Duration: The amount of time that a valve is open. It is specified in degrees of crankshaft rotation. Shorter-duration camshafts yield power in a lower rpm range. Longer-duration cams work better at higher rpm, at the expense of bottom-end power as duration time is increased. Camshaft duration is defined at a specified lift point, and confusingly, there are two types of duration usually specified: advertised duration, and duration at 0.050-inch tappet/lifter lift. Duration at 0.050 inch is the one to use when comparing camshafts.

Two values are critical in specifying duration: crankshaft degrees of rotation, and the point of the tappet/lifter rising at which the measurement is taken. Advertised durations are measured all over the map; that is, not at any regular tappet/lifter rise value. In our valve lift curve plots, the advertised duration is measured at 0.006 inch (the line just above the horizontal axis). It can vary from one cam manufacturer to another. Remember, too, that tappet/lifter lift is at 0.006 inch, and actual valve lift is this value multiplied by the rocker arm ratio. In this example, it is 0.006 inch x 1.5 = 0.009-inch lift at the valve.

The opening and closing points of each valve are circled in this plot (just above the horizontal axis), and counting the number of crankshaft degrees between them gives us an advertised duration of 270 degrees for both exhaust and intake lobes. See this value specified by the dimension arrows below the horizontal axis. When the duration is the same for both lobes, the cam is called a single pattern cam.

A dual pattern cam is one that has a different profile on the intake and exhaust lobes. The lift and duration on the intake may be smaller or larger than the lift/duration on the exhaust lobe.

Duration at 0.050-inch tappet/lifter lift in this example is specified farther up the valve lift plot line; and in this example it is 224 degrees duration. Keep in mind that actual valve lift at this tappet/lifter rise value is 0.050 x 1.5 = 0.075 inch. That is where the second set of circles is placed up the valve lift curves.

We can see that in the simplified marketing world of "mine is bigger than yours" that using advertised duration instead of duration at 0.050-inch tappet/lifter lift might be a perceived advantage. Always rely on duration at 0.050 inch to compare camshafts.

Lobe Separation Angle: The number of camshaft degrees separating the peak lift point of the intake and exhaust lobes. Lobe separation angle is fixed in the camshaft after its initial grind and can only be changed by regrinding the camshaft. In our valve plot, the lobe separation angle is 110 camshaft degrees-the number of degrees peak to peak.

Lobe separation determines where peak torque will occur in the engine's power rpm range. A lobe separation of 106 degrees is "tight" and causes maximum torque to start earlier in the engine's rpm range. It will increase quickly and then peak early. A lobe separation of 110-112 degrees is more "open," or broad, and spreads torque across the ideal operating rpm, and improves power at upper rpm.

Intake And Exhaust Centerline:
These centerlines are sometimes confused with lobe separation angle. We noted that lobe separation angle is fixed by the camshaft grind. Intake/exhaust centerline is referenced to piston TDC, thus it is adjustable by where the lobe's centerline is installed. This centerline is the position of the centerline-the peak lift point on the intake/exhaust lobe-in reference to piston TDC. It is the maximum lift point of the lobe related to TDC in crankshaft degrees. It can be changed-either advanced or retarded-and doing so is called degreeing a cam. Camshaft manufacturers supply a recommended intake centerline installation position.