Automatic Performance Control (APC)
Automatic Performance Control (APC) adjusts boost pressure constantly to match octane rating and engine load conditions. Maximum engine output is always available and damage due to pre-ignition (pinging) is eliminated. System consists of knock detector, pressure transducer, solenoid valve, and Electronic Control Unit (ECU).
Knock detector (below intake manifold) senses "pinging" in engine and transmits signals to ECU. Pressure transducer (under dash at left of steering column) senses boost pressure in intake manifold downstream of throttle valve. Pressure is converted to electrical signal which is sent to ECU.
The ECU (under rear seat cushion) also receives RPM signal from ignition system. ECU operates solenoid valve by regulating electrical current pulse ratio. Solenoid valve (right radiator support bracket) governs control pressure to boost pressure regulator. Boost pressure regulator operates boost pressure regulator valve (wastegate) based upon intake pressure.
Solenoid valve is fed by intake pressure downstream from turbocharger. Solenoid inlet tube "C" has fixed diameter orifice. When the solenoid valve is fully shut, wastegate is open fully and provides low boost since boost pressure regulator receives full intake pressure from outlet "W" of solenoid.
When solenoid valve is fully open, pressure is vented through outlet "R" of solenoid to intake upstream from turbocharger. Boost regulator pressure is reduced allowing wastegate to close and increase boost pressure.
Solenoid valve fluctuates between open and closed positions at fixed frequency of 12 Hz. Pulse width changes when boost pressure exceeds 6 psi (.4 kg/cm2 ). Pulse width is ratio of time solenoid valve is open to time solenoid is closed during each cycle. Solenoid valve cycles 12 times per second. Pulse width is changed by ECU, depending upon signals sent to ECU.