I had a '94 Jetta VR6. It used triple-prong Bosch plugs... I finally
had to change them out at 109,000 miles.
They were clean, clean, clean. Perfect combustion tell-tales.
The center (copper) electrode was worn down to a millimeter across at
it's widest point, and had a neat, triangular wear-shape to it. All 6
looked like this.
It's true that the spark will seek the easiest path to ground, so plug
life is prolonged by having extra prongs, but easiest path may change,
depending on engine load, fuel quality, presence of oil, head temps,
etc. Maybe the extra prongs provide an "easiest path" under these
varying conditions?
Anyway, I put copper triple-prongs into any vehicle I own that needs
new plugs. The 16V, the Audi 3A in the Van, the original wassercrapper
in the Van, and an assortment of other VWs over the years, all got
triples. Never a plug-related problem, and usually they run cleaner
than the plugs they replaced.

-Simon

On Jun 11, 2004, at 1:44 PM, Bruce Nadig wrote:

<clip>
> So, if you are a car manufacturer, and you have to make a part that is
> a
> wear item, which ultimately a spark plug is, last longer, you have to
> become
> inventive. If one electrode will erode away well before 100,000, why
> not add
> a second? If two won't make it why not add a third? Now you still have
> the
> issue of the single center electrode. That is handled by the
> increasingly
> exotic metals used for the electrodes (copper, then platinum, now
> iridium).
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