Abstract: Variable duration hydraulic lifters having either the conventional flat bottom surface or a roller mounted in their bottom end for engagement with a cam. The unique plungers have an upper portion and a lower portion whose outer cylindrical surfaces have grooves extending from their top edges to their bottom. The width of these grooves at their top end is less than their width at their bottom edges. Also these grooves may have a depth at their top edges that increases as it approaches their bottom edges. The benefits resulting from the unique grooves in the outer surface of the plunger includes reduced emissions, increased gas mileage, and an increase in the engines horsepower and torque.