Rally: the original motorsport
Photo Courtesy of Tim Winker
Photo Courtesy of Warwick Patterson
unknown photographer
Photo Courtesy of plaftaphoto
1985 900 turbo APC modification

APC box modified: P/N 7521826
main board p/n 5510779-01E. Sub-board p/n 55-11062-01A.

You will need a soldering gun, a desoldering kit and these components:
1 megohm resistor, 1.5 megohm resistor, 2.2 megohm resistor, .22 microfarad mylar film capacitor, 0.047uF Polyester or Polypropylene disc capacitor.

Resistor values ¼ or ½ watt, not critical.

Main board (all parts placements made on component side, plug at 6 o'clock position):

  • Add 1 megohm resistor between bottom pin of R47 and RH pin of R83.

  • Add 1.5 megohm resistor between top of R36 and jumper P03.

  • Remove R23.

  • Over (parallel) D24, add 2.2 megohm resistor (piggyback).


  • Substitute C1 with .22 microfarad mylar film capacitor.

  • Remove C5.

  • Substitute C6 with disc capacitor
        (0.047uF Polyester or Polypropylene (radial "473")

    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 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.

    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.

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