If the rotating assembly is the heart and soul of your engine, then the camshaft is the brain. The camshaft controls the movement of the entire valvetrain, telling the valves when to open and close and allowing your engine to breathe properly.
Like the human brain, the camshaft is a highly complex entity and even commands its own vocabulary. That’s why we’ve teamed up with some of our camshaft manufacturers to create a basic glossary of cam and valvetrain terms. Armed with this guide, you can understand the sometimes-complex language of cam manufacturers and translate it into increased performance by choosing the right camshaft.
In reference to engine timing, advance occurs when combustion begins before the piston reaches Top Dead Center (TDC). In reference to camshaft timing, advance allows the intake and/or exhaust valves to open and close earlier in the engine cycle.
AREA UNDER THE CURVE:
Describes what the valve lift cycle would look like if plotted on a graph. A graph depicting “area under the curve” usually includes the crank degrees running horizontally and valve lift (in thousandths of an inch) running vertically. The more quickly the valve opens and remains open, the greater the area under the curve.
A type of camshaft lobe profile where the opening and closing ramps (see definitions below) are not identical. Some camshafts are designed this way to achieve an opening ramp profile that has a high velocity and a closing ramp profile that has a slower velocity.
The base circle, also known as the heel, is the lowest point of the camshaft lobe and is the section of the cam lobe where the valve is in the closed position.
Located in the center of the timing gear, the cam button contacts the back of the timing cover and reduces camshaft endplay (the ability of the camshaft to move back and forth).
The process of measuring the exact degree position of the camshaft relative to the combustion cycle. This process allows cam timing to be adjusted to match the performance characteristics of the engine.
The actual shape of the camshaft lobes.
The portion of the camshaft lobe from the nose (highest lift point on the cam lobe) to the closed valve position.
The point at which a valve spring is no longer able to be compressed.
A camshaft design in which the intake lobe profile differs from the exhaust lobe profile.
Camshaft duration is the period of time, measured in degrees of crankshaft rotation, that a valve is open. “Advertised duration” measures duration from the point where the cam manufacturer believes the lobe starts raising the lifter. This is called the checking height and it varies from manufacturer to manufacturer. The best way to measure duration is “duration at .050” because it gives every cam the same standardized starting point for measuring duration. “Duration at .050” lift” is the deciding factor to what the engine’s basic rpm range will be. Lower duration cams produce the power in the lower rpm range. Larger duration cams operate at higher rpm, but lose bottom-end power.
The movement of the camshaft forward and backward within the camshaft galley. Setting the proper amount of endplay is important to avoid excessive valvetrain wear and timing chain failure and ensure proper lifter positioning.
The amount of vacuum generated by the engine within the intake manifold during combustion. Camshaft duration directly affects the amount of engine vacuum created.
FLAT TAPPET CAMSHAFT:
A camshaft that uses a lifter with a slightly curved bottom that slides against the cam lobes. Virtually every V8 engine built before the late 1980s used this type of camshaft.
Designed for use with flat tappet or roller cams, hydraulic lifters use an oil-damped, spring loaded plunger to help maintain valve lash. Ideal for street performance applications, this type of lifter offers quiet operation and requires virtually no maintenance.
The midpoint of the intake lobe.
Measured from spring seat to the top of the retainer, this is the overall height of a valve spring once installed.
The cross sectional diameter of the cam bearing journal.
The distance the valve travels to its maximum open position measured in fractions of an inch. The amount of lift you have and the speed at which the valve moves is a key factor in determining how much torque the engine will produce.
The initial pressure applied to the lifter, most often through the rocker arm system.
LOBE SEPARATION ANGLE:
The distance in cam degrees between the maximum lift point on the intake lobe and the maximum lift point on the exhaust lobe. This measurement provides a clue as to how the intake and exhaust lobes work in conjunction with one another to establish the engine’s breathing pattern. Generally speaking, a wide lobe separation angle broadens the powerband, and narrow lobe separation creates more low-rpm torque.
Lifters that have solid construction (no hydraulics). Mechanical lifters are ideal for high-rpm applications but require an adjustable valvetrain to maintain proper valve lash.
The highest maximum lift point on the camshaft lobe. It is where valves are kept open for as long as possible before making the transition to the closing ramp.
Responsible for opening the valve to its maximum lift point, this section of the camshaft lobe is located between the base circle and the lobe peak, or nose.
Spring pressure created when the valve is open. Open pressure is essential to creating proper valve control.
The amount of time that both the intake and exhaust valves are open at the same time and the piston is at Top Dead Center (TDC).
O.D and I.D.:
Generally relates to valve springs and refers to their outside diameter and inside diameter measurements.
The clearance between the valve and the piston when the two are at their closest point.
The angle of the camshaft lobe and how it relates to the opening and closing speed of the valves. The more aggressive the lobe ramp, the faster the valve opens and closes. The faster the valve opens and closes, the faster the ramp speed.
In reference to engine timing, retard occurs when combustion begins after the piston reaches Top Dead Center (TDC). In reference to camshaft timing, retard allows the intake and/or exhaust valves to open and close later in the engine cycle.
ROCKER ARM RATIO:
The differential between the pushrod and valve stem side of the rocker axis point, expressed as a ratio.
A camshaft that utilizes a lifter with a roller, or wheel, that rolls over the cam lobes. This design reduces valvetrain friction and wear.
Measured in pounds per square inch, seat pressure is the pressure exerted on the spring seat.
A thin flat disc in varying thicknesses used to adjust (compress) the valve spring’s installed height.
SINGLE PATTERN CAMSHAFT:
A camshaft that utilizes the same specifications (lift and duration) on the intake and exhaust sides, creating identical lobe profiles.
Refers to the profiles of the opening and closing ramps of a camshaft lobe and denotes that the ramps are identical.
TOP DEAD CENTER (TDC):
The highest point achieved by the piston travel within the cylinder bore.
A measurement taken between the tip of the valve stem and rocker arm tip. Valve lash is always measured with the lifter positioned off the lobe of the cam.
The distance from the face of the valve straight up the side of the valve to the bottom edge of the valve seat.