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.