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
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:
0.564" X 333 = 187.8
187.8 + 122.66 = 310.4lbs intake open pressure
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?