Date:         Thu, 13 Nov 2003 12:26:03 -0800
Reply-To:     Jack <john.cook58@VERIZON.NET>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         Jack <john.cook58@VERIZON.NET>
Subject:      Re: Cold Starts
In-Reply-To:  <3FB37668.4000202@fyi.net>
Content-type: text/plain; charset=US-ASCII

I've stayed out of this as I was hoping someone with specific technical
info on the Vanagon/Bosch ignition system would jump in & help put this to
bed.  The following is my attempt to clarify the technical situation, not to
step on anyone's ideas.

Eric's explanation of how the coil works is correct up to a point (do a
Google search on Kettering Ignition for a review) but I think that his
assumption that the heat build up in the coil is somehow related to the
spark plug design or quality is not, IMHO.

For purposes of this discussion, I'm going along with the reasonable
assumption that heat is the primary failure cause for the coil, but someone
has suggested that overvoltage may also be involved.  Maybe so, but that
suggests that the coil is poorly designed if it cannot withstand worst-case
voltage (open plug wire?).  Of course, we also know that insulation value
decreases with temperature, so there may be a combined effect.

As everyone already knows, the coil is basically a step up transformer with
an iron core.  The primary resistance is very low, only a few ohms (heavy
wire, few turns); the secondary resistance is many thousands of ohms (fine
wire, many turns).  Consequently, the primary current is several amps during
the charge (dwell) time.  This is when the temperature begins to rise!  Then
the actual operating temperature of the coil is related to many factors but
primarily to the dwell angle, number of cylinders, RPM, etc.

On  the secondary or spark plug side, it's perhaps useful to remember that
the resistance of a struck arc is very low (regardless of arc length), or
insignificant compared to the resistance of the coil secondary.  Also,
there's probably thousands of ohms intentionally inserted into the HV
delivery system for radio noise suppression (e.g., carbonized spark plug
"wire").

The bottom line is that I'm extremely skeptical that the choice of plugs has
anything to do with early failure of coils.  I think there has been ample
testimony that it is not provided already.

//Jack


On 13 Nov 2003, at 7:17, Eric Zeno wrote:

> Sorry tom, that's not how it works.  THe coil is charged to max. flux
> every cycle.  When the field collapses, the electronics have to jump the path
> of lease resistance, (the plug gap).   If this is bigger than normal where
> does the energy go? In to the coil as heat.   Smaller the gap the less is
> given off in heat.
>                                       Eric
>
> tom ring wrote:
>
> >I've seen a couple misunderstandings in this thread.
> >
> >A plug with a larger gap than specified will force the coil to produce
> >greater than normal voltage to fire.  Coil current in this case will
> >generally decrease unless the ignotion system can somehow force more energy
> >into each pulse, which a standard system will not do.  The coil will not run
> >hot, but may fail due to insulation breakdown.  This may be an intermittent
> >failure, and may be temperature sensitive due to interwinding distance
> >changing.
> >
> >A plug with a smaller gtap than normal will require less voltage than normal
> >to fire, and will draw greater current.  This will cause greater heating of
> >the coil.
> >
> >Follow old, worn, high gap plugs with new, too tightly gapped plugs...
> >
> >tom
> >
> >
> >
> >------
> >Tom Ring K0TAR, ex-WA2PHW  EN34hx
> >85 Westphalia GL Albert
> >96 Jetta GL The Intimidator
> >taring@taring.org
> >
> >"It is better to go into a turn slow, and come out fast, than to go into a
> >turn fast and come out dead." Stirling Moss
> >
> >
> >
> >