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Date: Wed, 09 Sep 1998 21:15:00 GMT
From: Ywan Mason <ymasonnopsamsouth.net>
Subject: Re: SPARK PLUGS


The DI uses the plugs as a sensor for a couple of inputs. The cassette measures ionization energy through the plugs to both synchronize cylinder firing and to detect knock. If the DI can't get a handle on which of two possibilities of cylinders to fire, it will fire both. The resistance in the plugs is critical to the readings the DI receives from them. Put plugs with the wrong resistances in and you change the voltage necessary to bridge the gap. This can overwork and over heat the coils inside the DI cassette. There have been a couple of revisions in the system since its inception and the readings it relies on from the plugs have changed, (DI, Di/APC to Trionic) but the overheating of the coils still applies. We use Bosch Platinum plugs in all the distributor equipped cars we service including our own. Application is critical, especially in turbos and even more so in modified high boosting turbos. Use WR7DP for 8 Vs and FR7DP for 16 Vs, makes no difference turbo or non. The plugs most catalogs give as the proper ones for Saabs are not these plugs. My experience with the listed plugs under Bosch Platinum for Saabs is that they will not fire under boost and will foul easily. The ones I listed above do fine however. I have seen them go more than 60K miles and still be fine, although I do recommend changing them at 30K intervals. Saabs are very responsive to plug gaps as well. On distributor cars, you can't go wrong with a .65 mm gap. On DI cars, use 1.0 mm. We recently received info about a Bosch Platinum plug that our Bosch rep says will work in the DI cars. We have yet to try it. All we use in the DI cars are the recommended NGKs in the appropriate heat range for the car (turbo or non). NGKs are great hi-po plugs. They are just about twice as expensive as the Bosch. If the right application is used, I see no difference in performance between the brands in the cars we drive and service..

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