ZeeOSix

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!

Sales2@Texas-speed

iTrader: (5)

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!

ZeeOSix

I understand what you're saying, and I'm assuming PAC has actually designed the spring to give the forces at the two points they have specified - 130 lbs @ 1.800" and 318 lbs @ 1.200".

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?

farmington

iTrader: (14)

If you're that paranoid about the springs, get em tested. Don't forget you must have a retainer on them when you test them.

ZeeOSix

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.

Darkman

iTrader: (2)

http://profblairandassociates.com/pdfs/RET036_Blair.pdf

TurboBuick6

iTrader: (2)

Beehive springs do noy have a linear rate to them. that is probably what is messing you up on your calculations. On a stock engine I would just throw those bad lads in there and call it a day.

kinglt-1

iTrader: (26)

Imo you ware way over thinking this... install them to the suggested height and forget about the rest. Its not like your installing them on some wild setup anyway.

99Bluz28

iTrader: (35)

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.

ZeeOSix

Interesting info ... thanks for the link. The paper does show that these springs have a near linear spring rate (stiffness) in the first part of their travel, and then the spring rate actually starts increasing with deflection ... so yes, they don't really have a "constant" spring rate.

Yes, I was assuming and using a constant spring rate in the calculations, which didn't match the two data points on the box when starting from the free height, because the spring rate is changing with deflection. The spring rate of 313 lbs/in on the box is probably the averaged constant rate between the two data points used (1.800" and 1.200").

ZeeOSix

I usually do over think this stuff, but doing so tends to result in a better understanding of what's really going on. Nothing wrong with knowing every detail IMO.

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?

vettenuts

iTrader: (4)

I actually found the stock setup was less than 1.800". It was 1.765" as I recall but would need to pull my installation data. As you found, the springs are non-linear.

SweetS10V8

iTrader: (4)

I think you should be setting up off of coilbind, not setting to a certain installed height.

ZeeOSix

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.

TurboBuick6

iTrader: (2)

I wouldn't worry about that little bit of difference. The 1518 is a low enough rate spring your not going to damage anything. Just take your time and do it right and have fun!

ZeeOSix

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.

ZeeOSix

Yeah, I think I'll be OK as long as GM setup all the closed valve heights the same & correctly in the heads during manufacturing. I'm taking my time ... way too much time, like doing brain surgery.

LilJayV10

iTrader: (39)

Post deleted. I understand what you are saying now.

vettenuts

iTrader: (4)

It shouldn't be an issue. I checked my files this morning and the 1.765" is an average when I rebuilt my stock heads (note that not machining was done on the heads). You just need to make sure you have the required clearances at full open. I have set a several sets of heads and haven't found one yet that actually comes in at 1.800". That number is just used by the spring manufacturers as a data point.

ZeeOSix

I just did an installed height measurement on one of the valves and got 1.775" ~ 1.777". Did this by using a digital caliper and measured between the spring seat and the bottom of the retainer - probably not the best method, but I seemed to get decent numbers. I installed the retainer and keepers without the spring installed, and pulled the valve up to seat it while taking the measurement.

This shouldn't be a problem since the PAC 1518 springs are rated for 0.650" lift ... well above what the stock LS6 springs were rated. Plus I'm using the OEM retainers and locks, so no dimensions should be changed by just dropping in the PAC 1518 springs.

Good to know.