SUBARU BOXER 2.5L SOHC engine SUBARU BOXER 2.5L DOHC turbocharged engine Active Valve Control System i-Active Valve Lift System: The 2.5-litre SUBARU BOXER engine powering the Impreza 2.5i features genuine green thinking: two-piece valve lifters that work separately at low rpm and together at high rpm. The Subaru Impreza features a standard 5-speed manual transmission linked to Subaru symmetrical full-time AWD through a viscous-coupling limited-slip centre differential. This transmission boasts a short-stroke feeling for maximum smoothness and efficiency.