Active Valve Control System (AVCS) is an automobile variable valve timing technology used by Subaru. It varies the timing of the intake valves by using hydraulic oil pressure to rotate the camshaft in order to provide optimal air flow in and out of the engine. The system is closed loop using camshaft sensors, crankshaft sensors, air flow meter, throttle position as well as oxygen sensors and/or Air-Fuel ratio sensors in order to calculate engine load. The ECU is programmed to operate control valves that adjust the delivery of the hydraulic pressure in order to move the camshaft into the position that will provide the engine with the best performance while minding emissions standards.
AVCS is used on the Version 7 and up EJ207 engines, the EJ255 and EJ257, the second generation EZ30D (2005+ in the USA Market) found in the Legacy Outback, Legacy 3.0R and the B9 Tribeca. A Dual AVCS system utilizes both the intake and exhaust camshafts on the EZ36 found in the 2008 Tribeca. Dual AVCS is also found on Japanese Spec engines, although a complete list of the engines with Dual AVCS has yet to be compiled.
i-Active Valve Lift System (i-AVLS) is a version that supports variable valve lift. It includes 2 intake lift profiles depending on engine speeds. AVLS-equipped Subaru engines have 2 intake valves per cylinder, but AVLS only controls one of them, leaving the other in high-lift mode at all times.
At low engine speeds, each of the two intake valves for a given cylinder run on a different camshaft profile (the engines are 4-valve per cylinder type). One valve is operated by a low-lift, low-duration lobe, while the other is actuated by a high-lift, high-duration lobe. This increases air speed causing a smooth idle, while at the same time promoting a swirl factor, increasing efficiency.
At high speeds, the ECU (Engine Control Unit) computer signals a solenoid to let oil pressure move a pin which locks the two lobes together. This means at high speed both intake valves are effectively driven by the high-speed camshaft lobe. This increases high-RPM power due to increased valve lift, duration and overall timing.
Cam locking is done by moving a pin with hydraulic pressure. As with other variable valve timing systems, it allows the engine to have a quiet low-speed operation with a smooth idle, while also using a camshaft lobe profile similar to "race cam" at high engine RPMs.
It was introduced with 2007 model year with 2.5-liter naturally aspirated Subaru engines.
Widespread use of AVLS began with non-turbocharged Subaru 2.5 liter four-cylinder engines in Model year 2006. AVLS was first used in the 2005 six-cylinder 3.0 liter Subaru Outback. Turbocharged engines and Subaru's six-cylinder engines continue to use Subaru's AVCS (variable cam phasing) system instead of AVLS. Reports generally agree the new engine feels like a brand new engine with flatter torque curve compared to previous non-AVLS, non-AVCS 2.5 liter engines. Initial applications raised maximum power from 165 hp (123 kW) to 175 hp (130 kW), but more significantly raised the power at most RPM levels. The goal is better fuel economy, improved driveability and increased power.