Rally: the original motorsport
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APC modification for 900turbo '86-'93

APC Part Number 75 24 127 or 75 66 599

You will need a soldering gun, and a desoldering kit. Also a 1k ohm resistor, ¼ watt, 5% tolerance

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

  • 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.

    In layman's terms 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]. It is more accurate to say that the "F" potentiometer controls the increment and/or Frequency that boost is stepped back up after detecting knock. The end result is that "F" controls the level of stepped back boost, after the initial surge of Peak boost. the best results are achieved by tuning both the "F" and "P" potentiometers CLOCKWISE. I used about a 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. If you are getting too big an initial spike and not maintaining boost then turn the "P" pot. down and try raising the "F". You want good initial build without too much backing off. Make sure you find a level you are comfortable with. If you tune the "P" pot too high you will overshoot on initial boost, bleed off a whole bunch and possible hit the fuel cutoff.

    At 14.7 PSI your overboost switch will turn the fuel pump off, you will KNOW if you trip this switch. If you are going to run higher boost levels it must be bypassed or recalibrated*.

    You can make your APC show you if it thinks that your engine is knocking. Connect a LED between +12v and APC pin 19. The 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.

    That being said, I managed to go through 2 gearboxes from too much torque and power. So start your soldering irons and good luck!

    * recalibration only on LH 2.2

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