Heel: The backside of the lobe, or where no lift exists.Base Circle: The smallest diameter of the cam lobe.Duration has a significant impact on performance, but basically, the more duration, the further up the scale peak power occurs.That popular lumpy idle that sounds so good is the result of a combination of longer than “stock” duration and increased overlap and is actually a result of a decrease in the engine’s efficiency at low speed.Additionally, such cams would cause the engine to have low manifold vacuum at idle, which interfered with the operation of some accessories, including power brakes.Over the years, as street rods became more accessory-laden, low vacuum at idle became more of a problem, and as electronic fuel injection became commonplace (with computer management systems that relied on manifold vacuum to provide the proper air/fuel ratios), those wonderful muscle-bound cams just sort of faded from the scene.What is important to understand is that the numbers generated by advertised duration and duration at .050-inch lifter rise are very different.As an example, a typical performance cam has an advertised duration of 290 degrees, duration at .050-lifter rise is 230 degrees, significantly different numbers for the same cam.
At one time, no self-respecting hot rodder would drive a car that didn’t have a cam that implied performance, which meant the engine had to have that unmistakable sound, along with an idle that would loosen the fillings in your teeth.
Lobe centers have a significant impact on performance.
Two identical cam lobe profiles ground at different lobe centers will have significantly different performance characteristics.
Cam Centerline: This is the relationship between cam timing and the position of the crankshaft. 1 cam lobe is positioned at a precise location in relation to the location of the No. Lobe Separation: This is the spacing between the intake and exhaust lobes of the same cylinder measured in degrees.
Typically, lobe centers run from around 108 to 115 degrees.