How Eccentric

Roller Lifters

The small wheel at the bottom of a roller lifter allows for very aggressive camshaft designs with more lift at a given duration. It also helps reduce friction between the camshaft and the lifter. Hydraulic- and mechanical-roller camshafts are available with profiles to match a variety of performance goals. Roller camshafts are typically more expensive than flat-tappet camshafts largely due to the lifters. Roller lifters can be reused, but flat-tappets should not be.

Single and Dual-Pattern

Camshafts with a single pattern are ground with identical profiles on both the intake and exhaust lobe. A dual-pattern camshaft has an intake-lobe profile different from that of the exhaust-lobe profile.

Centerlines

Each intake and exhaust lobe on a camshaft has a centerline--the halfway point of the duration curve. The intake centerline is expressed in crankshaft degrees after top dead center (ATDC), and the exhaust centerline is expressed in crankshaft degrees before top dead center (BTDC).

Camshaft companies use the intake centerline measured at the No. 1 cylinder to determine the camshaft's position relative to the rest of the engine. Some camshafts have a certain amount of advance ground in. By simply advancing or retarding the camshaft's installed position, the engine's torque band can be moved, which places the valve events farther ahead or behind the position of the piston in the bore. Camshaft intake centerlines are typically 104 to 116 degrees ATDC.

From this information we can calculate the lobe separation angle, which is measured in camshaft degrees instead of crankshaft degrees. First we add together the intake and exhaust centerlines (measured in crankshaft degrees). Then, because the camshaft spins at half the speed of the crankshaft, we divide this sum by two. For example, if the intake centerline is 108 crankshaft degrees ATDC and the exhaust centerline is 120 degrees BTDC, we have (108 + 120)/2 = 114 degrees of lobe separation.

Overlap

Camshaft overlap is the angle in crankshaft degrees at which both the intake and exhaust valves are open at the same time. This can be calculated by adding the exhaust closing point and the intake opening point. To demonstrate, let's use a cam with an exhaust closing point of 4 degrees ATDC and an intake opening point of 8 degrees BTDC. By adding these two values together, we find the camshaft has 12 degrees of overlap. But since these timing figures are taken at 0.050-inch lift, this therefore is overlap measured at 0.050-inch lift.

For engines that operate at high rpm, overlap uses the momentum of spent gases exiting through the exhaust valve to help pull the fresh air/fuel mixture into the cylinder through the intake valve. Altering the lobe separation by just a couple of degrees can have a significant effect on overlap, and this effect is increasingly felt at high rpm.

Conclusion

Camshafts play a big role in power production and in defining the performance personality of your engine. Understanding the terms may not make your engine run better, but it will help you chose a camshaft that best matches your performance goals. And as with any engine modification, knowledge is power.