Here is some general information and terms with regard to camshafts for those of you who get confused about these monster torque producing items. I teach auto tech at a local college and the information below came out of my cam notes.
Degrees Of Separation
A camshaft's lobe-separation angle (LSA) is ground into a cam and cannot be changed. Besides increasing or decreasing overlap, LSA has a variety of effects on how and where an engine makes power.
Narrower LSA:
Increased valve overlap
Increased low-rpm torque
Narrower powerband
Reduced idle quality
Increased cranking compression
Decreased piston to valve clearance
Wider LSA:
Reduced valve overlap
Improved torque top-end
Wider powerband
Improved idle quality
Reduced cranking compression
Increased piston to valve clearance
Essentials: AREA UNDER THE CURVE
If you've indulged in any bit of camshaft research or bench racing, chances are you've heard the term "area under the curve." It's the Holy Grail of cam design, but what does it mean? The term itself describes what a valve-lift cycle would look like if plotted on a graph, with time in crank degrees running horizontally and valve lift in thousandths of an inch running vertically. The more quickly the valve opens and dwells in that position, the greater the space from opening to closing, or beneath the curve. The payoff, according to Comp's Godbold, is lots of high-lift duration without low-lift duration, making for better performance at all rpm.
TERMS
Centerlines
The intake centerline is the point of highest lift on the intake lobe, expressed in degrees after top dead center (ATDC). The exhaust centerline is that lobe's highest point of lift, expressed in degrees before top dead center (BTDC). The centerline is used to tie the valve timing to the crankshaft's rotation.
Duration
As lift refers to how far a valve opens, duration refers to how long it stays open. The opening time is expressed in degrees of crankshaft rotation, so a cam spec'd out at 230 degrees duration means the valve is being held open for 230 degrees of crankshaft rotation.
Cam makers usually refer to two types of duration:
* Advertised duration is the number of crankshaft degrees that the cam follower is lifted more than a predetermined amount off of its seat. The SAE standard is 0.006 inch. So advertised duration is the number of crankshaft degrees that the valve is open at least 0.006 inch. Not all manufacturers, however, use the SAE standard.
* Duration at 0.050 inch, on the other hand, measures the movement of the cam follower, in crankshaft degrees, from the point where it is first lifted 0.050 inch off the base circle on the opening side of the lobe to the point where it descends to 0.050 inch off the base circle on the closing side. This industry standard is especially valuable for comparing cams from different makers.
Lifter
A cam lifter, also known as a follower or a tappet, makes direct contact with the cam lobes and follows the contour, or profile, of the cam. We'll discuss this in greater detail later, but there are four types of lifters: hydraulic flat-tappet, solid flat-tappet, mechanical roller-tappet, and hydraulic roller-tappet.
Lobe lift
An eight-cylinder Chevy camshaft features 16 eccentrics, or lobes, that are based on a circle (thus its name, "base circle"). The height of the lobe above this radius constitutes the cam's lobe lift.
Lobe-Separation Angle (LSA)
This measurement indicates the angle, in camshaft degrees, between the maximum lift points on the intake lobe and the exhaust lobe. Lobe separation is usually calculated by adding the intake centerline and the exhaust centerline, then dividing by two. For example, a cam with a 106-degree intake centerline and a 114-degree exhaust centerline has an LSA of 110 degrees [(106 + 114) / 2 = 110].
Overlap
Overlap is the amount of time, measured in crankshaft degrees, that both the intake valve and the exhaust valve are open. This situation happens at the end of the exhaust stroke and the beginning of the intake stroke. Increasing duration or decreasing lobe-separation angle--or doing both--increases overlap. A cam with an exhaust closing point of 4 degrees ATDC and an intake opening point of 8 degrees BTDC has 12 degrees of overlap.
single & dual
* Single pattern: In this type of cam, the intake and exhaust lobe profiles are identical, employing equal amounts of duration and lift. For example, a Comp Cams 270 Magnum (PN 12-211-2) cam has 270 degrees advertised duration, 224 degrees duration at 0.050 inch, and 0.470 inch lift on both the intake and exhaust sides.
* Dual pattern: This type of camshaft has different intake and exhaust profiles. In general, the exhaust lobe will employ more lift and duration to help evacuate exhaust gas from the cylinders. For example, Lunati Voodoo cam (PN 60130) has 261/267 degrees advertised duration (intake/ exhaust), 0.231/0.237 duration at 0.050, and 0.555/0.566 inch lift.
Valve Lift
This measurement refers to the distance, measured in thousandths of an inch, that the valve is lifted off its seat at the cam lobe's highest point. Remember, the cam's rotary motion is converted to linear motion through a lifter, pushrod, and finally a rocker arm. Therefore, valve lift equals lobe lift multiplied by the rocker-arm ratio. For example, if we have a lobe that measures 0.334 inch running a 1.5:1 rocker-arm ratio, 0.334 x 1.5 = 0.501 inch valve lift. Running a 1.6:1 rocker arm creates 0.534 inch valve lift.
