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Effect of diff. rocker ratios on cam duration

Discussion in 'The Hokey Ass Message Board' started by 461/2ton, Apr 29, 2009.

  1. 461/2ton
    Joined: Feb 17, 2007
    Posts: 76


    Is there a way to calculate the change in .050 cam duration with a change in rocker ratio? I am working with Ford 1.43 and 1.54 ratio rockers.
    If the relationship is a direct proportion (dont know if that is true), a cam with 240 duration using 1.54 rockers would drop to approx. 220 duration with 1.43. Right, wrong?
  2. Roothawg
    Joined: Mar 14, 2001
    Posts: 21,408


    Lift will change ,the duration should stay the same. Multiply the lift by the ratio of the rocker arm.
  3. buzzard
    Joined: Apr 20, 2001
    Posts: 4,335

    Alliance Member

    Lots of good info. on this_thread for you to check out.

    The quick version is this, the rocker ratio changes lift. The actual cam lob lift multiplied by the rocker ratio gives you total lift. (Ex. .326 cam lift x 1.5 rockers = .490 lift. .326x 1.6 rockers = .522 valve lift.) The durations, as I found out on that thread, changes very little to none. Hope this helps.
  4. btmatt
    Joined: Nov 15, 2006
    Posts: 227


    Duration at 0.002" wil be minimally affected. The more important number at 0.05 will be affected slightly. Because the profile of the lobe has some affect in the final number, there is no calculation to determine the change. You will need to install the cam and degree with each rocker arm to determine the difference.

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