From a November 1960 Road & Track article on the new four-speed transmission for the Chevrolet Corvair:
As a non-mechanic, I can describe how a four-cycle engine works; but I don’t know the specifics of the different techniques that make it work. I’ve always assumed that the valves were operated by a cam, or by push-rods attached to a cam.
What is a hydraulic lifter, how is it different from the way I assumed valves were lifted, and why would they make the engine ‘literally quit’ at a certain RPM?
Lifters are little cylindrical things just above the cam lobes, in holes in the block called lifter bores. They look like this: http://www.compcams.com/Technical/CurrentCatalog/HTML/280.asp.
The cam lobe pushes on the lifter, the lifter pushes the pushrod, the pushrod pushes the rocker arm, the rocker arm rocks and pushes the valve stem, and the valve opens. Sounds complicated, but it works.
Solid lifters are just that, solid steel cylinders. Hydraulic lifters are more complcated: the cam starts pushing and as the lifters rise in the bores a passage is uncovered (it´s the little hole in the photo about two thirds from the bottom) which lets pressurized oil in, which lifts an internal cylinder and pushes etc.
The main advantage of the hydraulic kind is you don´t have to adjust the valves; with the solids there has to be some clearance in the system to allow for expansion when things heat up, and that clearance has to be exact. (In VWs it´s usually .006¨.) Also the solids tend to make a thrashing sound which hotrodders like but most normal people don´t. (The next time your´re sitting next to an old VW at a red light you´ll probably hear a clicking noise over the various blattings and rattlings that old VWs make; lots of people don´t set their valves as often as they should and VWs are particularly prone to getting out of adjustment)
In the past hydraulics were prone to ¨pump up¨at high rpms, meaning they would
fill up with oil and not have time to bleed back down before the cam tried to push them again. This would keep the valve from closing, with results that ranged from the Corvair symptoms you described to bent valves or damaged pistons.
Nowadays they work much better; Corvettes and the like often turn over 7000 rpm with no problems from their hydraulics.
Simply, what happens is the action of the valve train can’t keep up with the RPMs of the motor. Since the lifters are hydraulic, they have to pump up with fluid and when you reach a certain RPM the lifters will float, which means they don’t fully fill with fluid or fully compress. They kind of stay half filled which means the valves don’t open and close fully and the engine looses power (won’t accelerate anymore). This floating problem can also be caused by weak and/or old valve springs. Here is a fairly good site about vavles and such.
The hydraulic pressure is NOT used to actually open the valve. It is used to automatically adjust the play in the valve train to zero, thus eliminating valve noise, and the need for periodic manual valve adjustments as required with solid lifters.
The hydrualic cylinder in the lifter is sized so that it will NOT overcome the force of the valve spring holding the valve closed. When the cam lifts the lifter body, a port may be closed, “locking” the adjustment in place while the valve is opened. Alternativly, the inlet port may be small enough that the leakage during the open event is not significant.
All lifters are suseptable to “float”. This is when the springs are no longer strong enough to keep up with the inertial forces. Since hydraulic lifters have extra mass (more parts, and some stored oil) they are heavier, and float at lower RPM than a lighter solid lifter would with the same strength valve spring.
Valve adjustment is critical. If the valves do not fully close, they leak combustion gasses and are eroded as a result (AKA “burning” a valve). To insure the spring fully closes the valve, some lash (slop) is needed in a “solid” valve train. As the valve and seat wear, this lash decreases. If it hits zero, the valve will quickly fail.
On the VW thing: Valves don’t get noisy for lack of adjustment. If you fail to adjust them, they get less noisy over time. Too loose is just noisy. Too tight is quiet and an invitation to a burned valve. Also, the aircooled VW owners I’ve known have paid MUCH more attention to maintainance than my experience of the general population.
All solid lifter engines have valve clatter. Aircooled engines have more because:
1)Due to the wide range of temperatures at which they operate, more valve clearance is required to allow for differential expansion of the various bits.
2)The valve train is closely coupled to finned cylinders and cylinder heads, which tend to resonate and radiate the valve noise. Contrast this with a liquid filled cooling jacket that tends to damp the noise.