Understanding Cam Specs

ROLLER VS. FLAT TAPPET
Cams come in two basic types, flat tappet and roller. Both of these types can be either hydraulic or solid. In recent years, roller cams have become much more popular and are often a good choice when building a high performance engine. The most obvious difference is the roller type uses a roller wheel at the end of the lifter to roll over the cam lobe, while the flat tappet appears to be just that--flat. However, a flat tappet isn't really flat, but has a large radius of curvature built into the bottom of the lifter, while a flat-tappet camshaft lobe is tapered to one side. Further, the centerline of the lifter bore in the block is offset slightly to the centerline of the lobe. The curved lifter face meets the tapered cam lobe at a slight included angle causing the lifter to "skate" over the cam lobe, rather than scrub or skid as is often thought. The lifter actually rotates in the bore as the cam lobe spins beneath it. Of the two designs, the roller is said to have lower operating friction.

Now, you might pull out that shinny roller cam and marvel with satisfaction over those fat, broad-shouldered, and brawny looking lobes. They certainly look a lot meatier than those puny, pointy, flat-tappet lobes. Somewhere you might have heard that a roller has a big advantage in "area under the curve," and now you can see the proof in those macho lobes. Really, guys judging what's going on in that way are totally fooling themselves. The way motion imparts on the lifter is completely different between flat-tappet and roller cams. A roller wheel's contact with the lobe is linear, along the length of the lobe, while a flat tappet's lifter base presents the surface of a geometric plane to the lobe. The easiest way to picture this is to imagine a roller lifter going over the nose of the lobe. As the lobe drops away, the lifter drops a like amount, in virtual lockstep; it is contacting in a line across the lobe and lifter wheel. A flat tappet, by contrast, has the width of the surface of its base acting upon the lobe. Going over the top, the flat tappet hangs with subdued motion while the nose of the cam rotates through an arc beneath it.

In practical terms, the roller design does have advantages, but they are not as clear cut as looking at a roller cam's wide lobes with smug satisfaction. A flat tappet is actually capable of higher initial acceleration than a roller, right off the base circle, since the diameter of the lifter base gives it a geometrical advantage. Check any cam catalog and compare a fast flat tappet to a roller. You'll find the flat tappet gets from the rated seat duration to the 0.050-inch number in fewer crank degrees. A major limitation of the flat tappet is the maximum velocity is limited by the diameter of the lifter. A roller, on the other hand, can keep accelerating, and reach higher velocities than a flat tappet. A roller provides the opportunity to design in higher velocity at higher lifts, which provides more high-lift duration within a given overall duration.The next huge advantage of a roller is its ability to withstand higher spring loads. A flat tappet can only tolerate a limited load between the cam and lifter, and then it's all over. In fact, with light spring loads, a flat tappet has a very long life cycle, but at high loads, its life is reduced, and there are definite limits on how much spring load can be applied. With very aggressive profiles, high-lift, and high-rpm, more spring load is typically needed for valvetrain control, and a roller becomes the natural choice. While rollers were originally found strictly the in realm of race cams, rollers have become very popular in less demanding applications. There is a tangible increase in area under the curve, or valve event window with a roller, due to the increased velocity at the higher end of the lift range, where the added time happens to do the most good--at higher lifts and flow rates. That's a performance advantage. The other advantage is doing away with the need to break-in a flat-tappet cam, and the virtual assurance of avoiding premature cam failure.

LOBE SEPARATION ANGLE (LSA)
A camshaft consists of intake and exhaust lobes, and a key consideration when designing a cam is the lobe separation angle, sometimes also called the lobe displacement angle, or lobe spread. The definition here is simply the distance in degrees, as measured on the cam, between the point of peak lift on the intake lobe and the peak lift on the exhaust lobe. There are several measurements found in a cam's specs, which give clues to the performance characteristics of a given grind. Some, such as lift and duration, are easy enough for even the neophyte camshaft connoisseur to understand. These are often the only specs considered when selecting a cam. Really, for any given lobe, that's all there is, lift and duration, and the two can be related to map the profile of a lobe throughout its lift cycle. Though lift and duration alone can fully describe an individual lobe, each cylinder of an engine has intake and exhaust lobes, and the timing of these events relative to each other have a significant influence on engine performance. Neither lift nor duration gives any clue as to this aspect of a cam's design. Lobe separation angle does.

Simply put, the lobe separation angle (LSA) is a measurement of how the intake and exhaust lobes are phased with each other. To establish the position of each lobe, the traditional reference point is where the lobes reach max lift. Picturing the end-view of a cam as a circle with 360 degrees; the lobe separation is a measurement in degrees of the distance between the max lift on the exhaust and intake lobes, respectively. Note that degrees of lobe separation angle are given in a simple degree measurement at the cam, in contrast to how duration is measured as degrees of rotation of the crank, which turns at twice the cam's speed. With this in mind, lobe separation angle is said to be in cam degrees, while duration is quoted as crank degrees.It's not astonishing to us that lobe separation will have a big impact on performance. After all, the valve timing events have to occur at the most advantageous moments to glean the desired results from an engine combo. Obviously a LSA of zero would have the intake and exhaust valves open and close at the same time and even we know this won't work. Cam grinders are pretty sharp on this subject, and have found the sweet range for LSAs in the range of 104-115 degrees for most applications. Typical off-the-self aftermarket cams will have a lobe spread between these values, with the greatest number of offerings falling toward the middle of this range. Coincidence? We think it's a pretty safe bet that they've got a handle on what works, and grind their cams accordingly.

Even within this relatively narrow range, the lobe separation angles will affect engine performance. The following chart gives some of the general haracteristics you'll see with two otherwise identical cams ground on narrow or wide lobe separation angles, assuming they are installed with the same amount of advance.A camshaft consists of intake and exhaust lobes, and a key consideration when designing a cam is the lobe separation angle, sometimes also called the lobe displacement angle, or lobe spread. The definition here is simply the distance in degrees, as measured on the cam, between the point of peak lift on the intake lobe and the peak lift on the exhaust lobe. There are several measurements found in a cam's specs, which give clues to the performance characteristics of a given grind. Some, such as lift and duration, are easy enough for even the neophyte camshaft connoisseur to understand. These are often the only specs considered when selecting a cam. Really, for any given lobe, that's all there is, lift and duration, and the two can be related to map the profile of a lobe throughout its lift cycle. Though lift and duration alone can fully describe an individual lobe, each cylinder of an engine has intake and exhaust lobes, and the timing of these events relative to each other have a significant influence on engine performance. Neither lift nor duration gives any clue as to this aspect of a cam's design. Lobe separation angle does.

Simply put, the lobe separation angle (LSA) is a measurement of how the intake and exhaust lobes are phased with each other. To establish the position of each lobe, the traditional reference point is where the lobes reach max lift. Picturing the end-view of a cam as a circle with 360 degrees; the lobe separation is a measurement in degrees of the distance between the max lift on the exhaust and intake lobes, respectively. Note that degrees of lobe separation angle are given in a simple degree measurement at the cam, in contrast to how duration is measured as degrees of rotation of the crank, which turns at twice the cam's speed. With this in mind, lobe separation angle is said to be in cam degrees, while duration is quoted as crank degrees.It's not astonishing to us that lobe separation will have a big impact on performance. After all, the valve timing events have to occur at the most advantageous moments to glean the desired results from an engine combo. Obviously a LSA of zero would have the intake and exhaust valves open and close at the same time and even we know this won't work. Cam grinders are pretty sharp on this subject, and have found the sweet range for LSAs in the range of 104-115 degrees for most applications. Typical off-the-self aftermarket cams will have a lobe spread between these values, with the greatest number of offerings falling toward the middle of this range. Coincidence? We think it's a pretty safe bet that they've got a handle on what works, and grind their cams accordingly.

Even within this relatively narrow range, the lobe separation angles will affect engine performance. The following chart gives some of the general characteristics you'll see with two otherwise identical cams ground on narrow or wide lobe separation angles, assuming they are installed with the same amount of advance.