At 02:44 PM 6/26/00 -0400, you wrote:
>sooooooo, someone kindly inform me as to what the centerline of a cam is,
>and what it accomplishes. i.e. the differences between 2 otherwise
>identical duration/lift, but one w/ more centerline
>
>--
>Jon Smith
You have lobe separation angle (LSA) and intake and exhaust
centerline. Both are important in figuring out the potential power a cam
can make in a given engine. There is a centerline for both the intake and
exhaust lobes. It's the theoretical maximum lift point of the lobe in
relationship to Top Dead Center in degrees of crankshaft rotation.
You can move the centerline by installing the cam advanced or
retarded. Advancing the cam will make the power potential of the cam lower
in RPM (come in at a lower RPM than if installed straight up) - usually
four degrees lowers it by 200RPM, and the opposite when retarded. This
movement of the cam (retarded or advanced) moves the "theoretical max lift"
point up or down in relation to crankshaft rotation (degrees). Basically
making power earlier or making power in the mid range and spreading the
band out and peaking out at a high(er) RPM.
Lobe separation is the distance (in camshaft degrees) that the intake and
exhaust lobe centerlines (for a given cylinder) are spread apart. Lobe
separation is a physical characteristic of the camshaft and cannot be
changed (unless the cam lobes are reground). This separation determines
where peak torque will occur within the engine's power range. Tight lobe
separations, such as 106 degrees, cause the peak torque to build early in
the RPM range of the cam. The torque will be concentrated, build quickly
and peak out. Broader lobe separations like 112 degrees, allows the torque
to be spread over a broader portion of the RPM range and has more power
through the upper RPM. The valve events on a longer separation angle cam
are not as "notchy" and tends to increase intake vacuum and reduces the
intake ripple/pulsation/reversion effect that is present in a shorter lobe
separation cam (ideal for 'puter controlled engines)
Both the centerlines and LSA play kind of the same role - they can tell you
when the cam will start to make power and when they start to wheeze, and
allow you to dial or tune in a cam to the engine's sweet spot for making
max power.
Duration is the time in degrees that a valve is open (from seat to
seat). It's measured at .050" of lifter lift/rise. For every 10 degrees
change in duration, the power band is moved up or down by 500 RPMs -
(Crower Cam Co.). Lift, of course, is the distance the valve opens. The
total lift is derived from the lobe lift of the cam and the rocker arm
ratio. The timing of how fast the valve is ramped up and down is a factor
in determining the amount of torque and engine will be able to produce.
Now the hard part is taking all the degrees, lifts, durations etc. and
actually making sense of them and making an imaginary model of the cam and
engine and their relationship. Master that and you can make a bunch of
money, if you don't get brain fry :)
Mike Schwall
mschwall@flash.net
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