The 1.5-litre DOHC engine powering the Impreza features a long stroke, which is useful for improving combustion efficiency and torque at low and medium speeds, and better gas flow in the combustion chamber and intake/exhaust ports, combined with variable valve timing to increase combustion efficiency across all speeds. Both fuel economy and dynamic performance were enhanced by reducing friction between the basic components through enhanced stiffness of the cylinder head and cylinder block.