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Lift
This can be cam lift or valve list. The latter being the the cam list multiplied by the rocker ratio.
Duration
This is the length of time, measured in crankshaft degrees that the valve is off its seat.
Overlap
The number of crankshaft degree where both the inlet and exhaust valve are open at the same time.
Cam timing
The position of the camshaft relative to the crankshaft. This is expressed as the number of degree that full lift occurs after top dead centre (TDC) in the case of the inlet, and before TDC for the exhaust.
Valve timing
The opening and closing position of inlet and exhaust valves relative to the crankshaft as figures before and after TDC and BDC.
Lobe Angle
The angle between the inlet and exhaust lobe, measured in degrees.
Ramp
The ramp is the part of the profile that takes up the valve clearance and slack in the valve train gradually, before the valve is actually lifted from the seat. It also rests the valve gently back to the seat after the closing flank. Mechanical profiles use a much larger ramp than hydraulic ones, as the hydraulic can follower should be in contact with the lobe at all times. The height of the ramp dictates what measurement the valve clearance should be set to.
Flank
This is the part of the profile between the ramp and nose. It is the most important part of the whole design. The flank controls the velocity and acceleration of the valve train. The acceleration / deceleration rate must be within the working limits of the valve spring, too much and valve float will occur. Generally high acceleration and velocity figures are beneficial to engine performance.
Nose radius
The larger the nose radius the better. Piper camshaft profiles are designed to utilize the biggest nose radius possible to keep the stresses to a minimum.
Dwell
As the valve reaches full lift it will stop moving for a few degrees before starting to drop back towards the seat, this period is known as the dwell. When checking the cam timing using the full tilt figure method the mid-point of the dwell should be taken as exact full tilt.
Rocker ratio
The ratio between valve motion versus cam follower motion. Push rod engines typically use a ration of 1.1:1 and 2.0:1. Over head cam, direct operating engines obviously have no rocker ration as the cam follower motion is exactly the same as the valve motion.
Overall height
The measurement from the nose of the lobe to the bottom of the base circle, in a straight line through the centre of the lobe.
Base circle diameter
The measurement across the lobe, calculated by measuring the overall height and subtracting the cam lift.
Information provided courtesy of Piper Cams and RPi Engineering
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