Lift And Duration
How far and for how long a cam's lobes push its valves open is expressed in the cam's lift and duration. These measurements can be taken either "at the cam" or "at the valve." Measured at the valve, lift is the linear measurement of how far off-seat a valve extends at its farthest point, which should be a direct relation to the measurement of the cam's lobe at its apex, or highest point out of round from its base circle. Duration is the amount time, measured in degrees of crankshaft rotation; the valve remains past a certain distance off-seat and likewise, should proportionately reflect the amount of degrees of cam rotation the cam's lobe remains above a certain amount out of round. Here's where things get tricky. Measurements taken at the valve are never consistent.
Imperfections or wear among parts of the valvetrain connecting the valve to the cam aren't accounted for, neither is lash (the "play" needed in cylinder head components to allow for thermal expansion). Even when taken at the cam, these measurements don't always result in consistent, relative, measurements at the valve for different applications. In the case of the Ford I4, lift measurements taken at the valve should maintain a 1:1 ratio with those taken at the cam, but in instances where a rocker is involved (think Honda D-series), a proportion exists between the cam lobe, the valve and the fulcrum of the rocker connecting the two. Ergo, two cams with the same lift values taken "at the cam" would measure differently at the valve for their respective application.
Since it's virtually impossible to standardize where a cam's lobe begins, published measurements of duration begin when the cam lobe rises to a certain point of lift-but that point isn't necessarily the same in every case. The SAE recommends .050 inches of lift as the starting and ending point for measuring a cam's duration (either at the valve or cam), but the Japanese mfg's use 1mm (.039 inches) as the standard for rating their cams, so American and Japanese cams with the same deg. duration won't be the same. Throw into the mix that some aftermarket cam specs are often published with no mention of at which lift point they're taken, and its easy to see the variables involved in explaining a cam's characteristics go beyond what's published. Most importantly: Neither lift nor duration express the total mass of a cam's lobe-and likewise, how much air it can make to flow around an engine's valves. To best understand this, refer to the graph below.
In this comparison, measurements of two aftermarket intake cams rated differently and an OEM intake cam and were graphed, overlapping. Both aftermarket cams advertise more lift and duration than the OEM, cam but while CAM A brags the most lift and duration, notice how CAM B features more "area under the curve." This increased area translates to its valve staying open farther longer, allowing greater movement of air. While both cams are rated absolutely the same, CAM B will flow more than CAM A, all things being equal.