A simple, passive, lost-motion mechanism for varying the timings of engine valve opening and closing events has been developed and evaluated. The mechanism was found to be durable and precise in its control. When applied to a high-specific-output engine, the mechanism allowed much lower idle speeds and reduced idle fuel consumption without compromising high-speed power. However, valve-train noise was increased moderately at low engine speeds. When applied to a low-speed passenger-car engine with a compatible camshaft to reduce valve-train noise, the mechanism provided idle quality, fuel consumption, and vehicle performance roughly equivalent to those for the same engine operating with conventional fixed valve-timing. Thus, the potential advantages of this type of variable valve-timing appear to be confined to an engine system which is designed for high power output over a wide speed range.