APC Modification for 900turbo with LH 2.2 Fuel Injection and APC Part Number 75 24 127 or 75 66 599
You will need a small length of 18 gauge wire, a soldering gun, and a desoldering kit. Also a 1k ohm resistor, ¼ watt, 5% tolerance [Radio Shack]
Note: R=resistor C=capacitor D=diode
Open up the APC box and remove the screws that hold the 2 halves together.
Remove R104
Remove C28
Remove R40, and replace it with 1k ohm Resistor
Remove C25
Solder 18 gauge jumper wire to pin 1 of the wire bridge [on the PCB without the Potentiometers] (striped wire) and to D33. Wire is connected to D33 closest to the edge of the PCB. You will know if you did it right if it is a dummy circuit- IE it does nothing but double up the connection made by pin 1 of the wire bridge. It should NOT bypass D33. I am fairly certain you can also leave the wire out all together.
Remove C26
Remove D36
Remove the wire patch between R103 and R51
(physically between, not electrical connection)
Remove C31
Adjusting the Potentiometers
This is where the real power comes from!
DO NOT touch the "K" potentiometer, only the "F" and "P" potentiometers.
We are fairly certain that the "P" potentiometer controls Peak boost [IE. The initial spike of boost] and the "F" potentiometer controls Full boost [IE. The level Peak boost is backed down to]. Some contend that the "F" potentiometer controls the increment and/or frequency that boost is stepped back up after detecting knock. Either way, the best results are achieved by tuning both the "F" and "P" potentiometers CLOCKWISE. I used about a ¼ turn on the "P" potentiometer and 1/8 turn on the "F" potentiometer for starters. You will be putting the APC in the car, driving around, and fine tuning for a few days. Make sure you find a level you are comfortable with. At 14.7 PSI your overboost switch will turn the fuel pump off, you will KNOW if you trip this switch. It can be unplugged or recalibrated*. I had my switch calibrated to somewhere right around 25 PSI. The overboost switch is located in the dashboard, driver's side, just above the APC transducer. To unplug it, just remove the vacuum hose from it and plug the line that went to it, or insert a straight vacuum connector where the T fitting was. You can make your APC to show you if it "thinks" that your engine is pinging. Connect LED between +12v and APC pin 19. APC connects pin 19 to ground when it notices knocking. I ran my car for nearly 100,000 miles with this mod, blowing 20+ PSI. I preemptively did the headgasket and it didn't leak a drop of coolant the entire time. The most important thing to remember here is your car should be in good condition having had a recent tune up and no problems like leaking seals, weepy headgasket, etc.
MAINTAIN YOUR CAR DAMNIT!!!
That being said, I managed to go through 2 gearboxes from too much torque and power. Please only do this modification to a stock 900turbo APC, the SPG boxes are different, and we are not too sure what the factory did to them, yet... Thank you, and good luck.
Note: '89 or later turbo - you're S.O.L on the overboost adjustment - it's handled internally by the F.I. computer. That's one difference between LH 2.2 and LH 2.4. Sorry.
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