Solid, Hydraulic, Flat-Tappet, and Roller Cams - How It Works

COMP Cams Explains the Pros and Cons of Solid, Hydraulic, Flat-Tappet, and Roller Cams, and the Advanced Technology that makes them all Work.

Stephen Kim May 6, 2013 0 Comment(s)
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With horsepower seemingly growing on trees these days, picking a set of high-flow cylinder heads for your engine combination is the easy part. A mother load of rotating assemblies and aftermarket blocks means that building a big-cube short-block to take advantage of all that airflow isn't exactly difficult, either. With the top and bottom end of an engine build covered, the wild card in the horsepower equation often comes down to the camshaft. While it's true that manufacturers offer a diverse range of off-the-shelf camshafts for every conceivable combination, several important decisions must be made before you even narrow the field of part numbers down to a list of finalists.

First off, which section of the catalog should you even turn to? Valvetrain technology has advanced so rapidly in the past few years that many of the old rules and generalizations no longer hold true. For instance, while it's true that roller cams generally offer both horsepower and longevity advantages over a flat-tappet cam, a flat-tappet valvetrain done right will handily smoke a roller valvetrain done wrong. Likewise, although hydraulic lifters were once too heavy and unstable to survive much past 6,500 rpm, they're now surviving in 7,500-plus rpm race motors, giving their mechanical lifter counterparts a run for their money. Consequently, where a mechanical-lifter cam was once the obvious choice, the decision isn't so clear-cut anymore. The lines are blurring, and anyone who doesn't get up to speed with the state of modern valvetrain technology risks getting left behind. To stack the odds in your favor and simplify the selection process, we solicited the help of Billy Godbold of COMP Cams. Widely regarded as one of the foremost camshaft authorities in the industry, we were lucky just to have the opportunity to pick his brain. Here's what we learned straight from the source.

Roller, Flat-Tappet, or Solid?

In the walk of performance camshafts, enthusiasts can choose from solid flat-tappet, solid roller, hydraulic flat-tappet, and hydraulic roller designs. The type of cam you elect to use is sometimes dictated by rules restrictions in whichever class you're racing in, but when this isn't the case, there are clear advantages and disadvantages of each type of cam. It was not that long ago that explaining the benefits of each type of cam required a rather complex answer, but now it is pretty simple. If your engine was ever equipped with a roller cam, stick with a roller unless the rules require a flat-tappet cam. If you have a flat-tappet motor, the cost to upgrade to a roller is generally a good investment unless there are clearly other aspects of the engine, like the cylinder heads, that might be limiting performance. Furthermore, it used to be that above 6,000 rpm, a solid-lifter system was clearly superior to any hydraulic cam. With modern short-travel lifters and lightweight valvetrain systems, we are seeing peak engine speeds migrating north of 7,000 rpm with hydraulic cams, and 8,000-plus rpm is not totally out of the realm of possibility with a next generation of valvetrain improvements. If someone had shown me those specs 10 years ago, I think you could have knocked me over with a feather.

Roller Advantage

Roller camshafts generally make more power than their flat-tappet counterparts, but contrary to popular belief, this isn't due to a substantial reduction in friction. The frictional differences between a flat-tappet and a roller lifter are typically under the measurable resolution of the dyno or Spintron. Roller lifters allow for far greater tappet velocity-up to 30 percent more-in addition to higher lift without resulting in nose wear. Another benefit is that rollers enable higher spring loads, resulting in more aggressive valve motion. While the common misconception that the OEMs went to roller lifters to reduce friction, the real reason they switched was to reduce lifter failures instead. The lower failure rate is also a major consideration today for both street and strip motors.

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Hydraulic Breakthroughs

Hydraulic roller cams have come a long way in recent years, and the thought of production engines like the LS7 that turn 7,000 rpm with a 100,000-mile warranty was inconceivable back in the '90s. The LS7's lightweight titanium intake valve in conjunction with the very stiff LS valvetrain layout and smooth cam design allow for this level of high-rpm performance out of an OEM engine. As impressive as that may be, however, COMP Cams is introducing a new hydraulic roller camshaft series capable of 8,000 rpm on old Gen I small-block Chevys, even with solid, 135-gram intake valves. Accomplishing this requires new valvespring technology, short travel lifters, COMP Pro Magnum or Gold rocker arms, and a whole new design philosophy on the lobe profiles themselves. All that said, five or ten years ago, we would have all thought that this kind of performance was impossible. Let's not forget that these are street grinds with over 0.600-inch valve lift!

Flat-Tappets, Big Power

Since many muscle car motors came equipped with flat-tappet cams from the factory, hot rodders sometimes underestimate their performance. As NASCAR Sprint Cup motors prove, however, the solid flat-tappet design is extremely capable. At a track like Pocono, these motors turn 9,800 rpm for 500 miles, so clearly you can make a flat-tappet cam that has the same horsepower and durability as a roller. The catch is that it is extremely expensive to do so. For instance, the coating on the tappets in these types of motors runs over $100 per piece in large batches. These are the same DLC coatings used in Formula 1 engines. The tappets are all custom pieces, typically made from material similar to the Maraging steels used in nuclear reactors. I'm not certain about the exact cost, but they are well over $250 per lifter, or $4,000 to $8,000 per set. The camshafts in these motors are ground from PM M4 tool steel, and the raw material alone costs about $500 to $600 per core. After making, heat treating, grinding, and super finishing the cam, you are almost assuredly over $2,000 per cam in batches of 10 or more. The valvesprings in these engines are about 1.200- to 1.400 inches in length and handle more than 0.800-inch lift. Those types of springs are not in a catalog anywhere and have to be custom ordered at four to fives times what you would pay for the best set of roller springs available. Moreover, the rocker arms in a Cup motor are typically custom systems, with ratios from 2.0:1 to 2.5:1, that typically cost over $10,000 per set. If you have the tappets, rockers, and valvesprings from a used Cup engine, it is still often cheaper to change over to a roller camshaft to use that engine in a different series of racing instead of sticking with a flat-tappet setup. As you can see, it can be done, but it costs a substantial amount of money to match the performance of a roller cam with a flat-tappet cam.

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Lifter Diameter

As camshaft profiles and spring pressures become more aggressive, it's standard practice to step up to a larger diameter lifter body. In addition to the increased strength of a lifter body itself, the biggest benefit is the larger wheel diameter. Larger wheels allow for faster lobe profiles and lower pressure angles. In high-end race motors with revised cylinder head port geometry and aftermarket blocks, sometimes a larger lifter body can allow for more offset as well.

Solid Roller Reliability

Many years ago, solid roller cams were perceived to be unreliable for extended street use, but several changes have been implemented in recent years to improve oiling and extend lifter longevity. Several years back, we noticed that bracket racers were putting more time on roller lifters with fewer failures than the guys who only drove their cars once a week to the burger stand for cruise night. The main difference in the two engines is that the cruise night engine rarely sees high rpm, but in a bracket motor there is often more oil getting thrown around all over the engine from windage. With that in mind, we began developing lifters with improved oiling for the needle bearings. The culmination of that technology is in COMP Cam's Elite Race lifters with pressurized tool steel axle. These lifters feature pressurized oil inlets that deliver oil through the axles and directly to the needle bearings.

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Modern Oil and Flat-Tappet Wear

In modern motor oils, many of the additives necessary for reliable flat-tappet cam performance have been removed. We try never to get tied up in environmental debates, but the EPA did place severe limits to the amount of zinc phosphates allowed on any road use oils because they believed those chemicals might get past the rings and damage the catalytic converters. The oil industry fought back because these ZDDPs are very inexpensive and effective soldiers that sacrifice themselves instead of the wear surfaces under high loads, such as those seen in flat-tappet cam applications. The EPA won, despite the fact that the industry would have preferred some more testing on the stability of different zinc phosphate combinations. We always recommend using either oils developed specifically for flat-tappet lifters or using special additive packages when running flat-tappet cams. Since not too many people have a stash of late '90s Rotella motor oil in their garage, COMP Cams offers a range of oils and additive solutions, and there are other good products on the market, such as those from Joe Gibbs Racing. At COMP Cams, we never really wanted to get in the oil business, but we had to get in to keep the cam failures to a minimum. COMP Cams' Hot Rod Oil and Joe Gibbs Racing Oil are excellent solutions for providing the protection that flat-tappet motors need. Another alternative is adding a bottle of COMP's "159" break-in additive to any off-the-shelf motor oil.

Offset Lifters

Some cylinder heads require offset lifters to kick the pushrod over to the side in an effort to optimize port geometry. That means that the pushrod is not centered on lifter body as it is with a traditional lifter. While this does affect valvetrain stability and wear, it's not as much as you might think. Anything under a 2-degree angle does not reduce stability. Past that point, it's safe to assume that every additional 2 degrees takes stiffness out of the pushrod equivalent to reducing the pushrod diameter by roughly 1/16 inch or more. The offset does not harm the stiffness, but does increase the wear on both the cam and lift as it tries to twist the lifter in the bore. How much wear is increased depends on the amount of load placed on the system.

Lifter Preload

As basic as it may seem, not everyone agrees on how to set the preload with a hydraulic cam. Many engine builders refer to this preload as "turns past zero lash". In production engines, manufacturers set the lifter near the middle of its travel. We have done considerable testing and with anywhere from approximately one turn from the top to one turn from the bottom, the valvetrain system behaves very consistently. Performance engine builders tend to fall into one of two schools of thought. Running light preload-zero to half a turn-allows the system to recover quickly if you go through an instability region. This is because the hydraulic system can try to hold the valve open after any bounce of more than a few thousandths of an inch. With solid lifters, the shape of valve bounce is a classic parabolic motion with symmetrical up and down patterns. With hydraulic lifters, the valve bounces off the seat and can then close very slowly on the way down, as the hydraulic system tries to take up the newfound clearance. To keep the system from hanging the valve open, some racers run minimum preload. On the other hand, the less preload you run, the more oil volume you have under the piston. The greater volume also means you have more aeration, or tiny oil bubbles in suspension, under the piston that can compress. Hence, some engine builders like to set the lifters from the bottom up, perhaps half a turn from the bottom, while using a longer pushrod to maintain good geometry. Clearly the first method allows the engine to go through mild "bad spots" better and the second method helps improve overall stability. The beauty of a short-travel lifter is that it allows the engine builder to choose both options and take advantage of the best of both worlds.

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Lifter Materials

The materials used to manufacture lifters have evolved over the years, and directly contribute to increased durability. Most mechanical lifters are made from very high-grade cast iron that has values of 55 or greater on the Rockwell hardness scale. Tool steel lifters are also becoming quite common. They can be used on cast-iron cams without coatings if the cam is either nitrided or super finished. On steel cams, the lifter foot requires a special DLC coating to prevent micro-welding between the mating parts. Beware that of the 100-plus companies that say they can DLC coat lifters, only about three have DLC recipes that will live with the impacts common to the tappet face in performance engines without flaking or wearing prematurely.

Roller Durability

In a high-rpm race motor with big valves and lots of spring pressure, the roller assembly in a solid-roller cam takes a lot of abuse. To enhance durability, several features can be incorporated into a lifter's design. Larger axles are pretty much a must, as the needles contact the axle at approximately the same angle every time on the opening side, as the valve opens and applies roughly 3,000 pounds of force for a few milliseconds. This will result in spalling of the axle unless the forces can be well distributed. For the same reason, more needles help spread the load between needles more evenly. With a larger axle, the wheel will become thinner unless you go to a larger outside diameter wheel. Using great materials in all three components is paramount. At the same time, direct oiling reduces surface temperature and scrubbing friction. We certainly prefer a 0.904-inch outside diameter COMP Cams Elite lifter with a 0.820-inch wheel over the 0.842-inch outside diameter lifter with a 0.755-inch outside diameter wheel when a customer can fit the larger body in the block.

Rebuildable Lifters

The cost of replacing lifters every season in a race motor can get expensive, so at COMP Cams we developed our Endure-X lifters with rebuilds in mind. Typically, most bracket racers will replace lifters about twice a season. During the rebuild process, we replace high-wear items like the axle, needles, and wheel. Since the lifter bodies also incur wear during race use, I would not recommend more than one or maybe two rebuilds. Now that the wheels, needles, and axles are so much better than even five or ten years ago, by the time you get to the second rebuild the body would really need to be replaced as well as the rolling assembly.

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