Automatic Performance Control
| Company type | Engine control technology |
|---|---|
| Industry | Automotive |
| Founded | 1982 |
| Founder | Per Gillbrand |
| Products | Engine knock and boost regulation system |
| Parent | Saab |
Automatic Performance Control (APC) represents the pioneering system designed for engine knock and boost regulation. This innovation was developed by Per Gillbrand at the Swedish automotive manufacturer SAAB.
SAAB first implemented the APC on their turbocharged Saab H engines starting in 1982, and it remained standard equipment on all 900 Turbo models through 1993 (including 1994 convertibles), as well as on 9000 Turbo models up to 1989. The technology was also licensed to Maserati for use in the carbureted Maserati Biturbo, where it was adapted with specific calibrations and marketed as the Maserati Automatic Boost Controller (MABC).
The APC enabled an increase in compression ratio—originally from 7.2:1 to 8.5:1, and later to 9.0:1 on the 16-valve versions introduced in 1985. This enhancement contributed to better fuel efficiency and permitted the use of lower-octane fuel without risking damage from knock.
Functioning as a boost pressure and performance regulator, the APC manages both the rate at which boost pressure rises and its maximum threshold while simultaneously monitoring for detrimental knock occurrences.
To regulate the turbocharger, the APC continuously tracks engine RPM alongside inlet manifold pressure using a pressure transducer. These inputs govern a solenoid valve that adjusts both how quickly boost pressure increases and its peak level by channeling pressure to the turbocharger’s pneumatic wastegate actuator.
Knock detection is achieved through a piezoelectric knock sensor, essentially a specialized microphone mounted on the engine block, which picks up characteristic frequencies produced by knocking. This sensor emits a small voltage signal transmitted to the electronic control unit, where it is analyzed to confirm knock presence. Upon detection, the control unit triggers a solenoid valve that directs boost pressure to open the turbocharger’s pneumatic wastegate, diverting exhaust gases away from the turbine and reducing boost until knocking ceases. Knock events managed by the APC can often be observed as subtle fluctuations of the in-dash boost needle. Additionally, the APC system includes a 'knock' output designed for an LED indicator, which illuminates when knock is sensed. Since knock sensor accuracy diminishes at higher engine speeds, the APC correspondingly reduces maximum boost beyond roughly 4,500 RPM.
APC boost gauge
In Saab’s Full Pressure Turbo (FPT) variants equipped with this technology, the instrument cluster features a non-numeric boost pressure gauge prominently labeled with the APC designation. While knock sensors have become widespread even on naturally aspirated engines today, Saab has maintained use of the APC branding as a distinctive hallmark.
The left portion of the gauge’s scale displays manifold vacuum during typical driving conditions, with a short white marker indicating atmospheric pressure (engine off). The orange segment represents safe turbo boost levels, whereas the red zone signals boost above approximately 0.5 to 0.7 bar, where either wastegate activation or fuel cut may occur due to overboost protection.
In 1990, Saab advanced APC technology by integrating boost control with ignition management through the introduction of the DI/APC system, initially available exclusively on 9000 models. This combined system not only mitigated knock by modulating boost via a solenoid valve but also retarded ignition timing, thereby optimizing knock control and managing fundamental ignition timing settings simultaneously.