Camshafts- Three-Way Cam Lobe Shootout

Cam Basics
Since there are readers new to this magazine every month, let's quickly roll through some camshaft basics to bring everybody up to speed. There are several ways to evaluate any camshaft, so we'll start with the simplest: lift. A cam lobe is nothing more than an eccentric on a shaft that rotates to create lifter movement. Lift is created when the lobe moves off its base circle, pushing up on the lifter. This lobe lift is multiplied by the rocker ratio to create total valve lift. As an example, with 0.330 inch of lobe lift multiplied by a 1.5:1 rocker ratio, the valve lift would be 0.495 inch.

Perhaps the most informative portion of lobe specs is duration, which is expressed in terms of crankshaft degrees. Duration is also most often delivered in terms of either advertised duration or duration at 0.050 inch of tappet lift. To be totally accurate, any duration spec should be accompanied by the amount of tappet lift where the duration is measured. This rarely happens with advertised duration, but we can tell you that Comp Cams measures both its hydraulic flat-tappet and roller camshafts at 0.006 inch of tappet lift.

Setting lash on any engine is relatively easy. The best procedure is to warm the engine and then use the exhaust-opening, intake-closing (EO, IC) procedure. With each lifter on the base circle, use a feeler gauge to measure lash at the rocker arm.

Where all this information can get confusing is when we move to mechanical lifter camshafts, either flat-tappet or mechanical roller, and talk about valve lash. Published cam specs are based on the numbers generated by the cam lobes and their effect on the valve. All mechanical lifter camshafts require a clearance between the rocker arm and the valve to account for expansion as the engine warms up. In the case of our XR286R roller cam, the intake lash or clearance is 0.016 inch with the engine hot.

Lash affects most of the published cam specs. The 0.576-inch gross intake valve lift figure on the cam card does not take into account the lash. This means we must use the equation 0.576 - 0.016 = 0.560 inch to come up with our actual net valve lift number. It's a small point, but worth noting for accuracy.

Lash also has an effect on duration. According to Comp Cams, the net change is that 0.001 inch of lash shortens the actual cam duration by 0.5 degree. So with a 0.016-inch lash on the intake, this effectively shortens the intake duration at 0.050 inch checking height by 8 degrees, creating a net duration of 240 degrees at 0.050 inch tappet lift. This explains why we chose a 248 at 0.050 roller cam, because the net duration after lash is actually 240 degrees.