valve float: A high-rpm engine condition in which the valve lifters lose contact with the cam lobes because the valve springs are not strong enough to overcome the momentum of the various valvetrain components. The onset of valve float prevents higher-rpm operation. Extended periods of valve float will damage the valvetrain.
valve lifter: also called a
"valve follower": the cylindrically shaped component that
presses against the lobe of a camshaft and moves up and down as the cam
lobe rotates. Most valve lifters have an oil-lubricated hardened face that
slides on the cam lobe. So-called "roller lifters," however,
have a small roller in contact with the cam lobe: thereby reducing the
friction between the cam lobe and the lifter.
valvetrain: the collection of
parts that make the valves operate. The valvetrain includes the camshaft(s)
and all related drive components, the various parts that convert the
camshaft's rotary motion into reciprocating motion at the valves, and the
valves and their associated parts.
viscous coupling: a particular kind of fluid coupling in which the input and output shafts mate with thin, alternately spaced discs in a cylindrical chamber. The chamber is filled with a viscous fluid that tends to cling to the discs, thereby resisting speed differences between the two shafts. Viscous couplings are used to limit the speed difference between the two outputs of a differential, or between the two axles of a car.