The beauty of hydraulic lifters is that they self-compensate for valvetrain clearances, doing away with the need for valvetrain adjustment. For a regular production vehicle, this reduction in maintenance is a definite plus, and from a manufacturing perspective, there was also the benefit of a simpler and cheaper valvetrain. Although the hydraulic lifter is more complex than a simple solid, it allowed manufacturers to do away with the provisions for valvetrain adjustment. Simple and dirt-cheap one-piece stamped-steel rockers were the inevitable result. Best of all, the travel in the hydraulic mechanism soaked variations in production tolerances with ease, undoubtedly streamlining the production process, eliminating the need to set valve lash at the engine plant, and down the road in service. The icing on the cake is that since hydraulics self-adjust to zero lash, they provide unrivaled quietness, a primary goal in OE engine design. Produced in the vast quantities required, hydraulic lifters became a relatively low-cost component, and even today, hydraulics are generally the cheapest lifters available.

Hydraulics for Performance?

All-out racing performance was never on the agenda when hydraulic lifters were conceived, however, the vast majority of performance cams sold are unquestionably hydraulic grinds. Some of the same attributes that made them a favorite with Detroit hold favor with many enthusiasts. Since most engines were initially set up with hydraulic cams, hydraulic performance cams are usually the most cost-effective replacement choice. Making a switch to a solid grind can come with quickly escalating costs, most commonly requiring the upgrade to adjustable rockers and compatible pushrods. Besides the cost, for dual-purpose applications, quieter operation and never having to adjust the valves make the hydraulic a tempting choice.

Hydraulics work extremely well in moderate rpm applications, the range of most mildly modified street engines. Move up the performance and rpm ladder, though, and the very hydraulic mechanism that makes them work so well in a milder application can create problems. Even in the height of Detroit’s love affair with the hydraulic lifter, auto manufacturers generally favored solid lifter cams in their most serious high-performance powerplants. Chrysler’s Hemi (to 1970); GM’s LT-1, LS-7, or L-88; or Ford’s “HiPo” 289, are just a few examples in which automakers spurned their favored hydraulics when outright high-rpm power was the goal. Why? Under the stresses of high rpm, the hydraulic piston, which serves to zero-out the clearances in normal operation, can either pump up or bleed down. These are two very different phenomena, both of which lead to valvetrain instability and hinder hydraulic lifter performance.

All hydraulic lifters can absorb a small portion of the cam’s lift profile in running, through fluid bleeding past the lifter’s plunger piston during the lift cycle. In stock or mild street applications, absorption is likely negligible. Very aggressive cam profiles and spring loads in a radical street or racing application can strain the hydraulic lifter’s mechanism to the point where some performance potential is lost through absorption. Lifters with tight internal clearances and valving most accurately follow the cam’s profile.

The second form of false motion is the better known problem of lifter “pump-up.” The hydraulic lifter’s plunger is continually under hydraulic pressure from the engine’s oiling system. Under demanding circumstances, such as at high rpm, the valvetrain can partially unload. This unloading can occur during the onset of valve float, during spring surge, with valve bounce on closing, or as the dynamic spring load on the valvetrain is drastically diminished while the cam’s lobe rotates over the nose at high rpm. The hydraulic lifter’s plunger will quickly gap-up any time the force of the oil acting on the hydraulic piston exceeds the force of the valvetrain on the lifter’s plunger. This will cause the lifter to be temporarily overextended, into a condition referred to as lifter “pump-up.” The overextended lifter will cause the valve to be held slightly off the seat when the camshaft is on its base circle, effectively hanging up the valves.

Special Hydraulic Lifters

The aftermarket has developed some variations on the standard-issue hydraulic lifters. One of the first refinements was the introduction of anti-pump-up lifters. The concept is as simple as it is effective. In an anti-pump-up lifter, the light-duty retaining clip at the end of the hydraulic lifter’s internal plunger travel is replaced with a heavier, more positive stop. When used in conjunction with an adjustable valvetrain, an anti-pump-up lifter can be set so that the internal plunger is at or near the top of its range of travel when the camshaft is on its base circle. In running, the anti-pump-up lifter is essentially adjusted so the piston is already pumped all the way up against the stop, eliminating the possibility of the plunger extending any further. An adjustable valvetrain is, of course, required to utilize an anti-pump-up lifter as intended. Anti-pump-up lifters may also include changes to the lifter’s valving or clearances to alter the bleed-down characteristics, although current theory holds that “stiffer” is better.

Another variation of the hydraulic lifter is the so-called variable duration designs. There are several of these types of hydraulic lifters on the market, all sharing the common principle of increasing the bleed-down rate of the lifter’s hydraulic plunger. The goal is to lose some of the cam’s lift and duration to hydraulic absorption, particularly at lower rpm, in an effort to help tame a big cam. Essentially, the bleed-down of the hydraulic plunger is dictated by clearance area available for the oil behind the lifter plunger to escape, and the cycle time. The theory holds that since the area open to bleed-down is constant, the amount of bleed-down will vary in accordance to the elapsed cycle time. At low rpm, the cycle time is longer, allowing more bleed-down to occur, while at higher rpm the cycle time is shorter, giving less time for the oil to escape, and thereby imparting a greater portion of a cam’s lift profile to the valvetrain. Critics contend, however, that fast bleed lifters will never reach the cam’s lift and duration potential.

At what point can instability with a hydraulic lifter begin to hinder performance? The answer, unfortunately, is very combination specific. Valvetrain weight and geometry, pushrod deflection, preload adjustment, spring load, the cam profile’s smoothness and intensity being some of the factors besides rpm that can upset a hydraulic lifter’s ability to maintain valve control. Even oil viscosity and temperature have been reported to make a difference. Though there are too many variables to absolutely pinpoint the rpm capability of a hydraulic lifter camshaft, long experience in the use of hydraulic cams can suggest basic guidelines. Depending on the camshaft/valvetrain/spring combination, standard hydraulic lifters can be expected to operate effectively to somewhere in the 5,500- to 6,000-rpm range. Typically, anti-pump-up lifters can raise the rpm potential by 500 to 1,000 rpm. Certainly some have far exceeded these numbers, while other combinations experience problems at even more conservative levels.

We wanted to test of some of the commonly available hydraulic lifters and see for ourselves what effects they would have in a performance engine. We also have seen guys with stock valvetrains fretting over getting the recommended lifter preload setting, a real problem if the non-adjustable setup fails to land on the recommended 0.020-0.030-inch preload. How much difference would we experience various amounts of preload using stock replacement–style hydraulic lifters? Would some of the special hydraulic lifters show any benefit in a typical 6,000-rpm street engine? Read on for the goods on what we found while dyno-testing our Mopar big-block.

Hydraulic Lifter Function

What separates a hydraulic lifter from a conventional solid flat tappet is the addition of an internal hydraulically operated plunger within the lifter’s body. With the valvetrain installed (or adjusted), the pushrod compresses the plunger within its range of travel. How far down the lifter plunger has been displaced at its base setting is called the lifter preload. Oil pressure enters the lifter through an orifice in the lifter body, and flows through another orifice into the hollow body of the lifter plunger. A one way check valve at the bottom of the plunger allows oil to fill the cavity below until all the clearance gone, effectuating the hydraulic self-adjustment to zero lash.

When the cam rotates into the lift cycle, the check valve at the base of the plunger closes under the pressure imparted by the valve spring, preventing the oil from being squeezed back out as the valve opens. At the top of the plunger of some hydraulic lifters is a metering valve or plate, which supplies oil to the pushrods for valvetrain oiling.

Dyno Table
SuperFlow Engine Dyno
1. PM 0.003 preload 429.5 387 445.0 @ 4,500 444.3 @ 6,000
2. PM 0.002 lash 431.7 389 448.0 @ 4,500 448.5 @ 5,900
3. HE 0.185 preload** 432.9 390.4 448.5 @ 4,700 455.0 @ 6,000 (1)
4. HE 0.150 preload 431.1 385.2 445.4 @ 4,600 453.3 @ 5,800 (1)
5. HE 0.100 preload 427.6 381.9 442.5 @ 4,600 443.1 @ 5,800 (1)
6. HE 0.050 preload 430.6 384.7 443.9 @ 4,800 451.7 @ 5,900 (1)
7. HE 0.025 preload 435.1 392.1 452.3 @ 4,900 451.9 @ 5,800
8. HE zero lash 436.4 393.4 457.4 @ 4,900 455.7 @ 5,800
9. HT 0.020 preload 434.4 390.4 452.7 @ 4,500 445.9 @ 5,900
Legend And Notes:

PM: -- Pro Magnum Lifter; Anti-Pump-Up Design
HE: -- High Energy; Base Design Standard Lifter
Ht: -- Hi-Tech; Fast Bleed Variable Duration Lifter
* -- Averages Calculated From 3,000-6,000 Rpm
** -- Tested At 0.010 Inch From Bottom Of Plunger Travel
1 -- Valve Float Limited Test To 5,900 Rpm

925 Tower Ave.
WI  54880
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TN  38118
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TN  38118
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VP Racing Fuels
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TX  78265