Valve Spring Math
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Valve Spring Math

  1. #1
    Sit N' Spin Jetaholic's Avatar
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    Default Valve Spring Math

    OK so I'm teaching myself how to do valve spring math to figure out what springs I need. Figured I'd post it up here and see if I got it right.

    Target Seat Pressure: 120lbs
    Target Open Pressure: 310lbs

    Measured Installed Height (between spring pad and retainer bottom): 1.975"

    Cam: Comp Cams XM278H - 0.564" lift intake/0.567" lift exhaust

    So in order to get a 310lbs open pressure from 120lbs on the seat, the pressure would have to increase by a factor of:

    310lbs - 120lbs = 190lbs

    In order to do that with this particular cam, we would need a spring rate of:

    190lbs / 0.567" lift = 335lbs per in.

    The only Comp Cams spring I could find that was close to the spring rate I need is a Comp Cams 950 dual w/damper. However, they spec the spring out with an installed height of 1.950", which puts the seat pressure at 116lbs. We need 120lbs, so we would need to shim it enough to give us the correct seat pressure + 0.025" (i.e. the difference between 1.975" and 1.950"). But the up side is that the spring has a spring rate of 333lbs per in., very close to our required 335lbs per in.

    Spring Choice: Comp Cams 950 dual w/damper

    950 Specs
    Coil Bind: 1.2"
    Spring Rate: 333lbs per in.
    Pressure @ 1.950": 116

    Obviously with 0.030" of shim, that would only put the valve spring installed height at 1.945". This would only give me a seat pressure increase of:

    1.975" - 0.030" = 1.945"

    1.950" - 1.945" = 0.005"

    0.005" X 333 = 1.665lbs

    Which would only increase spring pressure to:

    116 + 1.665 = 117.665lbs seat pressure

    However, if I shim it 0.045" it would put me at:

    1.975" - 0.045" = 1.93

    1.950" - 1.930" = 0.020"

    0.020" X 333 = 6.66lbs increase

    6.66lbs + 116 = 122.66lbs seat pressure

    So with a 0.045" shim, this puts our new installed height at 1.930" instead of 1.975". This puts me just over my target seat pressure. Now if I take the total lift and multiply that to the spring rate, then add that value to the new seat pressure, that puts me at:

    Intake -
    0.564" X 333 = 187.8

    187.8 + 122.66 = 310.4lbs intake open pressure

    Exhaust -
    0.567" X 333 = 188.8

    188.8 + 122.6 = 311.4lbs exhaust open pressure

    Now this spring specs out at a coil bind of 1.2". If I take the total lift of the exhaust side (cause it's more than the intake) and subtract that from the spring installed height, I get:

    1.930" - 0.567" = 1.363" open height

    Then I take that and subtract the coil bind spec from that:

    1.363" - 1.2" = 0.163"

    That puts me at 0.163" before coil bind.

    Did I do this math correctly? And if the math is correct and the spring measures right up to the above math, would this be the correct spring to run for my setup?
    Last edited by Jetaholic; 11-11-2008 at 10:08 AM.



    Quote Originally Posted by HammerDown
    Kendall L-427 Super Blu...extreme-pressure (ep) lithium complex keeps my thrusting balls happy
    Quote Originally Posted by back2k5 View Post
    Well I am putting in a nice stereo system in the Carrera...maybe Ill put on some rap and turn the scoop sideways

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  3. #2
    E-7 Sheepdog (ret) SmokinLowriderSS's Avatar
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    Quote Originally Posted by Jetaholic View Post
    OK so I'm teaching myself how to do valve spring math to figure out what springs I need. Figured I'd post it up here and see if I got it right.

    Target Seat Pressure: 120lbs
    Target Open Pressure: 310lbs

    Measured Installed Height (between spring pad and retainer bottom): 1.975"

    Cam: Comp Cams XM278H - 0.564" lift intake/0.567" lift exhaust

    So in order to get a 310lbs open pressure from 120lbs on the seat, the pressure would have to increase by a factor of:

    310lbs - 120lbs = 190lbs

    In order to do that with this particular cam, we would need a spring rate of:

    190lbs / 0.567" lift = 335lbs per in.

    The only Comp Cams spring I could find that was close to the spring rate I need is a Comp Cams 950 dual w/damper. However, they spec the spring out with an installed height of 1.950", which puts the seat pressure at 116lbs. We need 120lbs, so we would need to shim it enough to give us the correct seat pressure + 0.025" (i.e. the difference between 1.975" and 1.950"). But the up side is that the spring has a spring rate of 333lbs per in., very close to our required 335lbs per in.

    Spring Choice: Comp Cams 950 dual w/damper

    950 Specs
    Coil Bind: 1.2"
    Spring Rate: 333lbs per in.
    Pressure @ 1.950": 116

    Obviously with 0.030" of shim, that would only put the valve spring installed height at 1.945". This would only give me a seat pressure increase of:

    1.975" - 0.030" = 1.945"

    1.950" - 1.945" = 0.005"

    0.005" X 333 = 1.665lbs

    Which would only increase spring pressure to:

    116 + 1.665 = 117.665lbs seat pressure

    However, if I shim it 0.045" it would put me at:

    1.975" - 0.045" = 1.93

    1.950" - 1.930" = 0.020"

    0.020" X 333 = 6.66lbs increase

    6.66lbs + 116 = 122.66lbs seat pressure

    So with a 0.045" shim, this puts our new installed height at 1.930" instead of 1.975". This puts me just over my target seat pressure. Now if I take the total lift and multiply that to the spring rate, then add that value to the new seat pressure, that puts me at:

    Intake -
    0.564" X 333 = 187.8

    187.8 + 122.66 = 310.4lbs intake open pressure

    Exhaust -
    0.567" X 333 = 188.8

    188.8 + 122.6 = 311.4lbs exhaust open pressure

    Now this spring specs out at a coil bind of 1.2". If I take the total lift of the exhaust side (cause it's more than the intake) and subtract that from the spring installed height, I get:

    1.930" - 0.567" = 1.363" open height

    Then I take that and subtract the coil bind spec from that:

    1.363" - 1.2" = 0.163"

    That puts me at 0.163" before coil bind.

    Did I do this math correctly? And if the math is correct and the spring measures right up to the above math, would this be the correct spring to run for my setup?
    Simplification:
    Wants:
    Target Seat Pressure: 120lbs
    Target Open Pressure: 310lbs

    Spring needs:
    336#/" rate intake
    335#/" rate exhaust

    950 spring is JUST BARELY going to fill the bill. Which side can/will you hedge on?

    309.8 pounds ex, 308.8 pounds intake.

    You are only off on your seat pressure math (and thus minimum installed height)..
    333#/" = 3.33# per .010 (or 1.665/.005")
    You only need 4# from 116 to 120,
    0.015"=4.995#

    You will have to spring to to 1.935 to bring the 1.950 116# figure to 120# (3.33#/.010") .015" shimming = 4.995#, 116# +4.995# = 120.995#

    Thus, installed height # 121# is 1.935.

    Coil bind + 1.200, +0.567 = 1.767.
    1.767 from the installed height of 1.935 leaves you 0.168" coil clearance.
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    E-7 Sheepdog (ret) SmokinLowriderSS's Avatar
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    You can run a stiffer spring than that, I am.

    You will have to shim it ANOTHER .005" (total of .020") to get your target OPEN pressure, pulling your closed height to 1.930, and your clearance from bind to 0.163".
    Last edited by SmokinLowriderSS; 11-11-2008 at 04:07 PM. Reason: addition
    "As democracy is perfected, the office of president represents, more and more closely, the inner soul of the people. On some great and glorious day the plain folks of the land will reach their heart's desire at last and the White House will be adorned by a downright moron."
    H.L. Mencken

    OBAMA: Some people deserve this.
    The rest of us are being dragged along kicking and screaming.

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  6. #4
    steelcomp was here
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    You're going to need longer valves.
    Hopefully this is all "theoretical" and you're not seriously thinking of shimming for 5# of spring pressure. Springs vary more than that, especially these Comp springs, and unless you have an expensive digital spring checker, you'll never know. Granted, it is fun to figure this stuff out.
    Last edited by scott foxwell; 11-11-2008 at 05:29 PM.
    If God is your co-pilot, change seats!
    Acts 2:38, the perfect answer to the perfect question.

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    Sit N' Spin Jetaholic's Avatar
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    Quote Originally Posted by steelcomp View Post
    You're going to need longer valves.
    Hopefully this is all "theoretical" and you're not seriously thinking of shimming for 5# of spring pressure. Springs vary more than that, especially these Comp springs, and unless you have an expensive digital spring checker, you'll never know. Granted, it is fun to figure this stuff out.

    Yes it is all in fact theoretical. But if you look, the example head measures an installed height of 1.975", yet the spring is at 116lbs @ 1.950". So to even get it to that from the measured installed height, you'd have to shim 0.025". By shimming it 0.020" more to 0.045" you get the extra 6lbs.



    Quote Originally Posted by HammerDown
    Kendall L-427 Super Blu...extreme-pressure (ep) lithium complex keeps my thrusting balls happy
    Quote Originally Posted by back2k5 View Post
    Well I am putting in a nice stereo system in the Carrera...maybe Ill put on some rap and turn the scoop sideways

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    Senior Member wagspe208's Avatar
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    Quote Originally Posted by steelcomp View Post
    You're going to need longer valves.
    Hopefully this is all "theoretical" and you're not seriously thinking of shimming for 5# of spring pressure. Springs vary more than that, especially these Comp springs, and unless you have an expensive digital spring checker, you'll never know. Granted, it is fun to figure this stuff out.
    Guess what is wrong with all the theory??
    1) 4 pounds of seat pressure is nothing. Is the 116 new, or after the springs have urn. There will be a difference.
    2) What about manufacturing tolerances. If you think any spring manufacturer has a tolerance of less than 3% + or - you are kidding youself.
    3) Buy a valve spring tester, check prwessures, get damn close and be happy.

    PS, I love the theory stuff. There is a reason it is called theory, though, and not real world. In theory, I could date a super model and her sister at the same time. In reality, I have just one hot gf.
    Wags
    Good luck

  9. #7
    Sit N' Spin Jetaholic's Avatar
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    Quote Originally Posted by wagspe208 View Post
    Guess what is wrong with all the theory??
    1) 4 pounds of seat pressure is nothing. Is the 116 new, or after the springs have urn. There will be a difference.
    2) What about manufacturing tolerances. If you think any spring manufacturer has a tolerance of less than 3% + or - you are kidding youself.
    3) Buy a valve spring tester, check prwessures, get damn close and be happy.

    PS, I love the theory stuff. There is a reason it is called theory, though, and not real world. In theory, I could date a super model and her sister at the same time. In reality, I have just one hot gf.
    Wags
    Good luck
    116lbs is the pressure new. I am aware that you lose around 5% after they've been run.

    Theory is exactly what this is. I was just making sure I had the math all correct.



    Quote Originally Posted by HammerDown
    Kendall L-427 Super Blu...extreme-pressure (ep) lithium complex keeps my thrusting balls happy
    Quote Originally Posted by back2k5 View Post
    Well I am putting in a nice stereo system in the Carrera...maybe Ill put on some rap and turn the scoop sideways

  10. #8
    Strap on the JATO konajet74's Avatar
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    Jon, How'd your cam warranty issue go? Are they going to
    take care of the valvetrain also?

  11. #9
    Senior Member MACHINEHEAD1's Avatar
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    Comp recommends the 924. The 950 OD seems a little small 1.464. The 950 w/cup is plenty for 5500

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