A system for adjusting a compression ratio of an internal combustion engine, includes a piston position adjusting mechanism having a lever pivotably mounted on each of the connecting rods, adapted and configured to pivot when actuated, a cam, rotatably mounted with respect to each of one or more pistons and respective connecting rods, adapted and configured to maintain, at any moment, one of a plurality of spacings therebetween, rotation of the cam by a preselected angle causing a change in relative spacing between the head of the respective piston and small end of the connecting rod. The trigger is adapted and configured to cause pivoting of the lever, engagement between the cam and the lever, and rotation of the cam by the preselected angle.