a timing mechanism actuated by hydraulic fluid uses a means responsive to vacuum to relatively displace a drive member and a driven member . when incorporated in an internal combustion engine for advancing or retarding an engine camshaft with respect to an engine crankshaft , the relative displacement is selectively controlled to reduce smog - producing exhaust emission and improve engine performance and fuel economy . in the preferred embodiment , the drive member and driven member form a fluid pressure chamber therebetween and the vacuum responsive means regulates the hydraluic fluid in said chamber . a vacuum control means which is responsive to engine operating conditions governs the application of vacuum to the vacuum responsive means . when engine operating conditions are at certain specified levels , a signal is transmitted to actuate the vacuum control means to allow the application of vacuum to the vacuum responsive means . with the application of sufficient vacuum , the vacuum responsive means allows hydraulic fluid into the chamber whereby the drive member and driven member are relatively displaced .