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Cam Relationships

duration LCA overlap

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#1 scott87star


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Posted 30 October 2015 - 12:26 PM

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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#2 87redcat


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Posted 30 October 2015 - 03:16 PM

Good info. Thank you
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#3 creakyjoints


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Posted 30 October 2015 - 03:32 PM

Cams are all about the bump n grind.
the dude in the honda said, i thought you said your car was slow, my response was.. well you said yours was fast so i guess we both lied.. haha

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