It's purpose is to try to keep the boost pressure up while still protecting the engine from the disastrous knock that will occur if the boost is too high for the conditions the engine is running under at any moment. These conditions include fuel octane rating and ambient temperature, as well as a range of other variables.

In theory the MABC would allow you to run any octane fuel and still keep the performance as high as possible while not hurting the engine. I say "in theory" because this is only partially successful. The same concept has been incorporated in the microprocessor equipped electronic ignition control unit made by Weber Marelli for the later models with fuel injection. In these units the calculations used and the method of avoiding knock is much more refined.

• Manifold Pressure Sensor. (Input)
  This sensor measures the pressure in the intake manifold. If the throttle is wide open, this pressure can become positive, or in other words greater than ambient pressure. This is different from a normally aspirated car, where it will always be lower than ambient. Note that this is the pressure after the carburettor butterflies, and therefore can be a lot lower than current boost pressure. When the throttle is closed, the manifold pressure will be lower than atmospheric pressure, regardless of how high the boost may be at the moment.

• Engine speed "sensor". (Input)
  The engine speed is not measured by a sensor as such. It is calculated from the frequency of the ignition pulses which are picked off from the ignition amplifier in the same way as an electronic revolution counter.

• Knock Sensor. (Input)
  This is a piezoelectric microphone built into a metal stud that is bolted to the block in the bottom of the "V". It is mounted directly over one of the crank bearings, which will be an area that is greatly affected by the shockwaves that are results of engine knock. As these shock waves are the same as a sound, a microphone is a good sensor to pick it up. Remember that it will also pick up all other sounds present in the engine block. We'll come to that later.
  After reading the general description, you may not be surprised to learn that a SAAB knock sensor will fit the BiTurbo, and work as it is supposed to.

• Boost Control Solenoid. (Output)
  This is a solenoid air valve that will bleed off the boost pressure to the air filter or direct it to the wastegates, depending on whether it gets energized or not. Since it has only two possible states, it is pulsed by the MABC control unit. This exploits the slow mechanical reaction of the solenoid to position it in states between ON and OFF, and thus obtaining a continously variable valve from this simple solenoid. There are around 16 pulses/second, and the length of time it is energized during each 1/16sec interval will determine the position of this valve. This is what you hear as a "clattering" from the engine room at 2000RPM, and although some owners think something may be wrong, it is quite normal.

• Overspeed Relay. (Output)
  This relay is used by the control box to disable the ignition if it determines the RPM is above a set limit.

• Diagnostic port. (Input/Output)
  The MABC control unit has an input that can be used to disable parts of it for diagnostic purposes.
  It also has a couple of diagnostic outputs. One of these can be used to give a visual signal when the unit determines that the engine knock level is higher than it should be.