Abstract: A power plant has an internal combustion engine with piston backside pumping and a stroke of sufficient length to reduce the pressure of the combustion gases at the end of the power stroke to substantially a common inlet and exhaust pressure, extracting maximum power from the combustion gas expansion and reducing both power loss from expansion into the exhaust manifold and exhaust noise. For a cylinder the exhaust valve is opened at a specific time relative to the engine cycle, but exhaust closing is variable to account for changes in operating conditions and power settings. In a preferred embodiment the engine is a two-cycle compression-firing engine, and an exhaust valve for a cylinder is opened by cam action but closed at a variable time by an actuator controlled by a computer.