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Courtesy of Tony Sims


Camshaft 101


HEEL is the bottom-most portion of the camshaft's lobe.


RAMP is the ascending or descending side of the cam lobe coming off the base circle when lift begins to occur.


FLANK is the ascending or descending portion of the lobe past the base circle nearest maximum lift.


BASE CIRCLE is the portion of the path around the cam followed by the rocker arm that doesn't generate lift.


LIFT is the maximum amount the rocker can be raised off the base circle. Lift is measured in thousandths of an inch. (.000”) or mm. Lobe profile determines whether the lift comes on smoothly or abruptly. Lift is not necessarly the actual amount the valve will open, as rocker ratio and rocker clearance will affect that.


ROCKER RATIO is the measurement of the lever effect of the rocker arm. For example, if rocker ratio is 1.5, the valve will be lifted 1.5 x the amount of cam lift.


DURATION is the amount of time the valve is open (measured in degrees), beginning when the valve unseats. Duration begins when the rocker begins to ascend the ramp coming off the heel. “Duration at fifty” means that duration begins at .050, the industry standard for determining camshaft lobe duration. When you're reading camshaft specs you'll often see both "Duration" and "Duration at .050".


DURATION ALA' BMW is the number you are listing -- 284, 273, etc. This is the absolute duration figure for the intake lobe. The "duration at fifty" measurement is less, and varies based on lobe profile, i.e. the combination of heel, ramp and flank.


LOBE SEPARATION (also known as lobe centerline) is the distance (in degrees) between the intake lobe peak and the exhaust lobe peak.


INTAKE CENTERLINE is the position of the camshaft in relation to the crankshaft. For example, an intake centerline of 114 degrees means the intake valve reaches maximum lift at 114 degrees after top dead center (ATDC).


EXHAUST CENTERLINE is basically the same thing as intake centerline. It is when the exhaust valve reaches maximum lift degrees before top dead center (BTDC).


VALVE OVERLAP is the period of time when both the intake an exhaust valve are both open to allow for proper cylinder scavenging. Overlap occurs when the exhaust valve is closing when the piston is reaching top dead center. The intake charge from the opening intake valve pushes the exhaust gases out. Camshaft grinders can change low separation 'Valve overlap' to modify the performance of a camshaft. Sometimes they do this rather then change lift or duration.






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