[TR] Tidy, was Ignition wires

Randall tr3driver at ca.rr.com
Sat Mar 20 14:58:14 MST 2010


> Someone once told me 
> that the reason for using those little plastic spacers to hold the 
> plug wires apart was to avoid cross-induction between them.  I would 
> have thought maybe that meant spark jumping across the insulation of 
> two wires touching each other.

No, the insulation is supposed to be sufficient to prevent that.  In fact,
some of those separators are made of metal rather than plastic ... they are
usually plastic only because it's cheaper.

In this case, induction refers to the change in current in one wire causing
(inducing) a current in another wire.  Every time a spark plug fires, it
goes from being effectively an open circuit with 10 thousand volts or more;
to a direct short (the spark is actually a lower resistance conductor than
copper).  So the current in the plug wire goes from nearly nothing to a
fairly high value as most of the energy stored in the ignition coil's field
gets dumped into the spark.

> If either case is possible, and I'm 
> not convinced it is likely, it argues against braiding the wires 
> together.  

Induction is rarely a problem on a 4-cylinder 4-stroke anyway.  With, for
example, #1 firing, #2 will be at the end of the power stroke, and #4 at the
end of the exhaust stroke.  No problem with a spark, as what's in there
won't burn anyway.  (Some ignition systems even deliberately fire #4 at the
same time as #1.)  #3 will be the only other cylinder with even a
theoretically flammable mixture, but in practice it won't burn at that
point.

It's much more of a problem with a V8, where whenever a cylinder fires,
there is another cylinder with a flammable mixture under compression.  If it
gets lit, that cylinder tries to run backwards.

Randall


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