Engine building is about decisions--decisions that can make or break the outcome of a project. Nowhere is that more true than in the camshaft. There are a variety of choices in camshafts, but the only real way to differentiate one from the next is by the numbers. Essentially, everything you will know about a given camshaft and how it will perform in an engine combo comes down to the numbers describing it--the specifications. If these numbers are as foreign as the Da Vinci Code, you've got problems when it comes time to choose the right stick. Even if total mastery of a cam's numerology is a little deeper than you want to go, a fundamental understanding of the key specs will greatly improve your ability to avoid getting the shaft with the wrong grind.

We're not sure if the information presented here will qualify you as the neighborhood cam whiz, but we can say for certain an understanding of these concepts will put you more than a fender up on the guy in the losing lane.

LIFT
This one is the easiest terms to understand, with "lift" referring to how far the valve is opened off the seat in fractions of an inch. What creates the valve lift, as quoted on a cam card? The gross lift is the cam's actual "lobe lift," multiplied by the rocker ratio. As the camshaft comes around from the base circle to the lobe ramps, the lifter is displaced and rises, until it reaches the point of peak lift at the nose of the lobe. The amount of this displacement is the actual lobe lift of the cam. The lobe lift isn't the same as the amount of lift at the valve, since this lift is multiplied by the rocker ratio. The rocker ratio is created because the pushrod side of the rocker is closer to the rocker's pivot point than the valve side. The rockers in most factory V-8 engines ranged from 1.5:1 to 1.75:1, while aftermarket rockers can be had in a variety of ratios for many engine types. Usually the lift given in manufacturer's catalogs is based upon the cam's lobe lift multiplied by the engine's original rocker ratio.

Actually, although an engine's rocker ratio is given as a specification for that given style engine, the ratio when measured is often not exactly what the specifications will indicate. You'll find that the actual ratio can be a little more or less, and as a result, so will be the actual measured valve lift. Some things that will affect the measured lift, or rocker ratio, are the pushrod angles, pushrod length, and the particular geometry on the heads being used. Aftermarket rockers rated at 1.5:1 also do not usually deliver exactly that ratio, and sometimes deliver a slightly higher ratio. With all the variation in actual delivered ratio, valve lift as given is just an approximation (though usually it is pretty close), based on the rarely true assumption that the rocker delivers the exact ratio quoted. On the other hand, the "lobe lift" does not include the variable of rocker ratio, and is a very precise number. The "lobe lift" can be used to calculate the valve lift with any rocker ratio. If aftermarket rockers with a non-stock ratio are being used, the "lobe lift" multiplied by the rocker ratio will provide the valve lift number. For example, the popular COMP Cams XE 268H hydraulic flat-tappet cam for the small-block Chevy has a lobe lift of 0.318-inch on the intake side. The delivered lift with this cam will be 0.477-inch, 0.509-inch, and 0.541-inch with 1.5:1, 1.6:1, and 1.7:1-ratio rockers, respectively.

If you want to know exactly what the valve lift is going to be in a given engine combination, you actually need to mock-up the camshaft and valvetrain, and measure it with a dial indicator. Just doing the math with the lobe lift and rocker ratio will typically provide an approximation that is close enough, but as mentioned previously, there are a few other variables that will affect the actual valve lift delivered. These variations are of little consequence in a typical street or mild performance engine. However, no serious race engine should be put together without physically measuring the actual valve lift at the valve.

DURATION
While lift measures how far the valve is opened, duration tells us how long the open cycle lasts. How long here is given in degrees of crankshaft rotation, fundamentally measured from when the lobe starts raising the lifter, until the same lobe finishes by dropping the lifter back down to the start position. There is one complication, and that's just what procedure to use for making the measurement. The procedure here refers to the checking height, which is the tappet position at the start and finish of the measurement.

Unfortunately, when calculating advertised or gross duration, not all camshaft manufacturers use the same checking height, and that makes it difficult to directly compare advertised duration numbers from one company to another. For instance, some manufacturers measure advertised duration starting when the tappet moves up 0.006 inch from the base circle, and finish at 0.006 inch up on the closing side. Another company may use a tappet rise spec of 0.008 inch to calculate advertised duration. In this case, the measurement starts later (waiting for the tappet to rise another 0.002 inch before starting to measure duration), and finishes sooner (stopping the measurement 0.002-inch earlier), so the advertised duration ends up being shorter, even if the exact same lobe is being measured.

For the advertised duration numbers to be truly meaningful, the checking tappet rise used to calculate the duration needs to be known. Most cam manufacturers will provide this information in the specification section of their catalogs. Decades ago, most camshaft companies agreed to list the duration specification at 0.050-inch tappet rise. With "duration @ .050-inch," the checking tappet rise is given at 0.050 inch, so all of the companies are using the same yardstick for calculating their specs. These numbers provide a good basis for comparison when considering different camshafts.

SOLIDS VS. HYDRAULICS
Comparing specs of a hydraulic vs. a solid camshaft isn't as simple as it first may appear. While it would seem like solid and hydraulic lifter camshafts with the same lift and duration specifications would behave similarly, there are a few considerations not apparent at first. Beginning with the advertised duration numbers, solids and hydraulics are rated by completely different standards. For instance, in the COMP Cams line, hydraulics are rated for duration at 0.008-inch lifter rise, while solids are typically rated at 0.020 inch. Comparing a solid to a hydraulic by advertised duration is like comparing apples to oranges. In regard to lift, things are a little simpler, but again a direct comparison of specs would be misleading. The lash needs to be subtracted from a solid cam's specs to arrive at the true lift at the valve, which can then be compared to the hydraulic cam's specs.

Finally, we have duration at 0.050 inch. While both types of cams are rated in the same way, at the 0.050-inch tappet rise spec, again the numbers can't be directly compared between a solid and a hydraulic. Duration at 0.050 inch is measured in crank degrees at 0.050-inch lifter rise on the opening and closing side of a lobe. The engine isn't interested in how long the lifter is moved, but rather only sees what is happening at the valves. With a solid, the lash will take up some of the lifter's motion before there is any valve motion. In fact, with a 1.5:1 rocker ratio, the solid's duration at 0.050-inch reads as if the duration was taken 0.033-inch lifter rise in hydraulic terms. That's a significant difference. A solid cam will behave like a hydraulic with duration at 0.050-inch spec. of approximately 10 degrees less duration. All of this makes it very difficult to exactly match a solid and hydraulic lifter cam; it certainly can't be done by matching the numbers in a cam catalog or on a spec card.

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.

EFFECTS OF LOBE SEPARATION ANGLE
LSA NARROW WIDE
Intake Open Earlier Later
Intake Close Earlier Later
Exhaust Open Later Earlier
Overlap More Less
Cylinder Pressure Gain Lose
Idle Quality Worse Better
Idle Vacuum Less More
Torque Curve Peakier Flatter
Peak Torque More Less
High RPM Drops Off Hangs On

INSTALLED CENTERLINE ANGLE (ICA)
Sometimes, there is confusion between lobe separation angle (LSA), the topic above, and the cams' installed centerline angle (ICA). Both terms have to do with angles, lobes, and reference the peak lift point, and are somewhat related, but are two entirely different measurements. The LSA is a measurement of how far apart the peak lift points of the intake and exhaust lobes are ground into the camshaft. The cam has a LSA as soon as it's ground, or just sitting on the workbench. The installed centerline angle simply describes the installed position in the engine. It is a measurement the cam's phasing; the relative position of the cam vs. the engine's crank. This measurement is usually referenced by the crank degrees from TDC at which the cam's intake lobe reaches max lift.

Probably why these terms are so often confused, is that the numbers turn up in the same range of values. A cam ground with 110-degree lobe separation will read an installed centerline angle of 110 if installed "straight-up," or with no advance. Since the cam is connected to the crank via the timing chain, the cam's ICA, or phasing to the crank, can be adjusted forward or back, changing the installed centerline angle. This is called advancing or retarding the cam. The installed centerline is what is checked when a camshaft is degreed in.Most aftermarket cams are ground with some advance built in, typically about 4 degrees. Advancing the cam makes all the valve events happen earlier, and generally favors low rpm operation, helping idle quality, cylinder pressure, vacuum, and lower speed torque. Retarding the cam deteriorates these characteristics, though in some cases high rpm power may be enhanced. The real score here is that advance will almost always help improve performance as listed above, but most of the time retarding the cam will gain little if anything, even up top.

VALVE LASH AND LIFTER PRELOAD
What separates a hydraulic lifter from a solid tappet is the addition of an internal hydraulically operated plunger within the hydraulic lifter's body. The way the valvetrain is set up will depend upon whether the lifters are hydraulic or solid. Hydraulics generally run with pre-load, while solids must be set up with lash. Let's first consider a hydraulic. It's helpful to understand how the hydraulic mechanism works and what it does.

Oil pressure enters the lifter through an orifice in the lifter body, and flows through another orifice into the hollow body of the lifter plunger. A one-way check valve at the bottom of the plunger allows oil to fill the cavity below until all the valvetrain clearance gone, effectuating the hydraulic self-adjustment to zero lash. When the cam rotates into the lift cycle, the check valve at the base of the plunger closes under the pressure imparted by the valvespring, preventing the oil from being squeezed back out as the valve opens. At the top of the plunger of some hydraulic lifters is a metering valve or plate, which supplies oil to the pushrods for valvetrain oiling.

With the valvetrain installed (or adjusted), the pushrod compresses the plunger within its range of travel, and the hydraulic mechanism automatically zeros the lash. How far down the lifter plunger has been displaced at its base setting is called the lifter pre-load. The recommended pre-load with hydraulic lifters is usually in the range of 0.020-0.040 inch. Many stock engines came with non-adjustable valvetrains. Usually, this is not a problem when going with another hydraulic cam, but many things during engine building can alter the intended factory pre-load in these fixed systems. Things such as milling or decking the block or heads, the design of replacement lifters, gasket thickness, or the cam's base circle diameter can alter the pre-load. In these cases, the solution is custom-length pushrods, or making the change to adjustable rockers.With adjustable rockers, setting the pre-load is simply a matter of setting the lobe being adjusted to the base circle, and tightening the adjuster until the clearance in the valvetrain is just taken out (zero clearance). Then turn the adjuster in 1/2 - 3/4 of a turn and lock the adjuster down.

Solid lifters have no self-adjusting hydraulic mechanism, and need to run with clearance in the valvetrain. The lash specification is given on the cam card for a solid cam. The adjustment is made with the lobe being adjusted set on the base circle. A feeler gauge of a thickness matching the lash spec is inserted between the rocker and the valve tip, and then the adjuster is taken to zero lash and locked down. When the feeler gauge is removed, the lash will be set at the thickness of the feeler gauge. Lash is usually set cold when the engine is built, and then re-adjusted once the cam is run-in, with the engine hot. The hot setting will be more true to the engine conditions in operation.

UNDERSTANDING A CAM CARD
At the right, we have the information from a COMP Cams cam card for a Magnum 280H Mopar big-block hydraulic performance cam. The cam card provides the critical specs of the cam, and other useful information. The first section provides general information such as the part number and application, as well as other descriptive characteristics, such as the single-bolt nose for this particular cam. The second section give the basic lift and duration specs. The lift is calculated on the factory 1.5:1 rocker ratio. Note that the duration is listed along with the tappet rise at which it is rated. The next section provides the actual intake and exhaust lobe opening and closing points. As with duration, the tappet rise these points are derived at is also listed (0.006 inch in this case). These points can be used to check the cam's phasing or installed centerline when degreeing-in the cam. The next section lists the duration @ 0.050 inch, the lobe lift, and the lobe separation angle. These numbers can be used to compare other cams listing these same standardized specs. Finally, we have the valvespring recommendation. Matching the valvespring to the cam's requirement is critical to getting the most from a given combination.

* Part Number21-237-4
* Engine1959-1980 Chrysler, 383ci-440ci, Single bolt, 8 cyl.
* Grind Number CRB 280H-10
* Description

IntakeExhaust
Valve Adjustment00
Gross Valve Lift 0.4800.480
Duration At 0.006 Tappet Lift280280

Valve Timing At 0.006
OpenClose
Intake 3466
Exhaust7426
These Specs Are For The Cam Installed At 106 Intake CL
IntakeExhaust
Duration At 0.050 230230
Lobe Lift 0.320 in.0.320 in.
Lobe Separation 110
Recommended Valvesprings 920-974