Re: Spark plugs and DI's (long) - Saab 9-5 Bulletin Board - Saabnet.com
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Re: Spark plugs and DI's (long)
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Posted by zeke (more from zeke) on Mon, 14 Jul 2003 15:22:32 Share Post by Email
In Reply to: Spark plugs--intervals --sharp edges, TG, Mon, 14 Jul 2003 14:09:59
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Sharp edges are good - points are even better. There is a correlation between the intensity of the spark and the shape of the cathode, especially when running a narrower gap with higher cylinder pressures. A full pressure turbo should have a narrower gap for extended life of the DI. Saab investigated the short lived DI issue, a serious warranty problem, many years ago. Dealers were provided with special coloured cassettes to prove/disprove the cut-out/misfiring complaints before the cassette was changed. It was found that the cassettes were breaking down and failing due to heat and arcing problems. This led engineers to determine the cause: plugs gapped greater than 1.1 mm. Saab then ran many tests and found that .9 mm was an optimum gap and revised their gap recommendations from 1.0 +0.1/-0.0 to 1.0 +0.1/-0.2. Running at the smallest recommended gap (0.8mm) resulted in the least heat buildup in the cassette because it didn't have to expend as much electrical discharge energy to overcome BMEP and fire the plug. Below 0.8 mm would lower the temperature by the square of the induction current, but poor engine firing problems resulted because of the variability of fuel available to the general public.

I set my NGK's @ .034" (slightly modified boost) and change them every 12k miles. I know this isn't what Saab recommends but plugs are cheap and easy to change. NGK's are the only plug to use as they consistently operate between 5.6-6.5k ohms, which is just right for the capacitance of the DI. Their resistance does change, drastically, if they get too old.

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