Vac advance unit mod, boost results, plus aftereffects - Saab 900 Bulletin Board - Saabnet.com
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Vac advance unit mod, boost results, plus aftereffects
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Posted by MS (more from MS) on Mon, 10 Sep 2001 15:47:55 Share Post by Email
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The vacuum advance unit I've had on my car is of the kind that is not possible to modify, in that you can't alter the retard by removing stops. So I recently procurred one of the advance units which can be modified. So I did, mainly by drilling out the retard stop post. The first thing I noticed when pushing the accelerator during a test drive was the boost guage running well past the 10 psi that I normally get. But, because of the considerable timing retard, the actual acceleration felt (by me, the driver) was less than spectacular. So I advanced the timing to it's standard 20 BTDC mark (which ordinarily causes ping because of dumb low octane premium gas here in CA) and tried again. More felt boost to be sure, but lower psi, most likely because the APC registered ping. I still wasn't satisfied, and accepted that I didn't really know what I was doing, and may even be causing some problems, so ended up replacing the modified advance unit, plus returned the timing to the 16 BTDC. But then I noticed that I was only getting stock boost at around 5 psi. Why? No idea. Thinking i may have blown one of the vac lines because of that increased boost test, I checked everything and found nothing. Then while tracing an oil leak (the new turbo oil cooler) I noticed a few threads sticking out of the rubber coupling which connects the turbo and the outlet pipe heading to the IC. A closer look revealed a small slit. The boost test must have blown a hole in it! Wow. And if it caused that, I can only guess what would have happened to the engine itself (head gasket for one) if I'd kept the super "retarded" advance unit and drove around with 15 psi.

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