A system for deactivating half the cylinders of an internal combustion engine to reduce its capacity, as in converting a V-8 engine to a V-4, to reduce fuel consumption and atmospheric pollution and make the engine more economical to operate. The pistons of the deactivated cylinders are replaced by piston substitutes, which are equal in weight to the original pistons and do not affect balance of the engine, the piston substitutes being cut away to allow free air flow and avoid pumping action. Spark plugs in the deactivated cylinders are replaced by dummy conductive plugs, the valve push rods are removed and the tappets replaced by fixed substitutes.