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."