Understanding Cam Profile Terms

Annotated Cam LobeRAMPS The parts of a camshaft lobe that actually initiate the lifting and descending movement of the lifter are called “ramps“. Ramps include the lash ramp, the opening ramp, and the closing ramp. Camshaft lobe ramps are ground to have different rates of lifter movement in terms of velocity and degrees of duration, as measured in degrees of crankshaft rotation. The “lash ramp” of a camshaft lobe is a mid-point location between the opening ramp and closing ramp. The “opening ramp” of a camshaft lobe is the point where the lifter just begins to lift until the point that it reaches the nose of the lobe. The “closing ramp” is the point of the camshaft lobe from the nose back down to the lash ramp. NOSE The “nose” of a camshaft lobe is the top or the highest maximum lift point for the valve. It is where valves are kept open for as long as possible before making the transition to the closing ramp. BASE CIRCLE The “base circle“, also known as the “heel“, is the lowest point of the camshaft lobe and is the place where the valve is in the closed position. The “base circle” is the point where all valve lash settings are made. Cam Lobe SymmetrySYMMETRICAL is a term that refers to the “profiles” of the opening and closing ramps of a camshaft lobe. All “early technology” camshafts were ground on a symmetrical design, meaning both sides are exactly the same. That is to say the profile of the closing ramp is a “mirror image” of the opening ramp. ASYMMETRICAL refers to a camshaft lobe profile where the opening and closing ramps are not exactly the same. The reason some camshafts are this way is to try to achieve an opening ramp profile that has a high velocity and a closing ramp profile that has a slower velocity. In this way the valve can be set down more “gently” than the rate at which it was first opened. A DUAL PATTERN camshaft has an intake lobe profile design that differs from that of the exhaust lobe profile design. For example, camshaft “A” has intake lobes of 260 degrees duration while the exhaust lobes are 270 degrees duration. Camshaft “B“, has intake and exhaust lobes that are both at 260 degrees. Camshaft “A” is referred to as a dual pattern, while camshaft “B” is referred to as a single pattern. With the advent of emissions laws and the widespread use of computer systems, more modern single and dual profile pattern designs have been developed. A dual pattern camshaft is ground to “bias” the duration of either the intake or exhaust lobe. For example, if an engine is restricted on the exhaust side, compared to the intake side, the camshaft designer would try to compensate by grinding in more lift and/or duration on the exhaust lobe. Source: http://www.lunatipower.com/Tech/Cams/CamProfileTerms.aspx
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