Any time you go with non-OEM parts, you risk "going backwards" in some

respect(s).

However, 5-6 years ago, Volvo kinda proved themselves clueless WRT spark

plugs, as they went through (tried) a series of different plugs, including

copper and double platinums. Really clueless. Or unable to decide if they

should sell plugs that were technically optimal, had the lowest cost, or

the higest profit margin...

I'd heard that Bosch Platinums wouldn't/didn't work well (based mainly, I

believe, upon an iPd recommendation to stick with std copper plugs). This

intrigued me, as our cars' ignition and fuel systems are designed, and

under the control of an ecu made, by Bosch! And we know they did a pretty

good job on them. And this was their best spark plug... I wanted to try

for something better, but the most likely candidate was disrecommended by

a well-known Volvo tuner!?

But Volvo didn't make spark plugs, and obviously they didn't have the

answer.

So I called Bosch, and talked to some of their engineering types. They

all, without hesitation or qualifications, recommended their Bosch

Platinum spark plugs as a technologically superior plug for my car.

However, they equally promptly told that their "+4" plugs are NOT to be

used in turbocharged applications. And they offered how I could return

their Platinum plugs for a full refund if I or my car didn't like them.

And then I learned that those plugs were the least expensive ($2 ea, that

first set) of all the 'premium' plugs anyway...

That was more than 70K miles ago. I'm on my 3rd set of Bosch Platinums

now. I'm convinced they are the optimal plugs for this application (even

though I've not personally tried others):

- Their smaller-diameter center (Platinum) electrode is effectively

'pointier' (good for low firing voltage requirements), by design.

- The center electrode is totally fused to and protected by the porcelain,

so it is virtually totally resistant to the rounding that happens to plugs

in hot turbo applications (a good turbocharged engine can usually melt

down its 'normal' spark plugs in one good afternoon on the race track).

This rounding that is normal in normal plugs leads to elevated firing

voltage requirements, which leads to misfires, which leads to all sorts of

undesirable things. Bottom line: Bosch Platinums act like new for longer

than plugs with a normal-looking center electrode/porcelain arrangement

(and also those with funky 'split' or notched electrodes).

Other designs try to promote the same benefits, and some succeed fairly

well, but they can't protect themselves against high-temp (turbo!)

aging/rounding the way the BPs can.

Some folks say the Bosch Platinums are more prone to "lawnmower syndrome"

starting difficulties. And my car's had some starting problems -- but they

had nothing to do with the spark plugs! And while I do try to never shut

down my car before it's fully up to temperature, it's had numerous chances

to exhibit this syndrome and it never has. My belief is that cars who've

exhibited this syndrome had cold-starting chokes that were unconscionably

rich, and that no modern EPA-approved car should have the problem.

I have found one 'weakness' with Bosch Platinums: that little screw-on

top post terminal can work loose over the service life of the plugs. Make

sure you torque/lock them down a little harder/better than you think you

need to.

As for torquing the plugs themselves, I believe 15 lb-ft is recommended.

But if you use the Bosch Platinums, they recommend 1/4 turn after the new

gasket first makes contact with the mating surface on the cylinder head.

(I once kinda screwed up in this regard, and some of my clinders were

sucking air, which led to leaner mixture, which led to elevated temps, and

my BPs were unaffected by the higher temps. And this included some serious

"mountain climbing", where the engine was under boost nearly continuously

for extended intervals. Pix of plugs available at link below.)

I recommend a full application of anti-seize compound on the threads of

the new plugs before they go in, to help insure they'll come out nicely

next time.

- Dave; '95 854T, 111K mi