Valve Lifters – Hydraulic And Mechanical – So, What is the big difference.
Both hydraulic and mechanical valve lifters have the same job, But do it differently.
They both follow the contour of the camshaft lobe and transfer that motion to open and close the valves.
Both, hydraulic valve lifters and mechanical valve lifters look similar from the outside.
What’s inside the lifter makes all the difference. Mechanical (Solid) lifters are as the name implies solid. There is no internal mechanism to take up clearance, and in fact, they require clearance to operate properly. 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. In a hydraulic lifter, the seat moves by means of a hydraulic valve and oil pressure within the lifter.
When the lifter is filling with oil, it is pumping up. When the oil is exiting the lifter, it is bleeding or leaking down.
NOTE, Tappets or cam followers are just other names for the same thing.
Valve Lash Setting – What Is It
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. Mechanical valve 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.
Advantages Of Hydraulic Valve Lifters
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 valve train 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. 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.
Disadvantages Of Hydraulic Valve Lifters
Another potential drawback of hydraulic lifters is that at excessively high engine speeds; valvetrain inertia may open the valves further than intended. This results in additional valve train clearance.
A hydraulic lifter senses this clearance; the plunger begins to lengthen, and may actually extend far enough to prevent the valve from closing. This could result in valve to piston collision or burnt valves.
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.
There are times when the hydraulic valve lifter needs to be adjusted. But instead of setting lash, a hydraulic system needs to have the preload set since there is no lash. Finally, This is usually only required if the cylinder head has been removed.
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