Abstract: A variable length connecting rod (12) has a first locking mechanism (36) for releasably locking connecting rod parts in a first effective length setting (FIG. 7) for the rod, and a second locking mechanism (38) for releasably locking the connecting rod parts in a second effective length setting to change the compression ratio for an engine cylinder. When a length change is to be made, hydraulic fluid unlocks a locked one of the locking mechanisms, allowing inertial force to effect the length change during an engine cycle. At completion of a length change, the other locking mechanism automatically unlocks.