valve spring open pressure

[Greg Long]

Several months ago I had a custom solid roller ground for a customer. The spec sheet did not have spring pressures listed, so I contacted the grinder and was given seat pressure only, so I questioned the open pressure they wanted. Well the answer came back to get the seat pressure right and let the open pressure fall where it may.
This sort of went away from everything I had ever heard. I put the engine together with specs a little higher than I have on similar engines in the past.
It seems to be running well with no problems, but I would like to hear other opinions, ideas, or find out what I just don't know.
Thanks, Greg

[David Redszus]

Spring PRESSURE is an incorrect term. We should be considering the required spring FORCE.

Seat force is required to adequately seal the valve to its seat even as carbon builds up.
It is determined by valve contact area and valve seating characteristics.

Nose force is required to control the motion of the valve relative to the cam lobe.
It is determined by the valve train mass (valve, rockers, followers, push rod, etc),
and the acceleration G forces. For a given valve train mass, valve spring force becomes
a function of lobe shape and camshaft rpm.

Spring force determination is important to prevent valve float and subsequent valve bounce.

[PRH]

I def don’t agree with the “guidance” you got from the cam grinder.

I have dyno tested several motors built by others who had the heads set up with going by suggested seat loads...... with little or no consideration given to open loads......and when the open load is insufficient to maintain control of the valvetrain........ the motor just doesn’t want to pull past the rpm point where control is lost.

I saw a lot of that on some of the dirt oval track stuff, usually with faster rate SFT cams where they were worried about too much open load wearing out the cam.
Some of those cams need quite a bit of open load with the rpms they were running the motors in the car.

[BOOT]

With factors like lightweight retainers, valves, heat or other components and different spring types like conical or beehive. I don't think it's so much a lb/force or w/e you wanna call it as it takes what it takes. The only ideal way to have a set/goal open number would be with exp for that exact/sim combo, dyno, track or spintron testing.

I think a lot of people end up with more than they need because it's easier on parts than less

[David Redszus]

For a given valve spring rate, the spring force will increase proportional to lift.
A doubling of lift will double the force exerted by the spring.

But, for a doubling of rpm, the valve acceleration G force will quadruple; increase by a
factor of four.

While maximum spring force is reached at maximum lift, maximum valve acceleration will
occur much earlier and at a much lower valve lift. The valve acceleration across the
nose is much lower than peak valve acceleration.

Valve acceleration consists of two positive G force peaks and one negative G force peak.

Therefore, the term "nose force" is quite incorrect and should be replaced by
the term "open force".

[LSP]

Depending on spring rate, open pressure can vary widely, and not enough or too much spring rate can both cause issues.

With more information, I can give a recommendation, if interested send me a PM

[rebelrouser]

Several months ago I had a custom solid roller ground for a customer. The spec sheet did not have spring pressures listed, so I contacted the grinder and was given seat pressure only, so I questioned the open pressure they wanted. Well the answer came back to get the seat pressure right and let the open pressure fall where it may.
This sort of went away from everything I had ever heard. I put the engine together with specs a little higher than I have on similar engines in the past.
It seems to be running well with no problems, but I would like to hear other opinions, ideas, or find out what I just don't know.
Thanks, Greg

I have gotten some of the same run around when I call about a used cam, or a customer gives me a pile of parts to use. I really don't understand why it is not an industry standard to simple put what kind of spring pressures or force, the cam lobes require, and what RPM levels these specs would be good for. Not sure if the cam companies have the resources to do testing or what? It would seem to me they would have a vested interest in seeing that the products they sell would hold up and provide good results with their customers. And what do you do when you can tell the other end of the phone is pulling a number out of his ass, when you ask such questions?

[mt-engines]

Is this a drag application? Circle track? The most vague knowledge would be 100lbs/.100" as a starting point. Did you give the Cam guy the weight of the retainers and valves?

Post some specs or the incremental lobe lift #s

[Nut124]

Several months ago I had a custom solid roller ground for a customer. The spec sheet did not have spring pressures listed, so I contacted the grinder and was given seat pressure only, so I questioned the open pressure they wanted. Well the answer came back to get the seat pressure right and let the open pressure fall where it may.
This sort of went away from everything I had ever heard. I put the engine together with specs a little higher than I have on similar engines in the past.
It seems to be running well with no problems, but I would like to hear other opinions, ideas, or find out what I just don't know.
Thanks, Greg

I have gotten some of the same run around when I call about a used cam, or a customer gives me a pile of parts to use. I really don't understand why it is not an industry standard to simple put what kind of spring pressures or force, the cam lobes require, and what RPM levels these specs would be good for. Not sure if the cam companies have the resources to do testing or what? It would seem to me they would have a vested interest in seeing that the products they sell would hold up and provide good results with their customers. And what do you do when you can tell the other end of the phone is pulling a number out of his ass, when you ask such questions?

How would the cam grinder be able to specify the spring forces required when they have no control of the weight or dimensions of the rest of the valve train?

Seems like the best they could do is provide lift and accel curves for the builder to assess.

[frnkeore]

There is software out there to do this. While I don't have the software and don't know the name for it, I have had this calculated for me.

You do have to know the component weight and cam profile. This is the form I filled out for it.

[LSP]

There is software out there to do this. While I don't have the software and don't know the name for it, I have had this calculated for me.

You do have to know the component weight and cam profile. This is the form I filled out for it.

Software is just a prediction, real world can be different and usually is. There's some data being asked for in that form that isn't important, and some data missing that is.

If software was always accurate, engines and components would never have to be tested on a dyno, which is what championship engine builders and teams do day in, day out.

[Greg Long]

Thank to all responding. I feel everyone has valid points that I have heard of or read about and that is what brought the question to mind. I just can't understand how or why a cam guy can just seemingly write it off as just let it fall where it may.
I can really agree with the statement of cam grinders not knowing the components mass. I have never been asked about the variables involved in conversation or on any recommendation sheet. It makes you wonder if they have generic numbers for a solid roller, flat tappet, hydraulic, that they know will be close enough, just to ease their job. Over the years I have noticed the numbers are about the same from all grinders and most printed guides, magazine articles and books are in the same range for a certain cam type cam. Thanks again, Greg

[David Redszus]

Software is just a prediction, real world can be different and usually is.

Valve float prediction software can be very accurate...statically. I have written such a program.

But real engines operate in a dynamic world which is not as nice and smooth as are equations.
The problem becomes a matter of vibrations and poor design for dynamic conditions.
The actual lift curves in a running engine do not resemble those measured on a test bench.

If software was always accurate, engines and components would never have to be tested
on a dyno, which is what championship engine builders and teams do day in, day out.

Performance testing is perhaps the least important task addressed on OE and pro team dynos.
Measurement of dynamic parameters: fatigue, wear, temperature, vibration are of equal importance.

Factory race programs are often considered low priority regarding access to dyno testing, at the factory.

[Walter R. Malik]

I don't know of ANY cam designer "worth his salt" who couldn't give an accurate estimation of what his lobe profile would probably need for spring force for a given valve train weight.
The camshaft "supplier" might not be able to give those numbers correctly as he is usually trying to sell you a given valve spring with a certain rate and that is all he probably knows.

Personally, I believe OPEN force is more important with today's roller cams as long as seat pressure is simply enough.
Flat tappet stuff can have a very high acceleration rate near the seat so, it is probably the opposite for those.

[frnkeore]

I tell you what, I would much rather depend on a good spring program, that what is said on the phone at Comp Cam!

I've only called them 3 times but, 2 out of the 3, I got spring info that I KNEW was wrong, even after asking the person to confirm, what they were saying. The other time, the info sounded reasonable. The inquiry was about springs for a .875 lift rate SFT cam and their actual lobe, for it.

Yes, I'm sure that the actual person that makes their living designing cam lobes, can give you accurate spring info but, the average person, can't talk to that person at CC.