Tweaking the control unit!
Extreme caution should be taken when adjusting the control unit since there are three adjustment. If you adjust the wrong one, you might as well throw the control unit away since the chances of getting it back to the original setting is practically impossible. If you feel brave, dig in, it's not difficult.
First you'll have to take the four screws out of the top of the box. Remove the top cover from the box. You will see three holes in the top circuit board that gain you access to the three adjustments inside the box. Some of the earlier control units have the adjustment screws obviously marked with the letters K, F, and P (Knock, ?Frequency?, and maximum boost Pressure). If you don't have one of the earlier units, you'll have to look a little deeper to find the markings. Some units may not be marked at all. On my control unit, the screw on the right was marked with the letter P. I turned the screw 1/4 turn clockwise and test drove the car. There was a modest increase in boost pressure. I turned the screw another 1/4 turn clockwise and test drove again. Again a bit more boost. Finally, I turned the screw about 1/16 of a turn all the way to it's limit. This gave me about what I was looking for. The boost went right to the end of the gauge. This is about 14-15 psi.
To get rid of the boost taper, you'll have to consult one of the major Saab tuners and ask them about their modified control units.
If That's not
If that's not enough boost pressure for you, try jumpering the wires for the pressure transducer so that the APC control unit doesn't know what the boost pressure is and will rely solely on knock. I tried this on my car and did not like the results, however, several people who I know run their APC systems this way and have experienced no ill effects. One certain '86 Saab 900 turbo is running up to 20 psi with the transducer jumpered and a boost enrichment system.
For those of you with Garrett T-03 turbos, 16-17 psi is about where the efficiency of the turbo goes away and the air coming out begins to get really hot, so running the system with the pressure transducer may be advisable.
Another thought I have had is to bleed off the pressure to the transducer with a small bleed-off orifice to let the boost pressure get up to about 17 psi. You'll have to experiment with this idea.
Putting a resistor in series to the transducer won't work because as the pressure goes up, the resistance goes down. You could possibly install a variable resistor in parallel with the transducer though.
And when you get
really serious about making your car insanely fast...
Go visit the web site of Maximum Vehicle Performance for some serious cool stuff. I have had a chance to try a number of their products and have been 210% satisfied with every last part. I currently have a big bore throttle body, rising rate fuel pressure regulator, VX3 camshaft and adjustable timing gear, and the absolute hands down coolest 3" exhaust system ever built for any car (It's such a piece of artwork that you could even hang one on the wall in your house).
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