonohmic resistance.

dan

On Sat, 19 Jun 2004 11:03:05 -0700, LOREN BUSCH <labusch@VERIZON.NET>
wrote:

>I've stayed out of this till now but have to jump in.
>Two points that  seem to have been missed in the discussion.
>First, the emergence (I didn't' say origin, I don't know when they were
>first used or by who) of the multi electrode spark plug was at the end
>of WW2.  They were in use in aircraft to try to keep aircraft engines
>running when there was oil fouling in high time engines.  They were
>effective, if one electrode was fouling and resistance went up then the
>spark would appear at another electrode.  But still to just ONE ground
>point!  No change in 'load' on the system.
>And that is the second point.  The spark is provided by the back EMF
>from a collapsing field induced into the secondary winding of the coil.
>It is a collapsing field because the circuit has been opened.  Any
>attempt to analyze the circuit using Ohm's or Kirkoffs (sp?) law will be
>wrong, there is no closed circuit at the time of the spark.  The load on
>the system is solely a function of the characteristics of the primary
>winding on the coil (and resistance up to that point) and has nothing to
>do with whether or not the secondary is shorted to ground (through the
>spark plug) or not.
>The result is that the coil couldn't care less how many paths to ground
>are available as long as it is discharging into the resistance it was
>designed for and that is controlled by the SINGLE plug wire much more
>than the configuration of the electrodes on the spark plug.
>BTW, do I use multi electrode plugs?  Yes, because they should last a
>little longer before needing replacement.  Do I expect better
>performance? No and no reason that I should.