Understanding Cam Profile Terms
RAMPS 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.
SYMMETRICAL 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.