Thumb through the pages of any performance magazine and you'll find it all over the place--camshaft lingo. Lift, duration, base circle, dual-pattern, overlap, rocker-arm ratio; whether you're new to hot rodding or a seasoned gearhead raised on Duntov cams, fuelie heads, and Muncie rock-crushers, you have to admit that sometimes all that camshaft jargon can get confusing.
But there's good news ahead. In the following pages we'll explore various aspects of camshaft operation and review some of the more frequently used terminology. In the end, you'll not only be able to speak fluent camshaft, but you'll also be armed and ready to select your next bumpstick.
Our camshaft discussion centers on typical small- and big-block Chevys, which have the camshaft located through the top centersection of the engine block and incorporate the use of 16 respective lifters, pushrods, and rocker arms to actuate valve movement. Unlike many modern engines, the small- and big-block Chevy is not an overhead camshaft/valve design, but simply an overhead-valve engine.
The camshaft in Chevy V-8 engines is driven by the crankshaft and spins at half the speed of the crankshaft. By design, the camshaft has separate lobes for each valve (16 for Chevy V-8s). At the lowest point of each camshaft lobe is the base circle. This is the place where each valve will rest in the closed position and is the point where all valve-lash settings are made. Above the base circle, the lobes are ground in an eccentric fashion with ramps that initiate the lifting and descending movements of each lifter.
As the camshaft rotates, each lobe turns under its respective lifter while the pushrod is forced upward by the valve lifter. Next, one end of the rocker arm is moved upward by the pushrod while the opposite end of the rocker arm pivots downward to push the valve open. As the camshaft continues to revolve, the lifter moves downward on the lobe and the valve is closed with a valvespring. Altering a camshaft's characteristics can result in more power. Let's see how.
This Lunati small-block Chevy...
This Lunati small-block Chevy camshaft is a flat-tappet hydraulic-lifter design. For most street applications, a hydraulic-lifter camshaft can be designed to provide excellent performance without the need to periodically adjust the valves.
One way to change an engine's power profile is to increase valve lift. Valve lift is the amount (measured in thousandths of an inch) that the valve is lifted off its seat, and lobe lift is the amount that the cam lobe raises in radius above the cam's base circle.
Maximum valve lift is determined by multiplying the maximum lobe lift by the rocker ratio. For example, a cam with 0.350-inch lobe lift yields 0.525-inch valve lift with a typical small-block Chevy 1.5:1 rocker arm (0.350 x 1.5 = 0.525-inch valve lift). So if we simply install higher-ratio rocker arms, such as 1.6:1, the valve lift will increase (0.350 x 1.6 = 0.560-inch valve lift). However, an increase in valve lift is typically accompanied by an increase in duration, because ramps are limited in their profile, which is directly related to the type of lifters being used, such as a flat-tappet or roller lifter.
Look at a sideview of a cam lobe and you'll notice that it has a nose that gradually slopes away and back to the base circle. This is duration, and it's measured as the amount of crankshaft degrees that the valve is lifted off its seat. Camshaft manufacturers usually list two types of duration. The first is advertised duration, which is the angle in crankshaft degrees that the lifter is raised more than a predetermined amount (the SAE standard is 0.006 inch) off its seat. Duration at 0.050-inch lift is a measurement of lifter movement, in crankshaft degrees, from the point where it's first raised 0.050 inch off the base circle to the position where it is 0.050 inch from the base circle on the closing ramp side. This serves as a good value to compare cams. For example, Lunati's street/strip hydraulic (PN 07102) has an advertised duration of 285 degrees but is 235 degrees of duration measured at 0.050-inch valve lift.
The camshaft on the right...
The camshaft on the right is a roller design displaying its broader lobes. Typically, if a camshaft has more area under the curve, it can deliver more power.
Lifters, Tappets, or Followers
Camshaft lifters are sometimes called followers or tappets and are the components that make direct contact with the camshaft lobe. There are generally four different types of lifters: mechanical flat-tappet, hydraulic flat-tappet, roller hydraulic, and roller mechanical.
The hydraulic flat tappet is the most common lifter used in Chevy V-8 engines. It is self-adjusting and incorporates a valve-controlled plunger inside the lifter body. By design, engine oil pressure maintains a preload, and because there is no lash involved, hydraulic lifters are quieter than mechanical lifters. Today, most performance lifters can be operated to 6,000-6,500 rpm.
Mechanical lifters are commonly called solids; although they have oil-feed orifices, they are one-piece units. The benefits of solid lifters include exceptional rpm potential for engines designed to operate at high speed. To allow for expansion during engine warm-up (and because of mechanical camshaft design), solid lifters must maintain valve clearance to operate properly and require frequent lash adjustments.
The three lifters shown are...
The three lifters shown are (from left to right) solid, hydraulic, and roller. Notice the wheel at the bottom of the roller lifter that helps reduce friction.
Inside the hydraulic lifter...
Inside the hydraulic lifter (left) there is a plunger and spring. This is an antipump-up design and differs from the solid (mechanical) lifter's internals (right).
Here's an empty hydraulic...
Here's an empty hydraulic lifter (left) and an empty mechanical lifter. The hydraulic lifter has a larger area to accommodate the internal components needed for the spring and plunger.