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How to account for lash when calculating DCR

Discussion in 'Street/strip 400/430/455' started by accelr8, Jun 28, 2023.

  1. accelr8

    accelr8 Well-Known Member

    I'm getting ready to start my engine build and I'm looking at different cams. I'd like to calculate the DCR to help me select a cam. How do you account for lash when calculating DCR?

    Thanks,

    Jim
     
    accelr8, Jun 28, 2023
    #1
  2. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    From,

    http://cochise.uia.net/pkelley2/DynamicCR.html

    "The special case of solid lifter cams. Solid cams are usually speced at an abitrary lift value (often .015" or .020") determined by the designer to be a good approximation of the cam's profile. This lift spec is not always correct for a particular cam. The correct lift point to determine the seat to seat timing of the cam is: Lash / rocker ratio + .004". This accounts for the lash. A cam with a .026" lash (given 1.5 rockers) should be measured at .02133" (.026/1.5+.004= .02133>"). This cam lash, with seat timing speced at .020", is actually a bit smaller than advertised since the valve has yet to actually lift off the seat. How much is the question (.024" lash is the only lash that is correct at .020" with 1.5 rockers). Without knowing the ramp rate, and doing some calculations, or measuring with a degree wheel, it is impossible to know. Again, we have to use the mfg's numbers."
     
    LARRY70GS, Jun 28, 2023
    #2
  3. accelr8

    accelr8 Well-Known Member

    Thanks Larry! So it sound like you have to rely on the cam manufacture's recommended compression ratio when buying a solid cam and then dial it in when you degree it so the DCR is reasonable.
     
    accelr8, Jun 28, 2023
    #3

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