Closing up the gap between the tip of the rocker arm and the top of the valve stem reduces the pounding effect that can accelerate valve and rocker wear. Because of this, most of the push rod engines that have been built for the last 60 years have come factory-equipped with hydraulic camshafts and lifters. On newer overhead cam engines, hydraulic cam followers serve essentially the same purpose.
The basic operating principle behind hydraulic camshafts is truly old school technology. In 1910, a French car builder near Le Mans, France named Amedee Bollee invented the first self-adjusting valve tappets. Bollee’s two-piece tappets consisted of an upper and lower piston held slightly apart by a small spring. A port in the side of the lifter bore allowed oil to enter the cavity between the two pistons.
Oil pressure pushed the upper piston up to remove slack between the tappet and valve (this was a flathead engine where the tappets push up directly against the inverted valves). There was no danger of oil pressure pushing the valve open because the pressure exerted by the valve spring holding the valve shut was far greater than the oil pressure inside the tappet.
When the cam lobe raised the tappet, a one-way ball valve in the oil port prevented the oil between the pistons from leaking out. The oil trapped between the two pistons was incompressible, so the tappet acted like a solid member to push the valve open.
In the 1930s, General Motors developed its own “zero lash” tappets for some of its engines. By the 1950s, hydraulic lifters were common in most engines and are still used today.
Inside a Hydraulic Lifter
In a modern hydraulic lifter, a hardened steel push rod cup sits on top of a plunger mounted inside the hollow lifter body. A lock ring in the top of the lifter holds the assembly together. Under the plunger is a spring that holds the plunger up so oil can fill the cavity between the plunger and lifter body. A one-way check valve in the bottom of the plunger allows oil to enter the plunger cavity but traps the oil inside when the lifter moves up. This prevents the lifter from collapsing, which would not allow it to open the valve fully.
The clearance between the plunger and lifter body is extremely tight, typically 0.0002? or less. This is done to limit oil loss from inside the lifter (called the “bleed down” rate) when the valve opens and closes. A small amount of leakage (bleed down) must be allowed with each valve cycle so the lifter can readjust itself to maintain zero valve lash.
Valvetrain clearances change with temperature as the engine heats up and cools down, so the hydraulic lifters have to constantly compensate for thermal expansion in the block, heads, pushrods, valves and other valvetrain components. If this were not done, the lifters might retain too much oil, pump up and overextend themselves, preventing the valves from fully closing. This, in turn, can cause valve float, a loss of compression, misfire, and possible valve damage if a valve remains open long enough to hit a piston.
Features and Limitations
The very feature that allows hydraulic lifters to self-adjust and maintain zero lash can also work against the lifters at higher engine speeds. As engine rpm increases, the bleed down rate inside the lifters may be too great. There may not be enough time to refill with oil between each valve cycle, causing the lifter to collapse. Or, if the bleed down rate is too low and the lifters retain too much oil, they can pump up and overextend the valves. Either way, you can end up with valve float, misfiring and loss of power.
The rev limit for a typical set of stock hydraulic lifters is usually around 6,200 to 6,500 rpm. If you want to rev the engine higher than this, you either need solid lifters or modified performance lifters that can safely handle higher rpms without pumping up or collapsing.
The bleed-down rate of a hydraulic lifter can be varied by changing the internal clearances between the plunger and lifter body for other reasons too. Some aftermarket hydraulic lifters designed for street use have a higher bleed down rate that effectively reduces camshaft duration by 6 to 10 degrees and valve lift .020 to .030? at low rpm for increased intake vacuum and throttle response. At higher rpms, the higher bleed down rate has less of an effect allowing normal camshaft duration and lift.
Hydraulic lifters that have an “anti-pump up” design are made with tighter internal clearances and/or special valving to reduce bleed down. Anti-pump up lifters allow higher engine speeds and are a good choice for a dual-purpose street/strip engine. One supplier of such lifters says their anti-pump up lifters can handle engine speeds up to 7,500 rpm with no valve float, and can even be used with many camshafts that are designed for solid lifters.
Roller or Flat Tappet?
Aftermarket hydraulic cams and lifters are available in both roller and flat tappet configurations, with flat tappet cams and lifters being the most popular because of their lower price. Even so, roller cams offer many significant advantages over flat tappet cams.
The ramp profile on a flat tappet cam lobe is limited by the size and shape of the lifter body. The ramp cannot be too steep otherwise the bottom of the lifter will dig into the lobe. With a roller cam, much more radical lobe profiles are possible because the relatively small roller can follow a faster change in the lobe profile. This allows the valves to open and close at a faster rate to increase the “area under the curve” for more usable power and torque. Thus, a roller cam with the same lift and duration specs as a flat tappet cam will usually be ground with more aggressive lobes that increase airflow and power.
Comparing Cam Specs
Comparing the lift and duration specs of various camshafts is not as straightforward as it should be because of the ways different camshaft suppliers determine these numbers. Some cam suppliers measure lift when the lifters have moved .006? above the base circle, while others measure lift at from .004?. This can make the duration specs (valve open times) appear longer if the spec is measured from .004? rather than .006?.
It’s not a big deal, but it makes it harder to directly compare one brand of cam to another unless you read the fine print as to how they measure their cams. Comparing “advertised” duration numbers that are measured at .050? is a more accurate way to compare one cam to another.
Comparing hydraulic and solid lifter cams can also be tricky. The lobes on hydraulic cams are ground slightly differently than those on solid lifter cams to better accommodate the way in which hydraulic lifters operate. The opening rates on a hydraulic grind may be somewhat higher to overcome the slight bleed down that occurs inside the lifter, and the closing ramp may be more gradual for quieter operation of the valvetrain.
Can you use solid lifters on a hydraulic cam, or hydraulic lifters on a solid cam? Anything is possible, and the results will vary depending on the grind and application. Solid lifters cannot be run with zero lash, so they usually require anywhere from .004? to as much as .015? of lash depending on the grind and application. With some cam profiles, the lifters may become extremely noisy at higher engine temperatures and speeds. Thermal expansion will be greater on engines with aluminum heads than those with cast iron heads, so the “right” amount of lash can vary quite a bit.
As a rule, running solid lifters on a hydraulic cam will reduce the effective duration of the cam at lower rpm. This will move the peak torque and horsepower curve to a lower rpm range, which may or may not be advantageous depending on what you are trying to accomplish. If you use hydraulic lifters on a cam designed for solid lifters, the power curve will tend to move up a bit but there will also be an overall loss of power and torque compared to what the cam would normally deliver with solid lifters. The best advice is to use the type of lifters (solid or hydraulic) the cam was ground for rather than trying to change one for the other.
Choosing a particular type of camshaft for an engine build is an important decision that has to be made before any other parts are ordered or machined. Choosing a cam requires answering some basic questions, the most important of which is the engine application itself.
Are you building an engine for everyday driving? For towing? For street performance? For street/strip? For drag racing or a circle track car? Are there class rules that limit the type of camshaft and valvetrain components that are allowed?
What kind of vehicle is the engine going into (light car, heavy car, truck, race car)? What kind of transmission and gearing will the vehicle have (stick, automatic, wide ratio or close ratio transmission gearing, final drive ratio)?
How much money is your customer willing to spend on the cam and valvetrain? Is it a budget build that will require a flat tappet cam with solid or hydraulic lifters, or is the sky the limit?
The camshaft determines the engine’s horsepower and torque curves, so the cam has to match not only the application but all of the other components that go into the valvetrain, the cylinder heads, compression ratio and induction system.
The most commonly made mistake is over-camming the engine with too much lift and/or duration. Big numbers look impressive, and you may have a customer who insists having the wildest cam he can find for his engine. But is it the best cam for how he will actually use his vehicle? Probably not.
Most street driven vehicles seldom see the high side of 5,500 rpm, and most cruise at 1,800 to 2,500 rpm on the highway depending on how they are geared. The best all-around cam for a street performance vehicle, therefore, would be one that has its peak power and torque curve in the 1,500 to 3,000 rpm range.
On the other hand, if you are building an all-out race motor for a customer’s race car, you’d want a cam that produces peak power and torque in the rpm range where the car will be running most of the time.
As for lift, most stock and lightly modified heads won’t flow any more air once valve lift reaches about 0.550?. Pushing the valves open any further will not increase airflow or power, and may actually hurt power because of reversionary air flow. On the other hand, if you’re building a Pro Stock race motor with highly modified heads and CNC huge ports that can handle gobs of air, increasing valve lift to the physical limits of the engine is often necessary to maximize power.
Another decision that has to be made is how much lift do you want from the cam and rockers? For any given lift, you can use various combinations of cam lift and rocker arm ratio to achieve the same numbers. According to one cam supplier we interviewed, the best approach is to get more lift with the rocker arms and less with the cam. Why? Because higher lift cams are more highly loaded cams that experienced more wear.
Consequently, you are more apt to round off a lobe on a high lift cam that has big lobes than one which uses smaller lobes with higher ratio rocker arms. The valvetrain also tends to be more stable when a higher percentage of the valve lift is generated by the rocker arms rather than the cam lobes, lifters and pushrods.
Installation and Break-In
With flat tappet hydraulic cams, don’t mix old and new parts (same for flat tappet cams). New cams need new lifters and vice versa. With roller cams, that’s not the case because there’s no break-in period for the lifters to mate to the cam lobes. But on flat tappet cams, break-in is critical. The cam lobes and bottoms of the lifters must be coated with high-pressure moly lube, and the engine must not be allowed to idle after it is first started. Keep varying the engine speed from 1,500 to 2,500 rpm for the first 20 to 30 minutes after the initial start-up.
Here’s another tip: don’t coat the sides of flat tappet lifters with grease or assembly lube. Use motor oil. The thinner viscosity of the oil will allow the lifters to freely spin as they ride on the cam lobes.
With flat tappet cams, too much initial spring pressure can have an adverse effect on the cam lobes and lifters until the cam is broken in. For the initial break-in, one cam supplier said open seat valve spring pressure should be limited to no more than 280 lbs. If the engine has double springs, temporarily remove the inner spring for break-in, and use a lower rocker arm ratio to reduce the load on the cam. Some racers go even lower, using no more than 225 lbs. of open spring pressure for break-in.
Finally, all performance flat tappet camshafts (hydraulic and solid) require motor oil that contains adequate levels of ZDDP or similar anti-wear additives. SN and SM rated motor oils for late model engines that have roller cams or overhead cams do not contain enough of these ingredients to adequately protect flat tappet cams. Customers should be advised as to what type of oil or oil additive to use so they don’t ruin their cam.