Understanding Cam Cards

The cam card provides the critical specs of the cam. The first section provides general information, such as the part number, grind number, the engine family, and if there are any special instructions. Following that are valve lash adjustment recommendations since the example above is a solid lifter cam. The basic lift and duration specs are next, which are always calculated on the factory rocker ratio for that engine (1.6:1 in this case). Next are the exact timing of the intake and exhaust opening and closing events in reference to piston location in the cylinder Before (B) and After (A) Top Dead Center (TDC) and Bottom Dead Center (BDC). The next section lists the cam’s intake centerline, duration at the industry standard .050 inch, the lobe lift, and the lobe separation angle. These are numbers that can be used to compare other cams listing the same standardized specs.

Lift

This is the easiest term to understand. Lift refers to how far the valve is raised from its seat in the cylinder in inches. There are two types of lift commonly referred to: gross and lobe. The lobe lift is the actual size of the lobe on the cam measured at the nose of the lobe. The gross lift is the lobe lift multiplied by the rocker ratio to give the effective lift that the valve will see. A cam card will always refer to the engine’s stock ratio, but the gross lift can be altered by changing the ratio of the rockers, i.e. 1.5 to 1.6. So, yes, you can slightly upgrade your cam without actually changing it. 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 COMP Cams XE 268H hydraulic flat-tappet cam for small-block Chevy has a lobe lift of .318 inch on the intake side. The effective lift from this can be altered from .477 inch to .509 inch and .541 inch with 1.5:1, 1.6:1, and 1.7:1-ratio rockers, respectively.

Duration

While lift refers to how far the valve is opened, duration tells us how long it is open in degrees of crankshaft rotation. That measurement comes in two forms: advertised duration and duration at .050-inch lift. The advertised duration will always be a larger number since it starts lower on the lobe closer to the base, but the problem is that it is not measured from a standardized point that is always comparable to other cam brands. For example, Crane measures their advertised duration at .004-inch valve lift, while COMP measures at .006-inch lift, making the Crane cam sound like it has a longer duration when, in fact, the lobes could be similar. To address the situation, nearly all camshaft companies provide the duration at .050-inch tappet rise, which allow for a good basis of comparison.

Lobe Separation Angle (LSA)

Also sometimes called the lobe displacement angle (LDA), or lobe spread, the LSA is an expression of how the intake and exhaust lobes are phased with each other. Measured in degrees of cam, the LSA is the distance between the centerline point of peak lift on the intake lobe and the peak lift on the exhaust lobe. So while the lift and duration tell what you need to know about an individual lobe, the LSA tells how those lobes relate to one another on a cylinder and how much overlap is present (the brief window when both the intake and exhaust valves are open). Generally speaking, the vast majority of cams will fall in the 104-116 range with nice idling, a broader torque range, and automatic-trans-friendly cams having a wider angle. Racier, faster revving, rougher-idling cams with more midrange torque typical have a narrow LSA. Even more than lift and duration, the LSA really does create the attitude of the cam.

Installed Centerline Angle (ICA)

The installed centerline is a measurement of the relative position of the cam timing versus the engine’s crankshaft position. This measurement is referenced by the crank degrees from TDC at which the cam’s intake lobe reaches max lift. If a cam is installed straight-up, then its ICA will be equal to the LSA, however, the cam’s ICA can be advanced or retarded relative to the crankshaft timing to dial in the desired performance from the engine. Advancing causes valve events to happen earlier, while retarding delays them. Generally there is additional low-speed torque, vacuum, and idle quality to be found by slightly advancing the cam, but it is highly variable depending upon the cam’s specs and the engine family. This is one place an experienced builder with a degree wheel is invaluable. Beware that if you need to advance or retard a cam more than 2 degrees from what is recommended, you probably have the wrong cam to begin with.

Valve Opening & Closing Events

The lift, duration, and LSA combine to establish the valve opening and closing events. These events are what make an engine function and are expressed in degrees of cam rotation and referenced to piston location in the cylinder Before (B) and After (A) Top Dead Center (TDC) and Bottom Dead Center (BDC). We’ve heard Judson Massengill of the School of Automotive Machinists (SAM) express during bench racing at PHR’s AMSOIL Engine Masters Challenge that the intake closing point is by far the most important of the four valve event points. Considering SAM always has a strong showing, we’d say Massengill has a strong grasp of the concept of valve timing.