Saab: : : : APC Tweak/Hack : : :
Information on creating a Red APC.
I accept no responsibility for anything you do with this information.

This information is as compiled from various sources, including but not limited to: Parts and equipment required
  • Saab APC #7566599 (or equivalent)
  • 500 Ohm Resistor
  • 30.9K Ohm Resistor
  • 2.8 or 3.0 bar Fuel Pressure Regulator (3 bar is Bosch #0280160256)
  • Calibrated boost gauge
  • Digital Multimeter

    Assuming you want to wire a Knock sensor indicator:
  • Indicator lamp or LED
  • Wire
  • A connector for the APC plug, if you're not disabling the cut of boost while you're on the brakes.
Values from the common C900 saab boxes:

Part Number Description P03 (P) P02 (F) R42 R138
75-66-599 160 HP w/catalist, 165 wo/cat. 1987-93 12.1 Kohm 98 ohm 5.1 Kohm 26.7 Kohm
75-24-127 165 HP w/catalist, 175 wo/cat. SPG 12.4 Kohm 201 ohm 5.1 Kohm 26.7 Kohm
43-93-971 185 HP "Red Box" 23.2Kohm 295 ohm 510 ohm 30.8 Kohm
75-24-150 1988 9000T 18.8Kohm 55 ohm 5.9 Kohm 25.4 Kohm

Hacking:
: : : Redbox resistor mod
These changes don't increase performance, but rather seem to change reaction to knock.
Replace R42 with 500 Ohm Resistor
Replace R138 with 30.9K Ohm

: : : Knock LED
Pin 19 on the APC is grounded when the apc senses knock. Basicaly we want to wire an LED to this pin. In later cars, the APC receives 12v from the brake pedal switch, to cut boost during braking. Since you probably wanted to remove this feature anyway, we can kill two birds with one stone by using this wire and connector. First identify which wire goes to the APC at the brake switch, cut it free. Now at the APC end, remove the big plug from the apc, and unscrew the one screw that holds the cover on the plug. Locate the pin that the brake pedal light goes to (20 I think). I used an x-acto to free the pin from the connector, replace it in pin 19. You now have a wire from the knock pin of the apc into the cabin. Find a good 12v power source, and wire a lamp to it and the wire you cut from the brake switch. If your using a LED that's not 12v ready, wire a resistor in one side of it to keep it from blowing. 300 ohm should be sufficient. Now you know when the APC thinks you're pinging. With the car on (not running) you can get the led to blink, and the solenoid to go crazy, by tapping the knock sensor with something hard.

Tweaking:

WARNING: Your fuel system probably can't keep up, so don't tune your car unles you know what you're doing.
I recommend at the very least a 3.0 bar fuel pressure regulator from a non-turbo. Even so, at high rpm, I belive you will be lean anywhere over 14psi or so.

Before you do anything, read what Kevin has to say about APC operation.
Kevin K's notes on Dave N's site:
Below about 5-6 psi and under 3100 rpm (and at idle), the solenoid valve is 95% open, bleeding off the waste gate actuator to keep it closed. Once either of these limits is exceeded, the solenoid valve changes to a 'default' duty cycle, set by the 'P' screw. This partly closes the valve, and reduces the rate at which pressurized air is bled off the actuator.
....
THE 'F' SCREW sets the max pressure allowed before the solenoid valve quickly changes from the default 'P' duty cycle to a more closed condition in an 'active control' mode. This feeds more pressurized air into the actuator to open the WG more and reduce boost.
Consult the Red Box spec above, and Adjust P and F accordingly. Chances are you won't use the redbox spec, but it's a good place to start. I've found that the magic spot for P is between 25 and 27 Kohms (that is with the Pot. installed on the board). If P is set too high, the boost will not be stable: For example it will build up to say 16 psi, drop to 13, and climb agian, with no knock. By turning P down enough, you will probably get 13-14psi solid boost, then you can turn F up higher.
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