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Cam Relationships


scott87star
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Time to add something to the cam forums. Y'all have heard the terms, here's the math that defines the cam parameters. First, the definitions.

 

Overlap: period in crank degrees when both intake and exhaust valves are off the seat.

Duration: period in crank degrees when one valve is off the seat (seat to seat duration) or at a given lift value (0.050" typical).

LCA: lobe centerline angle or the angle in crank degrees between the peak of the intake lobe and the peak of the exhaust lobe. Note that in a single cam (overhead or in block) application the LCA is ground into the cam, in a dual overhead cam application the LCA can be altered using adjustable cam gears. Either way the math still applies.

 

Duration = (overlap/2 + LCA)*2

Overlap = (duration/2 - LCA)*2

LCA = (duration - overlap)/2

 

Point being if someone gives you two of the values you can calculate the third, they are all inter-related. Put another way you can't have big duration and overlap without having big LCA, it is forced into the design.

 

From the FSM, stock slipper lobe cam specs: Overlap 45 degrees, duration 264 degrees, the LCA must then be (264 - 45)/2 or 109.5 degrees.

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