What Does The Automatic Performance Control (APC) Do In My Classic Saab 900 Turbo?

The APC system was devised to manage the engine performance when 97 octane fuel was used. When fuel of this or a higher octane rating was present, the engine would deliver maximum power.

Handily, the APC system was also designed to ensure the engine could run safely on lower octane rated fuel.

Essentially, what the APC does is adjust the turbo boost to suit the fuel being used and the pinking/knocking tendencies of the engine. Short spells of knocking are perfectly normal when a heavy load is being exerted on the engine at about 3,000 revs per minute. The duration of the knocking depends on the grade of fuel being used.

Occasionally knocking of the engine is more likely when low-octane fuel, such as standard unleaded, is being used rather than the premium stuff. When this type of knocking occurs, there will be a noticeable reduction in the charging pressure which indicates that the APC is working normally and protecting the engine.

However if a constant knocking occurs every time a load is put on the engine, then this means you have a problem in the system that needs prompt attention.