Engine Tech: Flat-tappet Lifters Still Viable in Performance Engines
The wholesale conversion of almost all modern automotive engines to roller lifters or overhead camshafts may suggest that the flat-tappet lifter has outlived its usefulness. Yet, many great performance milestones of the last 60 years were accomplished with flat-tappet camshafts. Flat tappets in both mechanical (solid) and hydraulic forms have enjoyed a long successful run, and it’s far from over. Thousands of weekend circle-track racers and weekend bracket racers still race and win with simple, inexpensive flat-tappet camshafts.
“Enthusiasts still purchase a surprisingly high number of flat tappets,” says Chris Douglas of Comp Cams.
“There are also some engines that are not readily adaptable to use roller type lifters, due to physical constraints,” adds Chase Knight of Crane Cams. “The 272-312 Y-block Ford V8 has mushroom lifters with a small diameter upper stem, making roller installation challenging.”
Roller lifters provide clear-cut performance gains in many cases, but it all depends on how much is needed for a particular application? Flat-tappet cams are one of the most economical performance upgrades for budget-minded engine builders, and they still boast plenty of potential.
“A lot of people assume that hydraulic rollers make more power than a solid flat-tappet. Hydraulic rollers start getting unpredictable at higher revs and even could be worse than a hydraulic flat-tappet because they weigh more. All things being equal, a solid flat-tappet will make more torque and power, and it’ll also rev as long as you got enough spring pressure to control it,” boasts Steve Slavnik of Lunati Cams.
Friction reduction and profile shaping are the roller tappet’s distinct advantages over flat tappets. Rolling contact surfaces enjoy a lower coefficient of friction than sliding surfaces; which is an important factor in a factory production engine that needs to balance power, fuel economy and durability. But the overall friction reduction is not as great as expected, because the similar-style lifter bodies generate the same drag within their individual bores.
From a performance engine builder’s perspective, the primary advantage of a roller tappet is its ability to follow a more aggressive cam lobe without valvetrain distress. This allows much faster valve opening rates and provides more area under the lift curve. More lift can be achieved without the added duration required to “ramp up” to the desired lift, hence the area curve can be expanded without an increase in duration.
Steel roller cams and roller lifters are definitely more expensive, but undamaged roller lifters are generally reusable if you decide to change cams. Flat-tappet lifters can never be reused on a new cam, but they can be rerun in the same engine block as long as you match them to the same cam and return them to their original lifter bores. In the absence of excessive spring pressures, flat-tappet cams will also last a long time if broken in properly and supported by regular oil and filter changes.
Some racers discovered that a flat-tappet hydraulic can out-power a comparable solid-lifter cam, if the spring pressure is correct and the hydraulic lifter is adjusted one-quarter turn down from zero lash. This is partly because the hydraulic stick is effectively a bigger cam because it doesn’t require valve lash and clearance ramps. This can hold true up to about 6,500 rpm where the lighter solid lifter gains the advantage — provided that the inlet and exhaust components support the higher engine speeds.
“Each lifter establishes a precise relationship with the lobe it contacts as the break-in and early running processes occur. Very minute differences in lifter face radii and contour, lobe taper and the offset from the center of the lifter to the center of the lobe establish these unique characteristics,” explains Knight. “This is also why it’s not advised to put a used flat tappet camshaft and lifters into a different block, even if the lifters are going on to the same lobes.”
With hydraulic cams, the duration shrinks with more rpm due to both lifter compression of the air bubbles in the high-pressure side and deflection of the pushrods and rockers than it does in a solid-lifter system.
“Hence, I’d put the solid at a clear advantage above 6,500 rpm and a probably considerably better on any engine spending significant time over 6,000,” says Comp Cams engineer Billy Godbold.
The biggest mistake is not appreciating the fact that a flat-tappet does need a break-in.
— Steve Slavnik, Lunati Cams
Flat-tappet cam profiles vary broadly and according to application. Even old school single-pattern street grinds can still tear your head off with the right mix of cylinder heads, intake manifold and exhaust tuning. Incremental gains have been achieved by all manufacturers over the years, primarily in racing applications, but also in street grinds. Specific flat-tappet grind series have been developed to support a broad variety of engines and racing applications, depending on the original lifter size, usage and rpm range. Many of these lobe designs are meant for specific lifter diameters and 2:1 or higher rocker ratios. They differ in aggressiveness to the point that some grinds even specify increased oiling via EDM lifters, lifter bore grooves or whatever means possible to keep the lobes well lubricated. Other aggressive profiles are specified for max area under the curve or with tighter valve lash for quicker valve action.
Proper lubrication is critical with flat-tappet cams, particularly with contemporary motor oils formulated with less aggressive friction additives. Experts stress that greater loads place a premium on the lobe/lifter interface, so oil, assembly lube and break-in procedures become more critical than ever. They recommend that all flat-tappet cams be fully pre-lubed with high-pressure lubricant and broken in with quality ZDDP rich break-in oil (see Racing Oil versus Street Oil story). Once the cam is properly broken in, it is still important to run the correct oil for the application.
While there is nothing particularly new in flat-tappet lifter design, flat tappets have still gained performance because better springs, higher rocker ratios and stiffer pushrods have allowed designers to push flat tappet lobe designs well beyond what was previously thought possible for any given lifter diameter.
“With today’s stiffer, more stable valvetrains, an existing camshaft will appear to the engine to be more aggressive due to the lack of deflection,” says Knight. “This also allows the lobe designer to press the acceleration rates on new lobes a bit higher, with a lessened concern for durability. The same physics still govern the lifter velocity that can be used, as dictated by the effective lifter diameter.”
While one might expect technology to trickle down from NASCAR Cup engines, there’s nothing really applicable unless the lifter bores are opened up to a larger diameter, and that’s not practical for most street and sportsman applications. Also, much of NASCAR technology is devoted to special materials and exotic coatings.
NASCAR teams can also utilize very precise camshaft break-in rigs that condition the cam and lifters in a separate fixture from the actual engine. This performs the break-in function prior to installing them in the engine. Their level of precision is such that they can break in cams and lifters in batches and store them for future use without having to perform a break-in procedure on every new engine.
Most sportsman series that restrict roller cam usage also specify stock-type lifters to contain costs. In the best cases they may allow EDM (Electrical Discharge Machining) solid lifters that feature a small oil discharge hole (typically .010-.024-inch) drilled in the lifter face to provide direct lubrication to each individual cam lobe. The holes are drilled with an electrical discharge that bores a perfect pinhole. Most manufacturers center the hole in the lifter face, but others also offer an offset hole to place it closer to the lifter/lobe interface as the lifter rotates. The effectiveness is still debated by engine builders. As the lifter spins, the offset hole loses contact with the lobe for more than half its full rotation, effectively just squirting oil down the side of the lobe. A centered hole does not change its position as the lifter rotates, thus the oiling hole retains its maximum effectiveness.
The EDM racing lifter’s effectiveness depends on the particular engine and that similar results can often be gained by simply grooving the lifter bore to provide a constant oil feed to the rotating cam lobe.
“I prefer the groove as the tappets often loft a bit and at certain rpm will crash right at the nose, resulting in high stress focused near the EDM hole,” explains Godbold. “The groove seems more direct to the interface and does not compromise any part of the tappet face. However, both work well and either is far better than neither.”
Flat-tappet cam and lifter installation sometimes requires a little special attention, and there is one myth that needs to be addressed for any assembly begins. It’s unnecessary and even counterproductive to “pre-pump” hydraulic lifters full of oil prior to installation and valve adjustment. “Pumped up” lifters cause the valves to open during the adjustment process, preventing you from positioning the valve lifter plunger in the correct preload position.
I prefer the groove as the tappets often loft a bit and at certain rpm will crash right at the nose.
— Billy Godbold, Comp Cams
Pre-soaking hydraulic lifters in oil isn’t essential, but it does ensure that the lifter bodies are adequately lubricated prior to installation. It may also promote quieter engine start up if the oil in the oil bath displaces some air from the lifter’s plunger reservoir.
Cam and lifter installation generally assumes clean lifter bores and properly fit cam bearings. Use a commercially available cam installation tool if at all possible. They’re cheap! Otherwise screw the longest bolt you can find into one of the cam gear bolt holes to provide some leverage for maneuvering the cam through the cam bearings without damaging them.Builders have their preferences, but haste is not typically one of them. Lubricating the entire cam ahead of time is messy and invites contamination. It’s better to lube the lobes a few at a time as you insert the cam. Many builders use their fingers, but a more thorough job can be accomplished with a small brush. Coat the lifter bodies with oil and apply high-pressure lubricant liberally to the lifter faces, pushrod tips and guide plates before installation and break-in.
“The biggest mistake is not appreciating the fact that a flat-tappet does need a break-in,” warns Slavnik. “Light springs used to be the standard in the industry for break-in, but if you’re not an engine builder and don’t have the facilities, it’s a pain to change the springs.”
One options is removing the inner springs from dual- and triple-spring packs for the initial break-in. Many builders set up their spring packages in advance, but they substitute a used set of stock springs of the appropriate diameter for break-in. Once the break-in is accomplished they quickly change to the correct springs. Another trick is using low ratio 1.2:1 break-in rockers. These are especially popular for small-block Chevys as that engine is used in so many racing classes that require flat-tappet cams. The lower-ratio rockers barely depress the springs, so less pressure is applied to the cam lobes.
Each lifter establishes a precise relationship with the lobe it contacts.
— Chase Knight, Crane Cams
A widely recommended startup procedure is firing the engine and immediately taking it to 2,000-2,500 rpm. Check for leaks or other problems, then verify the timing to ensure that it won’t overheat. Monitor the engine constantly for problems. Check for oil pressure and watch for smoke coming out of the breathers which may indicate that something in the valve train is burning up. If you suspect a problem shut it down and investigate. You can always restart the process once you are satisfied that it’s okay.
After thirty minutes you can stop the engine, pull the valve covers and inspect the valve train with confidence that the cam has been properly broken in. Reinstall your preferred spring pack and rockers and re-fire the engine to build some temperature so you can re-lash the valves. Then you’re ready to tune the engine.
While rules may force the use of flat-tappet cams in some racing applications, advanced cam profiles and supporting hardware have given new life to flat-tappet cams for performance street engines. Check out the websites from the cam manufacturers for more information.