Performance Boats Forum banner

1 - 9 of 9 Posts

·
Sit N' Spin
Joined
·
1,423 Posts
Discussion Starter #1 (Edited)
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?
 

·
E-7 Sheepdog (ret)
Joined
·
6,834 Posts
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.
 

·
E-7 Sheepdog (ret)
Joined
·
6,834 Posts
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".
 

·
steelcomp was here
Joined
·
26,512 Posts
You're going to need longer valves.:p
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. :D
 

·
Sit N' Spin
Joined
·
1,423 Posts
Discussion Starter #5
You're going to need longer valves.:p
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. :D

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.
 

·
Registered
Joined
·
2,034 Posts
You're going to need longer valves.:p
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. :D
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. :D
Wags
Good luck
 

·
Sit N' Spin
Joined
·
1,423 Posts
Discussion Starter #7
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. :D
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.
 
1 - 9 of 9 Posts
Top