The cam lifter (also called a “follower” or “tappet“) is the component that makes direct contact with the cam lobes and “follows” the profile of the cam. There are generally four types of lifters:
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 “bleed down” the excessive oil pressure , and thus does not allow the valves to seat.
Mechanical Flat Tappet
The mechanical (solid) tappet is essentially a solid “link” 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 “bleed down.” Solid lifters do, however require lash or clearance to allow for part expansion as the engine heats up.
Mechanical Roller Tappet
The mechanical (solid) roller tappet allows for the most aggressive lobe designs. Roller tappets allow faster, “steeper” opening and closing ramps. This allows the cam to produce more lift for a given duration. They are not limited to a particular lifter diameter to obtain higher cam lifts. They also contain a roller that reduces friction between cam and followers. Roller cams require the use of higher valve spring forces making high engine speeds (over 10,000 RPM’s) possible.
Hydraulic Roller Tappet
The hydraulic roller tappet camshaft can provide the best of both worlds. Diesel engines and some motorcycle engines have used this design for many years. They provide most of the virtues of a solid mechanical roller tappet while providing the benefits of quiet operation and ease of valve lash setting.
This type of design still has the limitations of an oil bleed-off control type follower. If your application requires high RPM potential you should use a solid roller tappet design.