Abstract: A variable volume combustion chamber system for a four-cycle internal combustion engine features an engine having a cylindrical combustion chamber inside. An intake port and an exhaust port are located on a chamber side wall. An ignition plug port and a tangentially-mounted fuel injection port are also located on the chamber side wall. A cylindrical piston is pivotally located on a connecting rod in the combustion chamber. The system features an adjustable volume head assembly located on a chamber top featuring a solenoid connected to a cylindrical head and sleeve component via a positioning shaft. The cylindrical head and sleeve component adjustably extends through the chamber top into the combustion chamber and is movable via actuation of the solenoid.