You can also pick a cam based on airflow, but not on total flow numbers. Along with compression and operating range (engine speed at cruise, often dictated by gearing and tire diameter), the difference in airflow from intake to exhaust ports offers a guide for duration figures. For example, if your exhaust ports flow 60 percent of your intakes’ cfm figures (which is pretty restrictive), then use a lot more duration on the exhaust ports, regardless of lift. This applies to many street-driven cars and suggests choosing a split-duration cam, of which there are many to choose. Similarly, most dyno tests done both by manufacturers and sanctioned by magazines use dyno headers, which give much better exhaust flow than the corresponding car’s exhaust system. A dual-pattern cam with more exhaust than intake duration can make up for some of this backpressure.

Duration is also a concern when dealing with car weight. A 350ci engine in a light car with minimal exhaust restriction can get away with less difference in duration from intake to exhaust timing, whereas a heavier car will enjoy more of a difference in duration. Here’s an example: If we have a 2,800-pound ’68 Nova with a 350 and a fairly standard driveline and exhaust, it could use 222 degrees duration on both the intake and exhaust for good all-around street performance. However, in a 4,100-pound ’67 Impala with an engine of the same size and general state of tune and the same engine operating speeds, a split-duration cam with 222 degrees intake duration and 236 degrees exhaust duration would perform better (perhaps much better) than the single-pattern cam used in the Nova. In general, the lighter the car, the more free-flowing the exhaust; therefore, the less duration difference it can get away with. Not total duration, mind you, just duration difference from intake to exhaust. With some cams for late-model cars, 10 to 15 degrees of duration difference is commonplace.