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Hydraulic Lifter Selection and Adjustment

Jul 19, 2023

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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.

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 engine (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.

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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.

Our baseline lifters were Comp's Pro Magnum, an anti-pump-up design. The recommended preload for these lifters is 0.002 to 0.004 inch. The zero point can be felt by lightly spinning the pushrod while slowly turning the adjuster in, until exactly the point where a slight resistance is felt. This is zero. We set the rockers to specs, giving the output shown in row one of the accompanying dyno table (above). We found no noticeable valvetrain instability to 6,200 rpm.

The next test was with the Pro Magnum lifters set with 0.002-inch actual lash. With lash, power seemed erratic, though our best pull was up slightly on our previous test, as indicated in row two of the dyno table. For the 2hp and 2-lb-ft average gain, we don't think this setting is worth it, considering the additional pounding the valvetrain will see with lash and the inconsistency observed, pull to pull.

Hydraulic cams with conventional lifters will self-adjust over a wide range of lifter preload settings, although for performance use, adjusting preload to a minimal level is the standard recommendation. Comp specifies an ideal preload of 0.030 inch +/- 0.010 inch, or approximately half of a turn in from zero on a standard adjuster. Sometimes, we have been guilty of running much more, particularly in engines with stock non-adjustable valvetrains. How much power does this practice cost us? We wondered. We decided to test the full range of plunger travel, varying preload from just 0.010 inch off bottom, to all the way up, to see how much it would vary output.

We devised this rig on an arbor to measure the amount of plunger travel in a Comp High Energy lifter for our application. We found 0.195 inch of plunger travel from top to bottom, which is quite a range. Will we find any difference in power depending on the initial preload setting?

We tore down our test engine and fished out the Pro Magnum lifters to make way for the OEM replacement-style High Energy lifters. A strong magnetic retrieval tool is handy for removing the lifters from their bores.

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.

We wanted to run the lifter over the full range of plunger travel, starting near the bottom and working our way up. To help compensate for overextending the rocker's adjuster when running deep in the lifter's plunger, we used lash caps to make up some of the distance.

Our primary setting with the High Energy lifters was with the plunger adjusted down to 0.010 inch from fully bottomed. With the plunger practically all the way down, there's no room left for absorption or loss of cam motion through the hydraulic piston pushing back during the lift cycle. We found power was up over our previous test, but rpm capability was down compared to the Pro Magnums. The engine became unhappy over 6,000 rpm, at which point we limited the pulls. Really, with the plunger almost all the way down, there's precious little oil between the delicate valve at the base of the inner plunger and the bottom of the lifter body. We have heard engine builders warn that this setting can damage lifters.

From our initial setting of 0.010 inch off the bottom, we worked our way up in 0.050-inch increments, equating to preloads of 0.150, 0.100, and 0.050 inch, with the results in rows four, five, and six of the dyno table, respectively. At 0.150-inch preload, power was down from the previous test at nearly fully bottomed, and we had to drop the top rpm of the test to 5,900, due to the engine's audible valvetrain instability. At 0.100-inch preload, the engine was very unhappy, giving us the lowest numbers of the test and refusing to rev beyond 5,800 rpm with a fluttering miss. At 0.050-inch preload, things began to improve. We then set the preload to 0.025 inch, basically the recommended level, and found the engine would once again turn to our predetermined test redline of 6,200 rpm; power came up noticeably (dyno table, row seven). For our final test, the High Energy lifters were set to zero lash. Though this design of lifter isn't meant to run at zero due to the possibility of retainer clip failure, we found the best power of the day (dyno table, row eight).

Our last test was with the Comp Hi-Tech lifters, a dedicated high-bleed design. The Hi-Tech is a special-purpose lifter primarily designed to increase vacuum and throttle response with extremely large hydraulic cams or in racing applications in which hydraulic cams are required and there are vacuum rules in place. They are not recommended for extended street use. Hi-Techs use fast plunger bleed-down to absorb duration and lift at low rpm.

The recommended preload with the Hi-Tech lifters is 1⁄4-3⁄4 of a turn past zero lash, similar to a standard hydraulic lifter. We immediately found the Hi-Techs lived up to their promise. Idle vacuum increased from 7.2 in-hg to nearly 9 in-hg, a fairly dramatic change. Idle quality was noticeably better. Power pulls revealed output similar to the Pro Magnums, as shown in row nine of the dyno table.

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.

Editor's note: This story was originally published in the Summer 2014 issue of Engine Masters magazine.

On episode 10 of Engine Masters, you'll learn a simple combination for a 600hp big-block Chevy. Many people building engines like this are torn between the streetability of a camshaft that uses hydraulic roller lifters and the race-level rpm capabilities of the solid roller lifters. This episode will explain the tech and show you the real-world difference in power between the two styles of camshafts, so you can decide what's best for your next engine build. Sign up for a free trial to MotorTrend+ today and start watching every episode of Engine Masters, plus much more!

To celebrate HOT ROD's 75th anniversary, we teamed up with CASTROL GTX to bring you some of the stories that exemplify the core of what HOT ROD is and reflect the brand's influence on America's car culture. Click here to learn more about CASTROL GTX. Related: