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Technical Is there a formula to figure out what rocker arm you can use?

Discussion in 'The Hokey Ass Message Board' started by El Caballo, Nov 20, 2019.

  1. El Caballo
    Joined: Mar 3, 2001
    Posts: 6,283

    El Caballo
    ALLIANCE MEMBER

    I'm interested in the idea of using 1.7 ratio rocker arms on a 302 (stock 1.6 ratio). I would want to avoid any piston to valve interference of course. I'm sure the camshaft grind and lifter rod length make a difference if there is an aftermarket cam used. However, for this question let's assume stock:

    Part Description : Camshaft
    Opens : -22 BTC/12 BBC
    Closes : 26 ABC/-8 ATC
    Duration : 184/189
    Opens : 22 BTC/45 BBC
    Closes : 70 ABC/40 ATC
    Duration : 272/265
    CamLift : .230/.238
    Gross Valve Lift : .368/.380
    Center Line C/L : 114/104
    Rocker Arm Ratio : 1.6/1.6
    Valve Lash Clearance : HYD/HYD

    Has anyone tried this? I was thinking it would be a different way to achieve more lift and duration using a smaller cam.
     
  2. razoo lew
    Joined: Apr 11, 2017
    Posts: 536

    razoo lew
    Member
    from Calgary

    Valve lift is cam lift multiplied by rocker arm ratio. The 1.7 ratio will obviously provide more valve lift, but there is no way of predicting piston/valve interference without more information.
     
  3. El Caballo
    Joined: Mar 3, 2001
    Posts: 6,283

    El Caballo
    ALLIANCE MEMBER

    If we take the .238 x 1.7 = 0.4046 valve lift.

    I guess I need the maximum allowable lift, but trying to find out without pulling the head and testing with clay, etc.
     
  4. Mike VV
    Joined: Sep 28, 2010
    Posts: 3,029

    Mike VV
    Member
    from SoCal

    It's all really simple.
    Plus...are we to assume that you are talking about a...Chevy 302 ?
    For all the hassle, the cost of some ill fitting rocker arms, just buy the cam that you really want..!
    1.7 rocker arms on a small Chevy...na..!

    Mike
     
    Oldioron likes this.

  5. El Caballo
    Joined: Mar 3, 2001
    Posts: 6,283

    El Caballo
    ALLIANCE MEMBER

    I've read where people have recommended anywhere from 0.480 (tappet) to a high of 0.520 (roller).

    Assuming the 0.480 - 0.4046 = 0.0754, is that enough clearance to avoid any problems?

    Ford 302 circa 1974
     
  6. squirrel
    Joined: Sep 23, 2004
    Posts: 55,934

    squirrel
    Member

    Ford 302, it appears.
     
    Johnny Gee and Fordors like this.
  7. saltracer219
    Joined: Sep 23, 2006
    Posts: 1,071

    saltracer219
    Member

    It would be a Ford, they have 1.6 ratio rockers in stock form, Chevrolets have 1.5
     
    Fordors likes this.
  8. oldiron 440
    Joined: Dec 12, 2018
    Posts: 3,299

    oldiron 440
    Member

    I agree with buying the cam that you need/want and passing on the rockers. I've seen problems with strange harmonics in the valvtrain using 1.7 rockers on standard cams.
    Be aware that you might need push rods and valve springs to run the 1.7 rockers.
     
  9. El Caballo
    Joined: Mar 3, 2001
    Posts: 6,283

    El Caballo
    ALLIANCE MEMBER

    Valve spring bind is what I was thinking about as well. This was more of a theoretical exercise than anything that I would start dumping money into. I'd rather get a crate engine on a roller block than invest the same money or more trying to massage stock stuff into waking up. Right now I'm satisfied with the dual exhaust and aftermarket intake and carb. Just put a Summit four barrel on there and I'll update how it does after I have driven it for a while.
     
  10. El Caballo
    Joined: Mar 3, 2001
    Posts: 6,283

    El Caballo
    ALLIANCE MEMBER

    80781d33-ad89-4676-9e81-e77db7e361da.jpg


    Sent from my iPhone using H.A.M.B.
     
  11. The point of contact will be at tdc in overlap. If you knew how much clearance you had before changing rockers, you could make a calculated guess as to what you would end up with. People have been swapping ratios for decades. The correct geometry and push rod length is what's important. As an example, the 93 mustang cobra has 1.7 rockers where all the standard ho's have 1.6. I have the 1.7's on my 32. Better than 20,000 miles without issue.
     
    El Caballo and Mark Yac like this.
  12. sunbeam
    Joined: Oct 22, 2010
    Posts: 6,213

    sunbeam
    Member

    If you are checking the intake is the one most likey to hit ,10 degrees or so after the valve will be closer, the piston doesn't move much in that 10 degrees less than the valve.
     

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