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For years there has been a lot of contradictory information surrounding
the results of using Bosch Platinum spark plugs. These myths and legends
had bled over into Volvo 850-dom, which I found odd, as the 850's engine
management system is by Bosch, and also because they worked so well for
me! Further, even after I'd proclaimed the Bosch Platinum plugs fit for
(even severe) service in my 850 Turbo, it seemed like some owners of these
cars had a remaining aversion to or doubts about Bosch Platinums, likely
due to lingering doubts over these persistent myths and legends and mechanics' tales.
After a bit of digging, I came up with a phone number for Bosch technical
support: 800-521-5462
(While I was "only" asking spark plug questions, I was left with the impression that this might be a useful resource for solving a stubborn
Motronic problem as well.)
I will now attempt to debunk for you the myths and legends surrounding Bosch Platinum spark plugs:
Bosch acknowledges a problem with using their Bosch Platinum spark plugs
in engines with ignition systems that employ 1 coil for each pair of spark
plugs (also known as a "waste spark ignition system").
In cars with conventional (1 coil, distributor/rotor) ignition systems
(including the 850 Turbo with Bosch 4.3, 4.4 engine mgmt systems), there
is no problem. In fact, not too surprisingly, according to Bosch, the
maker of the 850's engine management system, the recommended plug is the
Bosch Platinum FR7DP, which is what passed with flying
colors my tests-under-fire in my engine (see for yourself).
The problem that Bosch has acknowledged when using their Platinum plugs
in engines with waste spark ignition systems stems from the tendency of
electrode metal to get transferred depending upon the polarity (direction
of current flow) of the spark. In waste spark ignition systems, 1/2 of the
plugs always see reverse polarity sparks. Given the very narrow platinum
center electrode of the Bosch Platinums, performance will be degraded more
significantly when reverse polarity sparks will cause material to be
transferred from the large ground/outer electrode onto the narrow platinum
center electrode's exposed end surface.
In cars with conventional (1 coil) ignition systems, every spark is of
the same polarity, for which the Bosch Platinums are optimized, leaving
them to deliver their designed-in advantages, which include:
o reaching self-cleaning temperature faster ("than any other plug")
o maintaining the spark gap and low firing voltage requirement throughout
their service life. (even in my high-stress turbocharged application)
While there remain some cars with conventional ignition systems which
do not seem to get along very well with Bosch Platinums, which seemingly
no one can explain (maybe all their sparks are of the 'wrong' polarity
in some cases), they did and do good work in my '95 854 Turbo.
- Dave 
"Unidirectional tires are a mixed blessing on cars that go equally fast in both directions."
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