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.