Hydraulic-Mechanical Valve Lifters-FAQ. Valve lifters follow the contour of the camshaft lobe and transfers that motion to open and close the valves. Tappets or cam followers are just others names for the same thing.
Let’s start with understanding the basics, Hydraulic and Mechanical.
A domestic V-8 engine from the muscle car era will have either solid (mechanical) or (hydraulic) valve lifters. A solid lifter is a rigid component. A hydraulic lifter is designed to accommodate variations in valve train clearance in order to automatically maintain zero lash. It does this by filling and emptying the lifter with engine oil through a metering orifice and a check valve. When the lifter is filling with oil, it is pumping up. When the oil is exiting the lifter, it is bleeding or leaking down.
The term Valve Lash setting.
This describes the amount of clearance between the rocker arm and the valve stem. This happens when the lifter is on the base circle of the cam. (valve closed). Solid lifters are different. They have a predetermined lash or clearance. When adjusting the valves on an engine with hydraulic lifters, you are not really setting the lash or clearance. In reality you’re actually setting the preload on the lifter through the pushrod and rocker arm. The traditional adjustment on a hydraulic lifter is zero lash. Usually followed by a predetermined amount of turns on the hold-down nut.
Not all engines with hydraulic valve lifters have a preload adjustment.
If the rocker arm is stud-mounted, such as on a Chevy, then proper procedure involves attaining zero lash plus the specified amount of preload. Some engine designs, such as the Pontiac V-8, use a rocker stud, but are considered non adjustable. On these engines, the rocker stud needs to be changed to make the valvetrain adjustable with a hydraulic camshaft. Other designs use a pedestal mount for the rocker. They may require shims or an aftermarket rocker arm with an adjustment screw to set the preload. Some engines, including Buick V-6s and V-8s and many Chrysler V-8s, use shaft-mounted rocker arms. If the valve train is noisy, there is a wear issue or mechanical problem.
Hydraulic valve lifters offer several advantages,
- No more maintenance valve adjustments
- Quieter operation due to having no lash
- Longer valve and cam life due to eliminating the pounding of a solid lifter
- Smoother engine operation due to precise valve control and automatic compensation for expansion and contraction of the valvetrain parts caused by temperature change.
To understand how the hydraulic lifter is able to accommodate valvetrain slack while maintaining zero lash, we should look at its inner workings. When the valve is closed, the plunger spring in the hydraulic lifter takes up all clearance in the valvetrain. Oil enters the lifter body through feed holes and flows inside to the plunger. The oil continues to flow down through the hole in the bottom of the plunger, around the check valve and through the holes in the check-valve retainer to completely fill the cavity below.
As the lifter begins to ride up the cam lobe, the oil below the plunger tries to escape past the check valve. This sudden flow of oil forces the check valve to seat, which seals the hole at the bottom of the plunger. Now the full load of the valvetrain is on the lifter. It is extremely hard to compress any fluid. This makes the lifter now act almost as if it were a solid design.
Read more on Hydraulic Valve Lifters,
A predetermined and closely held clearance between the lifter’s plunger and its body permits a minute amount of oil to escape from below, moving past the plunger. This movement of the plunger with respect to the lifter body after the check valve is seated is called leak down or bleed-down; it consists of the oil draining out.
As the lifter returns to the base circle of the camshaft, oil fills the high-pressure cavity and the cycle begins again. When the engine temperature change requires shortening the lifter’s effective length, a hydraulic lifter automatically compensates: the valve spring forces the plunger down. The return spring raises the plunger when lengthening the lifter is required and causes more oil to flow into the spring cavity.
One of the disadvantages of a hydraulic valve lifter is that it cannot follow as aggressive a cam profile as a mechanical design. This limits the engine’s power and operating speed. In addition to the cam profile being milder, a hydraulic valve lifter requires a certain amount of time to respond to changes in the engine. In turn limiting engine power when compared to a mechanical design. Aftermarket performance valve lifters are designed to pump up and bleed down at a different rate. The disadvantage is they can than often sacrifice quiet operation and longevity to do so.