Lobe Separation Angle (LSA)
Lift and duration adequately describe a given lobe of a camshaft, and indeed we can map the profile of the entire lift curve of an individual lobe by just plotting lift versus duration, however, a running cylinder requires two lobes, an intake and an exhaust, and these events must happen at the appropriate time relative to each other in an operating engine. While many enthusiasts will only look at the lift and duration specifications, these specs tell nothing of the missing piece of the puzzle, the phasing of the intake and exhaust lobes relative to each other. To define this important aspect of the cam's design, we use the lobe separation angle, sometimes called the lobe displacement angle (LSA) or spread.

The lobe separation angle is the simple angle between the peak lift point of a cylinder's intake and exhaust lobe pair. This direct angular measurement at the cam is specified in degrees at the cam, in contrast to degrees of crankshaft rotation, which turns at twice the camshaft speed. As you might imagine, the relative phasing of the intake and exhaust valve events has a serious affect on engine performance. A camshaft with a lobe separation angle of zero would have the intake and exhaust valves opening and closing at the same time, and that clearly won't work. In fact, typical lobe separation angles run in a range from about 102 to 116 degrees, with most performance aftermarket cams ground between 106 and 112 degrees. Even within this relatively narrow range of common LSAs there is quite a dramatic difference in how the engine will behave when comparing a narrow LSA such as 106 degrees, to a wide one such as 112.

Let's look at how a change in LSA will affect the valve events (assuming a fixed installed centerline). As the separation is narrowed, the exhaust opens and closes later, while the intake will open and close earlier. The later exhaust closing and earlier intake opening directly adds to the cam's overlap, and the overlap effects are significant (See: Overlap). Rather than individually looking at all the subtle interrelated effects created by a change in lobe separation angle, it is more useful to just bottom-line it here. As the lobe separation is narrowed, expect the cam to exhibit a nastier idle with more lope. Typically, once "on the cam," the peak torque and horsepower are improved, however, the engine will drop off more quickly past peak horsepower rpm.

EFFECTS OF LOBE SEPARATION ANGLE
LSA: NARROW: WIDE:
Intake open Earlier Later
Intake close Earlier Later
Exhaust open Later Earlier
Exhaust close Later Earlier
Overlap More Less
Cylinder pressure Gain Lose
Idle quality Worse Better
Idle vacuum Less More
Torque curve Peakier Flatter
Peak torque More Less
High rpm Drops off Hangs on

Lift
Of all the specifications related to a camshaft, "lift" is the easiest to understand. Lift simply refers to how far a valve is opened off the valve seat, with the specification given in thousandths of an inch. Where does the lift specification come from? The gross lift is the cam's actual "lobe lift," multiplied by the rocker ratio. As a cam lobe rotates from the base circle to the ramp, the lifter is displaced upwards by the eccentricity of the lobe until the point of maximum lift is reached. This action at the lobe of the cam is called the "lobe lift," and the lobe lift is generally given as a base specification on a cam card or in a catalog. The lobe lift isn't the same as the amount of lift at the valve, since valve lift is the product of the lobe lift and rocker ratio. Typically, the factory rocker ratio for most production V-8 engines ranged from 1.5:1 to 1.75:1, depending upon the engine type, but aftermarket rockers are available in a wide selection of ratios. Usually the lift given in manufacturers' catalogs is based upon the cam's lobe lift multiplied by the engine's original rocker ratio.

While the gross valve lift specification for a given cam is generally derived from the lobe lift and factory rocker ratio, the actual lift delivered may vary substantially from this spec. Clearly, if the rockers are a ratio other than stock, the valve lift will change. The gross valve lift can be determined for any rocker ratio by simply multiplying the lobe lift specification by the rocker ratio. For instance, a cam with .320-inch lobe lift will provide .480, .512, .544, or .576-inch lift with rocker ratios of 1.5, 1.6, 1.7, or 1.8:1, respectively.

So is the gross lift as given above exactly the true lift at the valve? Not necessarily, since the actual ratio any given rocker delivers may be somewhat off the quoted ratio specification, and subtle valvetrain geometry factors can also have a measureable affect on the true valve lift. The only way to precisely know the valve lift is to take a direct measurement using a dial indicator on the valve stem with the valvetrain fully assembled and adjusted. Still, this being the case, the gross lift as calculated by the lobe lift and rocker ratio is generally a good approximation when considering lift.

Now the big question: How much valve lift is right for my engine? Generally, more lift equates to more power and torque, however, the trade-off is component life, reliability, and the ability to control the valvetrain at higher rpm. It is important to consider the cylinder head's flow characteristics when contemplating lift, and target a valve lift that is well matched to use the head's capabilities. Your returns diminish rapidly when using higher lift on a stock-style head that may see no flow increase or even a drop in flow as the lift is increased. Conversely, applying less-than-optimal lift on a high-flow aftermarket cylinder head capable of very good high-lift flow simply is not taking advantage of the flow potential presented by the cylinder head.

Installed Centerline Angle (ICA)
All of the camshaft specifications listed so far-lift, duration, and lobe separation angle-are attributes of the camshaft itself. The installed centerline angle specification (ICA), in contrast, is a specification that describes how the cam is installed in the engine. What we are referring to here is the camshaft timing, or phasing, and this is measured relative to the crankshaft, and given as a point in degrees of crankshaft rotation.

When a cam is installed, it must be phased to operate the valves in the correct relationship to piston position. A simple reference is provided by the marks on the timing chain that will normally put the camshaft in the ballpark. Since the installed centerline angle will have a direct affect on every valve event relative to piston position, the true installed centerline will have a significant impact on the running characteristics of the engine. The process of "degreeing-in" the camshaft measures the installed centerline.

When a camshaft is installed at a position (in degrees of crank rotation) that is equal to the lobe separation angle, the cam is said to be installed "straight-up." For instance, a cam ground on a 108-degree lobe separation angle will be "straight-up" when installed at an intake centerline angle of 108 degrees after TDC. This will also put the exhaust centerline angle at 108 degrees before TDC, and split the overlap event evenly over TDC. This "straight-up" or "split-overlap" camshaft position is the zero reference for the cam position, and any advance or retard is relative to a "straight-up" installation. Advancing the cam will move all of the events to an earlier position relative to crank rotation, while retarding the cam will have the opposite effect. Advancing the 108-degree LSA cam from the example above by 4 degrees will put the intake installed centerline angle at 104 degrees ATDC, and the exhaust at 112 BTDC, and move each valve event 4 degrees earlier relative to crank position.

Normally, we find positive effects from moderate amounts of cam advance, while retard can produce negative results. Consequently, most aftermarket cams are ground with a small amount of advance relative to the keyway or pin, usually 4 degrees. An advanced position favors low-rpm operation, helping idle quality, cylinder pressure, low-rpm torque, and vacuum. Retarding the cam deteriorates these characteristics, and in many instances may not show any benefit at higher rpm.

Overlap
Overlap is a measurement of duration, given in degrees of crank rotation, specifying the period during which both the intake and exhaust valves are both open off their seats. During overlap, there is open through-flow between the intake and exhaust ports of the engine, and this can be a very useful situation for the production of power. As with any duration measurement, the actual checking height used to determine the overlap will directly impact the measured overlap specification, so the checking height needs to be known to make meaningful comparisons of overlap.

It is useful to touch on the dynamics of overlap, to gain some perspective of how it affects the engine's output. In a running engine, the exhaust flow within a header system produces significant scavenging force in the form of negative pressure at the exhaust valve. This is a combination of the inertia of gas flow and reflected pressure waves. These forces combine to clear the cylinder and chamber of residual exhaust gasses, and draw intake charge to improve cylinder filling. The downside is that the negative pressure generated by both inertial and pressure wave effects are rpm sensitive. At low rpm, the open area (overlap duration and lift) can work in the opposite direction, causing reversion and the subsequent lope, rough idle and even misfire.

The amount of overlap is not something that can be altered independently in a camshaft design, but rather it is determined by the duration and lobe separation angle. The flow area during overlap is also directly influenced by the overlap lift, and as the overlap duration increases, so does the lift during overlap, multiplying the effect. As duration is increased, and/or the lobe separation is narrowed, overlap increases as a direct result.

SOURCE
Comp Cams Inc.
3406 Democrat Road
Memphis
TN  38118
800-999-0853
www.compcams.com