PAC 1518 Spring Questions
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PAC 1518 Spring Questions I've got a set of brand new PAC 1518 valve springs I'm going to install in my stock 2002 Z06 w/LS6 this weekend.
Box says:
Batch # 38367 / 4-1-11
Installed: 130 lbs @ 1.800"
Open: 318 lb @ 1.200"
Rate: 313 lbs/in
If you do the math based on the numbers above, the force between the two spring length points of 1.800" and 1.200" correlate with the 313 lbs/in spring rate.
I measured the un-installed free length of the springs with a digital caliper and came up with an average of 2.315".
If the spring is compressed from 2.315" to 1.800", that is a compressed distance of 0.515". That equates to a force of 161 lbs @ 1.800" (using 313 lbs/in), which doesn't match the PAC spec of 130 lbs @ 1.800" on the box.
Likewise, if compressed from 2.315" to 1.200", that equates to 349 lbs @ 1.200" (using 313 lbs/in) instead of the PAC spec of 318 lbs @ 1.200" on the box.
Is the spring rate really not linear in the first part of compression? I don't have a spring tester.
Is the free length of these PAC 1518 springs too long? Why don't the force numbers vs. compressed distance correlate closer? Maybe I'm missing something on how PAC comes up with their force vs. compressed length specs (?).
I recall seeing a PAC Tech here on the board (don't recall your user name) ... could you please also chime in - thanks!
Box says:
Batch # 38367 / 4-1-11
Installed: 130 lbs @ 1.800"
Open: 318 lb @ 1.200"
Rate: 313 lbs/in
If you do the math based on the numbers above, the force between the two spring length points of 1.800" and 1.200" correlate with the 313 lbs/in spring rate.
I measured the un-installed free length of the springs with a digital caliper and came up with an average of 2.315".
If the spring is compressed from 2.315" to 1.800", that is a compressed distance of 0.515". That equates to a force of 161 lbs @ 1.800" (using 313 lbs/in), which doesn't match the PAC spec of 130 lbs @ 1.800" on the box.
Likewise, if compressed from 2.315" to 1.200", that equates to 349 lbs @ 1.200" (using 313 lbs/in) instead of the PAC spec of 318 lbs @ 1.200" on the box.
Is the spring rate really not linear in the first part of compression? I don't have a spring tester.
Is the free length of these PAC 1518 springs too long? Why don't the force numbers vs. compressed distance correlate closer? Maybe I'm missing something on how PAC comes up with their force vs. compressed length specs (?).
I recall seeing a PAC Tech here on the board (don't recall your user name) ... could you please also chime in - thanks!
Last edited by ZeeOSix; Sep 24, 2011 at 05:10 PM.
You have to take a measurement to get your base height to calculate anything off of. Free height minus 1.800 times spring rate DOES NOT equal pressure at 1.800"! You would have to use a bench top pressure tester to take a reading at 1.800, or 2.310", or wherever else to have a starting point to calculate from!
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You have to take a measurement to get your base height to calculate anything off of. Free height minus 1.800 times spring rate DOES NOT equal pressure at 1.800"! You would have to use a bench top pressure tester to take a reading at 1.800, or 2.310", or wherever else to have a starting point to calculate from!
But if the spring rate of 313 lbs/in was truly linear and constant from zero compression (ie, free height) to the 1.800" installed height, then theoretically the force at 1.800" should be equal to free height minus 1.800" times spring rate.
Maybe the spring rate isn't really that linear?
I'd like to know what the nominal free height of the PAC 1518 springs are supposed to be just to feel good that this set isn't out of spec for some reason.
Anyone have a new set of PAC 1518s sitting around where you could measure the free height of a couple and post the results?
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Even is they were made a little too long in free height, it only adds ~30 lbs to the specified forces. Probably isn't going to hurt the valves or seats, although the stock yellow springs are supposedly only 90 lbs at installed height of 1.800". These PAC 1518s could potentially be around 161 lbs at 1.800".
Just want to make sure I don't use something that's not right.
Again ... if anyone has some new PAC 1518 springs laying around, I would appreciate it if you could measure the free height of a few and let me know what you measure.
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I'm not that paranoid, and don't know who in my area would have a good spring testing machine. I'm just trying to understand why the numbers don't come out assuming the spring rate of 313 lbs/in is linear and constant.
Even is they were made a little too long in free height, it only adds ~30 lbs to the specified forces. Probably isn't going to hurt the valves or seats, although the stock yellow springs are supposedly only 90 lbs at installed height of 1.800". These PAC 1518s could potentially be around 161 lbs at 1.800".
Just want to make sure I don't use something that's not right.
Again ... if anyone has some new PAC 1518 springs laying around, I would appreciate it if you could measure the free height of a few and let me know what you measure.
Even is they were made a little too long in free height, it only adds ~30 lbs to the specified forces. Probably isn't going to hurt the valves or seats, although the stock yellow springs are supposedly only 90 lbs at installed height of 1.800". These PAC 1518s could potentially be around 161 lbs at 1.800".
Just want to make sure I don't use something that's not right.
Again ... if anyone has some new PAC 1518 springs laying around, I would appreciate it if you could measure the free height of a few and let me know what you measure.
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Yeah, the info on the box is just your basic info needed, not all the info you'd need for your calculations.
IMO, just set them to the proper seat/installed height and don't worry about it.
IMO, just set them to the proper seat/installed height and don't worry about it.
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Last edited by ZeeOSix; Sep 25, 2011 at 02:50 PM.
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Anyway ... about the installed height. I'm just replacing the springs and using all the OEM retainers, locks and seats (seats are the metal base of the valve guide seals). So I take it the installed height on these springs will be exactly what the OEM springs were at, which I believe is also 1.800".
The spring's installed height is defined by the distance between the seat surface and the retainer surface when the valve is closed, which hasn't changed in this case. So I shouldn't have to worry about the installed height needing any adjustments ... right?
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What ever the installed height is from the factory based on the distance from the valve guide seal "seat" to the bottom surface of the spring retainer is what I'm stuck with. The distance from the spring seat to the bottom surface of the spring retainer (ie, installed height) is defined by the assembled configuration of the retainer and locks on the valve stem when the valve is closed. None of that can be changed with OEM parts.
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I found the on-line 2011 PAC catalog and plotted the deflection vs. load data PAC shows for the PAC 1518 valve spring. See plot below - you can see the slightly non-linear deflection vs. load curve, which indicates the spring constant is slowly increasing with increased deflection.
Other calculations (not shown) indicates that the spring stiffness slowly changes from 260 lbs/in at the installed height of 1.800" to a rate of 360 lbs/in at a compressed height of 1.200". The 'average' spring rate between 1.800" and 1.200" is the advertised 313 lbs/in you see on the box.
Other calculations (not shown) indicates that the spring stiffness slowly changes from 260 lbs/in at the installed height of 1.800" to a rate of 360 lbs/in at a compressed height of 1.200". The 'average' spring rate between 1.800" and 1.200" is the advertised 313 lbs/in you see on the box.
Last edited by ZeeOSix; Sep 30, 2011 at 02:38 PM.
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Correct me if I'm wrong, but I don't think there is really anything I can do to make the installed height 1.800" if it's less than that (1.765" stock, if that's the case). On the LS1/LS6, the only valve spring seat "shim" is the built in base/shim provided by the OEM valve guide seals. There is no way to remove shims and increase the installed spring height.
What ever the installed height is from the factory based on the distance from the valve guide seal "seat" to the bottom surface of the spring retainer is what I'm stuck with. The distance from the spring seat to the bottom surface of the spring retainer (ie, installed height) is defined by the assembled configuration of the retainer and locks on the valve stem when the valve is closed. None of that can be changed with OEM parts.
What ever the installed height is from the factory based on the distance from the valve guide seal "seat" to the bottom surface of the spring retainer is what I'm stuck with. The distance from the spring seat to the bottom surface of the spring retainer (ie, installed height) is defined by the assembled configuration of the retainer and locks on the valve stem when the valve is closed. None of that can be changed with OEM parts.
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Good to know.