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Spark Plug Recommendations, Classic 900 turbo

I know what you're thinking, how can I give advice on something as disputed and personal as spark plug choice. Well I've noticed that a lot of people are just outright SOLD on all those silly claims made by the people that make such neat things as 4 prong spark plugs. I have also fixed enough problems caused by these so-called wonder plugs that I am going to tell you what plugs to put in your 79-93 900 turbo. Sue me if you don't like it. These work great for me in racing and my street cars, so they might work great for you too...

First some quick background because not many people seem to know why you would put one spark plug in versus another. And the reason isn't just brand difference, it's heat range and electrode composition and placement. And yes, brand difference does matter because DAMN, there are some really crappy spark plugs out there.

The spark plug numbers all tell a tale: They denote electrode placement, clearance of electrode and grounding prong in the head, shape of the electrode and prong, heat range, etc. Your car was designed to work with a spark plug that projected a certain amount into the combustion chamber (eg, not far enough to whack a piston!) and had a general overall shape to the electrode to provide good combustion for the shape of your chamber and had a certain amount of pre-gap. Most numbers in the plug deal with the fitment issue (will it screw into my head and not whack a piston?) but we're not going to discuss those. We're going to discuss plugs that all fit correctly but have a different electrode composition and a different heat range. Heat range can have the biggest effect on your performance, beyond the obvious one of putting the wrong shaped plugs in.

Your engine typically operates at a specific heat range. The manufacturer spec'd plugs for that heat range. But that's not the whole story. Different driving styles can change the operating temperature of your engine: push hard all the time and your combustion temps are hotter. Do a lot of short distance driving, like running errands in town, and your combustion temps are not as hot. If your combustion temps are outside the temperature range of your spark plugs problems arise. If the plug is not getting hot enough it may foul, leading to incomplete combustion, poor emissions and poor gas mileage. If the plug is getting too hot you can run into incomplete combustion, predetonation and meltdown. Yikes!

Here's how spark plug heat ranges work:

The colder a plug is the more it will withstand hard driving but the easier it can be fouled, leading to misfiring. this happens when the plug cannot heat up enough to burn off carbon and combustion deposits. This plug does not heat up fast, it dissipates heat quickly into the head to maintain a colder temperature at the tip.

The hotter a plug is the easier and faster it is to burn off carbon and combustion deposits but the faster it becomes overheated when pushed hard (affected by hotter combustion temps). it dissipates heat slower and therefore has a higher temperature at the tip. overheating the spark plug tip will lead to incomplete combustion and pre-detonation.

So let's see about what to put in your car. Do you need a cold plug for hard driving, or a hotter plug that won't foul around town? Heat ranges are given in the numbers but they can be confusing in that they are not standardized.

Bosch heat range numbers start at 1 and get hotter the higher the number.
So 4 = pretty cold and 7 = pretty hot.

NGK heat range numbers start at 9 and get hotter the lower the number.
So 4 = pretty hot and 7 = pretty cold.

For *really* hard driving like a track day or heavy sustained hauling (speed not cargo) try using bcp8ev, or bcp8es. these plugs will be too cold to get good running while pussy-footing around town but damn they work well when you really push them. Like race-track push them. You can get as good, sometimes better, results using copper plugs in a turbo. The downside (and reason platinum is spec'd) is you will have to check and replace them more frequently. I actually run Bosch copper plugs in my rally car (the horror!) and the plugs used are w5dc (a 5 heat range in Bosch being roughly equivalent to an 8 in NGK). So if you're an experimental kind of guy you can give those a shot. The copper are much more reasonably priced so this offsets the need to replace more frequently. For what it's worth I've never had any luck with the Bosch platinums and the reason people stay away from Bosch plugs in Saabs is the poor performance of the platinums. They seem to work well in other cars though. Go figure.

So to recap:

NGK:
BCP6ES or BCP6EV for city/town driving (stop and go)
BCP7ES or BCP7EV for regular/hard driving
BCP8ES or BCP8EV for very hard driving/racing (this plug will foul with normal driving)
where the 's' denotes copper and the 'v' denotes platinum

Bosch:
W6DC (for regular/hard driving)
W5DC (for very hard driving/racing)
where '6' is a heat range equivalent to the ngk '7' range and where '5' is a heat range equivalent to the ngk '8' range, which is a colder plug than stock NGK spec (being a '6') or even the factory NGK 'hard driving' recommendation of a '7'.

Hopefully you've made it to this point with an understanding of *why* I'm saying you should use these plugs and enough knowledge to start doing your own spark plug thinking and not just regurgitating "buy my plugs because they will give you 5hp" marketing crap about a fancy new plug. If not, c'est la vie as the French say.

James
Team Saabworks




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