Editing Z31/camspecs (section)

=== DURATION ===

Duration refers to how long a valve is opened in relation to crankshaft rotation. This open valve time period is expressed in degrees of crankshaft rotation. So, a cam specification of 220 degrees duration simply means the cam holds the valve open for 220 degrees of crankshaft rotation.

As strange as this may sounds, more duration can be helpful in high RPM engines but not low RPM engines. The extra degrees of open valve time in high RPM engines gives the air flow a little more time to get into (or out of) the cylinder in spite of the piston's stroke. However, at lower RPMs, more duration can cause less power because the valves will be open at the wrong time in relation to the piston's stroke up or down in the cylinder.

In order to measure a camshaft's duration, you must use a degree wheel in conjunction with a predetermined lift point. The most common duration references are advertised duration measured at 0.050-inch lift. While the advertised numbers are supposed to be measured at 0.020-inch lift, some cam grinders measure farther down the lobe to make their camshaft look more radical than it really is. Because of this confusion, most engine builders use the 0.050-inch duration figure,