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cam lift question

  1. #1
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    Default cam lift question

    If I have a pair of heads where the valve springs/valve combo has a max lift of .700, the solid roller cam has .715 lift, and valve lash is set at .025, is it possible that it will work, or does it not work like that?

    .715
    - .025
    .690

    .690 actual lift with a valvetrain combo rated for a max of .700

    I realize that it would need to be checked during engine assembly to make sure that it is all working correctly, I'm just wondering if the math of it works like that.

    Is there a relative easy fix if the valve train doesn't work right due to slightly too much lift, or are you supposed to replace springs and valves? (valve spring pocket machining? re-grind cam for lower lift? <-- how much lower lift?)

    Thanks again!
    Last edited by hotrod56cars; 05-25-2008 at 10:50 PM. Reason: more

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    Senior Member bordsmnj's Avatar
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    interesting. where does rockerratio play in this? .715 x 1.7?
    <----also wants to learn.
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    Default NO

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    Senior Member bordsmnj's Avatar
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    yeah, basically its your cam(.715) x rocker ratio(example 1.7) =1.215"
    then subract your lash for net lift. hers where i got this
    http://www.circletrack.com/techartic.../photo_06.html
    losta good info here -Jas
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    Quote Originally Posted by hotrod56cars View Post
    If I have a pair of heads where the valve springs/valve combo has a max lift of .700, the solid roller cam has .715 lift, and valve lash is set at .025, is it possible that it will work, or does it not work like that?

    .715
    - .025
    .690

    .690 actual lift with a valvetrain combo rated for a max of .700

    I realize that it would need to be checked during engine assembly to make sure that it is all working correctly, I'm just wondering if the math of it works like that.

    Is there a relative easy fix if the valve train doesn't work right due to slightly too much lift, or are you supposed to replace springs and valves? (valve spring pocket machining? re-grind cam for lower lift? <-- how much lower lift?)

    Thanks again!
    Hey hot rod, The cam spec you are listing being .715 lift of valve off the seat. On that cam the acctual or net amount of lift at the cam would be .420. The rocker factors in as the lift at valve. Therefore .420 x 1.7 = .715. Here is a double check for you. .715 devided by rocker ratio. A dial indicator at each ind of the rocker tells these numbers. Lastly a cam of that size wants a minimum .040 coil clearance. YOU MY FRIEND ARE IN THE DANGER ZONE!!!!!!!!!!!!!!! I don't know if you have purchased your'e rockers yet but if you do the math with the info above a slower ratio rocker is a easier fix. IE .420 x 1.6=.672 lift at valve. Very good spec cam.IMO M

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    Ol guy is right, and that would be a VERY tight combination, best dealt with by less lift, one way (different cam) or another (lower ratio rockers).
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    ok not to hijack but to understand, my cam is .680 lift how would i know what rocker to use to atchieve that lift would it be in the part number?
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    Quote Originally Posted by bordsmnj View Post
    ok not to hijack but to understand, my cam is .680 lift how would i know what rocker to use to atchieve that lift would it be in the part number?
    Hey bord, not knowing the motor Ford or chevy it's a hard call But the stock ratio is the factor the cam grinder comes up with for the lift at the valve,Chev uses a 1.70 rocker so the lift at cam is .400. Ford 429-460 uses a 1.73 rocker so the lift at cam would be .393 Then you multiply by rocker ratio and that determines the lift at valve. Again the cam grinders design cams to work with stock ratio. then its up to the motor builder to take some lift out or add some by rocker ratio changes. I was racing a 429 ford and would order my cam choice knowing I was going to machine the head for chevy 1.70 rockers, The chevy valve train is much easier to play with and less costly in the long run. Hope this helps. M

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    ah ha! beautiful. thanks for clearing that little mystery up. i looked at a cam i have here and said to myself there's just no way those lobes are .700 tall,lol. i'll keep that 1.7 in my mind. -jas
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    Quote Originally Posted by hotrod56cars View Post
    If I have a pair of heads where the valve springs/valve combo has a max lift of .700, the solid roller cam has .715 lift, and valve lash is set at .025, is it possible that it will work, or does it not work like that?

    .715
    - .025
    .690

    .690 actual lift with a valvetrain combo rated for a max of .700

    I realize that it would need to be checked during engine assembly to make sure that it is all working correctly, I'm just wondering if the math of it works like that.

    Is there a relative easy fix if the valve train doesn't work right due to slightly too much lift, or are you supposed to replace springs and valves? (valve spring pocket machining? re-grind cam for lower lift? <-- how much lower lift?)

    Thanks again!
    That's a bit tight and would likely be a problem. Depending on the application, you may be able to buy +.100" longer retainers to give you a bit more room. You'll likely need different springs to get proper seat/over the nose spring pressure. Likely cheaper than new valves.
    If I can, I like to set my spring height about .100" taller than my valve lift. This gives you room to shim the spring if it starts to get a bit soft and still allow enough room to avoid coil bind. Good luck.

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    Someone might add the insignificant fact that valve spring PRESSURE may not be correct for the application. Required pressure is dependant on lobe profile (how radical the ramps are), application, rpm's it regularly sees, valve weight, etc.
    You need .060" minimum before spring coil bind. Just because a spring is spec'ed at .700 has little to do with how much lift it will handle before coil bind. Different manufacturers (of springs) rate their springs different (before coil bind). (I don't even think that sentence made sense)
    But, what do I know.
    Wags

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    Quote Originally Posted by wagspe208 View Post
    Someone might add the insignificant fact that valve spring PRESSURE may not be correct for the application. Required pressure is dependant on lobe profile (how radical the ramps are), application, rpm's it regularly sees, valve weight, etc.
    You need .060" minimum before spring coil bind. Just because a spring is spec'ed at .700 has little to do with how much lift it will handle before coil bind. Different manufacturers (of springs) rate their springs different (before coil bind). (I don't even think that sentence made sense)
    But, what do I know.
    Wags
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    If the heads came as a package from say Brodix for example, and the spring package was for .700 lift, this would typically be .075-.100 from coil bind. You say your cam has a gross theoretical of .715, factor in a typical lash of .020, plus valve train deflection (with some decent parts) and you will be lucky if your getting .650 actual at the valve.
    I would say you will be more than fine, but need to check the heads to make sure they are indeed set-up for .700 lift (coil to coil bind and seal to retainer clearance) and are of the proper pressure for the cam.

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    Valvetrain. I'm looking at a set of Merlins, 030620-3, assembled, new, recomended for a solid roller cam, max valve lift .700

    Originaly I was going to go with 030620-4 with a max valve lift of .800, but unfortunately the Summit Credit Card doesn't have the balance for that.

    My cam is .715 lift with a 1.7 rocker arm ratio.

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