Lobe CenterlineCam lobe centerline is another factor in camshaft design. The lobe centerline is an imaginary line that simultaneously passes through the point of maximum lift on the lobe's nose (tip) and the camshaft's center of rotation (axis). The point at where the actual centerline of the intake lobe occurs in relationship to TDC position of the piston is defined in degrees of crankshaft rotation after top dead center (ATDC).
The valve is closed when the tappet is moving along the cam's base circle surface. The ram
The intake lobe centerline is calculated by dividing the intake duration by 2, then subtracting the intake valve opening. For example, using the Andrews 64G cam specs in Table 2, it has 272 degrees intake duration and the intake valve opens at 30 degrees BTDC and closes at 62 degrees ABDC. This would result in an intake lobe centerline of 106 degrees: 272/2 = 136, 136-30 = 106. Dividing the exhaust duration by 2, then subtracting the exhaust valve closing calculates the exhaust lobe centerline. Again using the Andrews 64G cam specs, the equation would look as follows: 276/2 = 138, 138-30 = 108. The values of the cam's intake and exhaust lobe centerlines are used in the calculation of another specification called Lobe Separation Angle.
Lobe Separation Angle (LSA)A major specification many racers note when selecting a camshaft is lobe separation angle (LSA). Lobe separation angle (also known as lobe displacement angle) is the distance measured in camshaft degrees between the centerline of the intake lobe and the centerline of the exhaust lobe for the same cylinder. Lobe separation angle is related to lobe centerline and both may be the same value, but they are not the same because they refer to different reference points.
As an example, LSA is one of the few occasions where cam specifications are specified in cam degrees instead of crankshaft degrees. Cam degrees are different from crankshaft degrees since the cam turns at half the speed of the crank. This results in twice as many crankshaft degrees for a given number of cam degrees. Additionally, unlike lobe centerline, LSA is ground into a cam and cannot be changed without regrinding the cam.
The importance of Lobe separation angle is that it has a direct relationship on overlap. Essentially, valve duration and cam lobe separation angle (LSA) determine the overlap period. When lift is added to the duration and LSA factors, the "overlap triangle" is defined. The overlap triangle represents a more accurate way of viewing overlap than only considering the amount of overlap duration. For a given LSA, the greater the duration and lift, the greater the overlap triangle will be. In addition, the wider the LSA (more degrees), the less overlap there will be. In contrast, the narrower the LSA is the greater the amount of overlap. However, keep in mind that two camshafts having the same duration and lift figures can be ground with different lobe separation angles, which results in different amounts of overlap.
Although there are exceptions for any rule, a tighter LSA usually improves midrange torque and usually results in a faster revving engine. A tight LSA also tends to produce a narrower powerband since the torque and horsepower peaks usually occur closer together. An engine with a narrow powerband usually requires higher ratio gears for optimum acceleration. In contrast, a wider LSA produces a broader powerband and more peak power. A wide LSA also improves idle quality and fuel economy while reducing exhaust emissions.