anesthes

iTrader: (12)

Valve spring math.

I wanted to do this writeup to help others understand something I ignored about engines for many years. I never had a problem (until recently) with
valve springs, so I never cared to learn much about them. I always skipped over that chapter in any performance magazine I read.

First, I want to thank two folks on TGO who helped me understand this, and answered all my questions: <B>Stekman</B>, and <B>RB83L69
</B>


I'm going to explain, in the best way possible, how valve springs work and are installed/upgraded from a non experts (thats me) point of view.

Valve springs have two primary functions. The first is the <B>seat pressure</B>.

<B>Seat pressure</B> is how much pressure is applied to the valve in its closed state, when the valve is seated. The main purpose
of seat pressure is to keep the valve from bouncing on the seat when the valve closes. When running a radical cam, with to little seat
pressure, the valve can actually bounce a few times on the seat when closing.

The second function of the valve spring is the <B>open pressure</B>.

<B>Open pressure</B> is the amount of pressure applied to the valve at various points of lift, and until fully opened. Various camshaft
and lifter combinations require minimum pressures to keep the valvetrain tightly on the cam lobe. Otherwise the valve can float when open.

The general consensus is that hydraulic flat tapet cams require about 100lbs of seat pressure. Hydraulic roller camshafts need around 120-125 lbs
of seat pressure.

But that is not all that effects valve spring selection. You need to ensure your valves have a safe margin to avoid coil bind at max lift. At max lift,
the spring is compressed. The minimum safe distance is .030" between coils at full lift. This ensures that even if you do float a little bit, the spring
will not bind.

Lets look at the specs for a popular Comp cams spring:

* Outside diameter: 1.430 in. two spring
* Inside diameter: .697 in.
* Seat load: 121 lbs. at 1.800 in.
* Open load: 388 lbs. at 1.200 in.
* Coil bind: 1.150 in.
* Rate: 344 lbs.

<B>Outside diameter</B> is the diameter required of the spring pocket (whre the springs rests on your head), and of the retainer (the steel plate
on top of the spring.

<B>Inside diameter</B> is the min distance inside the spring for the valve guide boss and guide seal.

<B>Seat load </B> tells us two things. 1) the spring is rated at 121lbs when seated. Ideal for a hydraulic roller camshaft. 2) The recommended
installation height of this spring is 1.800 inches. We will discuss this later.

<B>Open load</B> This is the amount of pressure at the max safe open point of the spring (max lift).

<B>Coil Bind</B> Is what happens when the spring compresses too far. bad bad bad.

<B>Rate</B> is how many lbs of force per inch this spring is rated at.

Lets talk about installed height. The installed height is the distance between the spring pocket in the head, and the retainer. These springs want
an install height of 1.800".

The machining in the top of the head, and the length of the valve will play a role in the install height of your current setup. To measure the install height
of your heads, you need a tool called an "install height micrometer". Basicly, you remove your current spring, put the mic in place of the spring (with retainer and locks) and turn it until its tight in the pocket. Then you look at the side for the measured height.

So lets say you have a set of aftermarket heads, and you need to use these comp cams spring. You take your factory springs out, you put the mic in, and turn it up until its tight. You look and read.. 1.900"

OK! This means our heads are machined with an install height of 1.900".. But wait. The spring we want to use require 1.800" of install hight. Either our valves
are too long, or the heads are machined too deep. In this scenario, you would need to remove .100", by adding a shim (or two) to the spring pocket. Basicly,
put one .100", or two .050" shims under your new spring, and assemble.

Ok, what if you measure and find out your heads are machined for 1.750" ? Are you doomed?

Comp cams makes something called an offset locker. This locker moves the retainer up .050" on the valve stem, thus creating .050" more clearance for you.
So folks who are .050" short, can add this with the offset locker.

Now lets talk about coil bind and lift. Your camshaft is generally measured by valve lift, with 1.50 ratio rockers. Lets use my hydraulic roller camshaft as an example.

My intake valve lift is: .502"
My exhaust valve lift is: .510"

So lets pretend I have the springs above, and they are installed at 1.800".

When my intake valve is at max lift, it will compress that spring by .502". So subtract the cam intake lift, from the install height. The answer is 1.298". No problem here, we have .098" left until we hit our safety margin!

Now lets subtract exhaust lift from installed height. The answer is 1.290", so we have .090" left of our safety margin.

So if your following along with the math, at the install height of 1.800, we can safely run up to a .600" lift camshaft.


And that really is all there is to it. Amazing huh? Just remember the basic guidelines: You need a spring that fits your pocket, so measure your pockets <B>DIAMETER</B> and make sure you have retainers to match. The spring needs to fit over the guide + seal, so measure that too. The spring must have enough
pressure for your camshaft, but not too much, so follow the recommendations for flat tapet vs roller. And lastly, make sure you subtract the <B>Open Load</B> height from the install height, to find out how large of a camshaft you can use.

DuronClocker

iTrader: (1)

*VERY* informative. That answered most questions I had about springs. The only one I have left is...are the springs the only thing that holds us back from using bigger cams on stock heads? If I get the head machined for bigger springs, then I can run bigger springs and a bigger cam? I don't need any other clearancing, correct?

Also, how much does it usually cost to have heads machined for larger springs? I'm considering buying a set and using 1.6RRs..

anesthes

iTrader: (12)

Well, other than piston to valve interference. I run flat top pistons with valve reliefs.

Hard core race builders, with huge lift cams and pop up pistons use modeling clay on the top of the piston when the piston is TDC, then they lift the valve to check for interference.

-- Joe

Stekman

To add to what anesthis has said (great info, btw), coil bind is when the coils actually come into contact with eachother, physically preventing the spring to be compressed any further. An added word also about the spring rate. This is the amount of pressure required to compress the spring 1". Meaning, if the spring has a 300lb/in rate, then 300 lbs of force is needed to compress the spring 1". Valve springs are also linear in calculation. Meaning, to compress the same spring .25", only 75lbs are needed and for .5" compression, only 150 lbs are needed.

Perhaps another topic, not widely known, but still very important (and one of the reasons i cannot emphasize enough, the importance of matching the spring to the cam, usually the recommended spring). Spring surge. What is it? Thanks to their coiled design, valve springs have a few key characteristics about them. These are called the "resonant frequencies." Another name is "natural harmonics." A lot like sound waves at a high enough frequency can be used to shatter glass, springs also operate and a set frequency. The glass itself can only take a set frequency. When this frequency is met, it is destroyed. The same is true for the valve spring. And , if left undampened, these frequencies can destroy or severly impair the spring, limiting its overall effectiveness. The job of the valve spring is simple, yet so complex. It must overcome all the harmonics put out by the cam and the crank, being able to maintain control of the spring itself, and more importantly the valve. They need to deliver the power to resist the inertial forces of the valve train. When the spring vibrates at its set resonate frequency, it looses control of the valve and things go crazy. This is why it is so important to match the spring to the cam. Not only for the obviousl reasons, but cam manufacturers have gone through the process of picking out components whose RPM ranges do no upset the spring frequencies. Anything that can dampen the spring frequencies is good. This is why MOST aftermarket springs, even the single outer coils, such as a Comp 981 spring, is sold with an inner damper coil. Damper springs work much like a crankshaft damper, they absorb harmonics via friction. They narrow spring surge to a very narrow RPM band. Usually, if the spring is matched to the cam, the surge RPM is above the camshafts powerband.

How spring surge can be prevented, or at least, hindered: Like mentioned, damper springs can be used to absorb some of the frequencies by using friction. Variable rate springs also exist. Variable rate springs have one end more closely wound than another. This design somewhat resists surge, due to the fact that there is no constant resonate frequency throughout the entire spring. A less common spring is the barrel wound design. These are smaller on the ends, with a larger center portion. Example: the ends may be 1.26" (factory sized) but the center portion may be 1.45". This gives you the effect of a larger diameter spring. By changing the diameter, you change the frequency of the spring throughout, thus resisting surge. However, dampers cannot be used with these springs. Perhaps the most common is the multiple spring design. By adding an inner spring, you add a seperate resonate frequecy. This in itself can act as a damper, or, a damper coil can be slid in between and further dampen harmonics.

To run a larger cam lift-wise on factory heads, you need to do a few things. First and foremost, make sure you have adequate clearance between the top of the guide boss and the bottom of the retainer. Factory heads top out at about .480" or so of max lift between these 2 points, which is the main issue (factory springs are actually *good*, and I use that term loosely, to about .510" before coil bind, from the installed height). That's the first thing. The second is whether or not the spring will actually support it.

installed height - coil bind - safetly clearance value (I use .060", there is no SET value) = what the spring will physically support.

Assuming there is adequate clearance between the bottom of the retainer and the guide boss, which, if machined, there SHOULD be, then I take the above value and determine just how much valve lift I can run. I always like to leave a bit extra, as a "fudge factor" if you will. This leaves room for shims or whatever.

DuronClocker

iTrader: (1)

*edited because the previous post was posted before I got around to posting this reply and answered all questions*

DuronClocker

iTrader: (1)

I want to put an LT1 cam in my motor and am trying to go cheap, but if I have to do something, I will. You say stock springs can be good for up to .510" though sketchy, but they should work for .480" it sounds like.

The LT1 cam would put me about right at .480" lift. If needed, would I be able to just file down the bosses a tad bit, or does the machine shop do it a special way?

Stekman

Just run it to a shop. You could, in theory, do it with a file. However, you can also try to carve stone with a leather awl.

About the springs, yes, using the formula to determine how much lift the springs themselves can take, factory springs can end up taking about .510". However, the clearances between the top of the guide boss and the bottom of the retainer come first, in this case. I have heard (and seen, once) of people filing down the bottom of the retainer to add clearance. This is one way, probably not the smartest way as it can weaken and even decrease the grip between the lock and the retainer. Just take it to the shop. They will rig it up on the bridgeport (at least my local shop does) and attach a special cutter and knock them off easy. Cam companies do sell the tools to do it yourself. Crane I know sells one, I'm sure other places like Comp and Isky also offer them. But they are the cutters with a pilot that centers it. They use a 1/2" drill. If done carefully, I think you could do it yourself.

anesthes

iTrader: (12)

[QUOTE]Originally posted by Stekman
[B]

And the above picture shows exactly why Comp 987's dont work on my sportsman-II heads. See the taper of the bottom of the guide boss, it extends past the outter diameter of the seal, so the inner spring is sitting on a taper, not the flat spring pocket.

-- Joe

kdrolt

Some additional comments:

1. Spring pressure is a misnomer. Springs exert force, not pressure. This is obvious for anyone with an ME background, but apparently not for everyone else.... especially for writers for automotiove magazines.

2. Cams don't put out harmonics, stable or unstable. Those descriptors are more verbal hogwash.

3. A valve spring will do a proper job of keeping most of the static force on a valve (so that the motion is controlled precisely by the cam lobe profile) for most engine rpms except for two conditions:

(A) when the rpms are high enough, the inertia of the valve train (mass*acceleration) can overcome the spring force, and that's called valve float.

(B) the so-called "spring surge" is one of the resonance vibration modes of the valve spring. Using stiffer springs raises the frequency (which can be converted to engine rpm) for resonance, as well as raising the ability to use a larger static spring force preload (mistakenly called seat pressure). There are three ways to raises the rpm for resonance: increasing the stiffness of the spring, or lowering the mass, or some combination of both. "Beehive" valve springs do both: they have lower overall mass at the retainer end, and they are wound to a smaller radius, which in turn increases their stiffness in torsion.

There are engineering and math meanings behind words like resonance and harmonics, but they are seldom used properly, especially in auto rags. FWIW.

Stekman

Considering as how pressure is a force per area, such as pounds per square inch, yes, pressure is the wrong term. Springs are technically measured in pounds of force applied to hold the the valve to the seat or the rocker. However, too many people call it "pressure" to go about trying to change it.

Correct, cams do not exert harmonics. I worded that badly. The crankshaft exerts torsional vibrations that get transmitted to the valve train, via the timing set, and eventually ends up getting to the springs. The opening and closing of the valves are the other source of harmonics.

kdrolt

I wasn't aiming my comments at you -- they were more generally aimed (and specifically targeted at the auto writers). Marlan Davis and Hib Halverson are two exceptions. Some of what gets read in those magazines winds up being used again on forums, like this one.

Crankshafts don't exert torsional vibrations on anything except the torsional damper at one end, and the flywheel/trans at the other end. Crankshafts experience torsional vibrations. Some of those vibrations get into the valve train via propagtion through the tensioned half of the timing chain.... but that still has NOTHING to do with harmonics.

And how can you say that valve opening/closure has anything to do with harmonics when you haven't defined what harmonics are? Valve opening/closure events, which act as small force impulses and not as "harmonics", can be mitigated by using ramps to smooth out the actuation. Cam and valvetrains are almost always designed to have smooth transitions from off-on-off the lobe, and that means smooth displacement, velocity, acceleration, and jerk (all d/dt related).

Tossing around pseudo-technical terms like harmonics without saying what they are repeats the same offense made by the writers.

Example: an oscillator vibrates at 100 Hz. If the vibration is measured with an optical non-contact probe, we can measure the vibration output at all frequencies including 200 Hz. The vibration at the 2nd harmonic (200 Hz) will usually be much less than at 100 Hz (the fundamental). If the oscillator were an audio amp, we wouldn't want any output at 200 Hz if the signal being amplifier were at 100 Hz. That would be zero percent 2nd harmonic distortion, at least for this one example. So in using the term "harmonic", there is an implied factor of 2 for the second harmonic, and 3 for the third, and so on. So in this example, I am being very specific about what a harmonic is.

Now apply this to "harmonics of an engine" or to a valvetrain. What exactly does that mean? I sure don't know, and I'm not looking for an answer -- I am pointing out that you are still using the jargon of the auto writer(s) without knowing what it means.

Reverting to the valvetrain discussion, there is no need to drag "harmonics" into the discussion because it has no place, and it only proves that the user of such a term doesn't know what it means.

Again, this isn't aimed at you (or anyone else at TGO)..... it's a criticism of the writers who foisted the terms on us first, and thereby cause those same terms to get carelessly repeated here.

Stekman

Yes yes. Perhaps harmonics is a bad term overall. i can't word thigns right. I think "natural resonate frequencies" would be better.

Can't we all just get along?