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Hydraulic Lifters
A hydraulic lifter looks very similar to a solid lifter, except for one important internal difference: It is designed to be a self-adjusting shock absorber, using engine oil as the working fluid. When the camshaft lobe pushes up on the bottom of the lifter, an internal pool of oil pushes up on a piston and spring assembly. The internal piston pushes on the push rod. Due to this process, no rocker arm adjustment is needed since the lifter self-adjusts the entire valve train. The upside to hydraulic lifters is that the initial shock of contact is taken up by the oil, eliminating the constant need for rocker arm adjustment. The downside is that because it absorbs shock, the engine loses horsepower through the valve train. Moreover, hydraulic lifters have internal mechanical components that wear out, causing oil bypass or bleed-down. When it does wear out, the lifter no longer lifts. This condition is known as a "collapsed lifter."[LEFT]
Solid Lifters
Solid lifters are made of a solid rod of hardened steel. Most are about 1 inch in diameter and 2 inches in length. The bottom of the lifter rides on the camshaft, and the top has a small depression where the push rod sits. According to Ball State University, solid lifters were commonplace in the 1960s and the 1970s. The advantage to a solid lifter is that it does not lose horsepower in the process of lifting. The drawback is that the rocker arm can go out of adjustment quite often. If you hear a car engine make a clack-clack-clack-clack sound when going down the road, it's because the rocker arms are out of adjustment and hammering the valves. Currently, solid lifters are used in racing engines due to the horsepower loss factor of hydraulics.[LEFT]
Read more: Types of Engine Lifters | eHow
Hydraulic Flat Tappet
The hydraulic flat tappet is self-adjusting, due to the valve controlled plunger within the tappet body. It operates to pre-load the push-rod by using the oil system pressure to maintain this pre-load in the closed valve position. Hydraulic tappets are quieter than mechanical tappet lifters since there is no lash or free-play. However, it is generally agreed that they fall short of offering optimum performance above 6,000 - 6,500 RPM. Many cheaper designs fall even shorter than this. This poor performance at high RPM is due mainly to the inability of the lifter to "[I]bleed down[/I]" the excessive oil pressure , and thus does not allow the valves to seat.
Mechanical Flat Tappet
The mechanical (solid) tappet is essentially a solid "[I]link[/I]" between the cam lobe, and the push-rod. In most cases it is a simple heat-treated cylinder with a radiused contact face. It allows more RPM potential than that of the hydraulic tappet since there are no worries about the inability of the lifter to "[I]bleed down.[/I]" Solid lifters do, however require lash or clearance to allow for part expansion as the engine heats up.
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