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Valve Spring Truth?

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Valve Spring Truth?

Postby Cammer » Tue Jan 08, 2008 11:03 am

I am asking members to take a serious look at valve spring pressures.

Members of this forum regularly recommend very high spring pressures.

I am questioning the validity of this practice.

IMHO, out of control spring harmonics and related spring surge are responsible for alot of valvetrain problems and adding spring pressure is often used as a band-aid to mask poor engineering! This added spring pressure can increase heat and friction, and cost power.

I am playing the devil's advocate to get members to examine the science they are using in spring selection.

If racing is to grow, we must also grow and challenge the practices we employ!
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Postby axegrinder » Tue Jan 08, 2008 1:03 pm

I only build two or three motors per year and I'm still learning all the time so I'm far from an expert. What matters the most to me is the distance to coil bind. If my seat preasure is a little low but the valve is opening to within .050 of coil bind, then I'll run it.

Too much spring can also cost you power if your pushrods cant handle the preasure. 330lbs on the seat and .080 wall pushrods were like a rev limmiter at 7400rpm!
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Postby CamKing » Tue Jan 08, 2008 1:23 pm

The whole reason we started balaning cams was to allow for less spring pressure. Most of the development work I do with teams, is related to running less spring pressure.
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Postby DavidHarsay » Tue Jan 08, 2008 4:20 pm

From what I understand, the basic reason people are on the higher rate spring bandwagon is the spring pressure with the valve closed, to avoid bounce and the resulting reversion.

There are many other factors of course.

Many of us would agree that blindly recommending ultra high spring rates to control spring surge or harmonics is inappropriate.

Just as a discussion... not trying to counter anyone's opinions.

The coil bind (or rather close to it) setup is something new I personally learned here, and actually ignored in the past (Thanks Speedtalk!!!)

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Postby stock z/28 » Tue Jan 08, 2008 4:33 pm

Hello,


I think its a very relative question. I have wondered the same thing many times

I used to race a 1969 Camaro with the 302 in stock eliminator for several years back when actuall "stock" spring pressures, as well cam lift and duration, were required.

The seat pressure was a round 75lbs and open pressure was about 225 at about .450 lift.

These cams had very fast ramps, and I would have thougth that valve control would have been fairly difficult.


These motors would generally spin to about 8800-9000-RPM. If the motor would not run past 8000 it was time for a new set of springs, about 12-20 passes.



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Re: Valve Spring Truth?

Postby SStrokerAce » Tue Jan 08, 2008 6:36 pm

Cammer wrote: I am asking members to take a serious look at valve spring pressures.

Members of this forum regularly recommend very high spring pressures.

I am questioning the validity of this practice.

IMHO, out of control spring harmonics and related spring surge are responsible for alot of valvetrain problems and adding spring pressure is often used as a band-aid to mask poor engineering! This added spring pressure can increase heat and friction, and cost power.

I am playing the devil's advocate to get members to examine the science they are using in spring selection.


The basics of the science are F=ma.... where your spring load is relative to the acceleration and mass it has to control. If you take mass out of the valvetrain be that spring, retainer, lock, valve or rocker tip (lower MOI) then you can either ADD acceleration or SUBTRACT spring load. Both obviously have advantages.

Spring rate is a big part of this not because what it gives you over the nose but that it raises the natural frequency of the springs as well. Resonant frequency is also a product of spring OD as well. The smaller the OD usually the higher the frequency. This all comes about because a smaller OD and higher rate also lead to lower mass of a spring, and the less mass a spring has the higher the resonant freq given the same rate.

Quick calculator for you, it's not 100% right but it gets you in the ballpark

http://www.efunda.com/DesignStandards/s ... e.cfm#calc

You just can't use that calculator forever unless you become a member.

The other thing to look at is if the spring is right for your application. If you have excessive coil clearance then it's probably not. Setting the coil clearance up for your needs will prevent you from having excessive surge during opening and closing and higher natural freq helps when the cam is on the base circle.

Cammer wrote:If racing is to grow, we must also grow and challenge the practices we employ!


AWESOME QUOTE!

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Postby Ed-vancedEngines » Tue Jan 08, 2008 7:46 pm

Actually at the exact times when the valve is on it's seat at rest or when the valve is fully open there is not much if any spring presure needed. But O'boy when the lifter jumps off the ski slope of a decelleration ramp that strong spring rate really comes into play and is severely needed.

When the lifter goes over the top of the nose of the cam and suddenly you want the valve to quit any additional movement is when that high open pressure really helps. If it is not enough to overcome the inertia with the moving weight you end up with what is called valve float or a valve lofting.

When the valve is first slammed shut with the spring rate with seat pressure is where you really need good valve control to have a controlled bounce of one to two times maximum the valve will leave the seat after the initial closing. The valve bouncing repeatedly after the initial closing is resulting from insufficient seat pressures and/or excessive free clearances between coil bind which allows and even further promotes spring surging. When the valve spring is in a surging state there is very little presure holding the valve closed at the exact point when it needs controlled the most. Do not confuse spring surge with spring occilations caused from harmonics, sometimes called fuss points.

Of course as early mentioned as the moment of mass of weight goes down on the valve end of the equation, using ligher components and smaller springs allows requirements for spring seat and open tensions to go down.

The shape of the actual cam lobe has a greater influence in what is needed for spring rates and pressures more than just about anything else, in my opinion.

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Postby SWR » Tue Jan 08, 2008 7:47 pm

So,with the same valve weights etc and the same rates/pressures on springs,a smaller OD spring will hang in there a little bit further than an equvivalent spring of bigger dia??
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Postby Rick360 » Tue Jan 08, 2008 8:13 pm

Is there an engine speed where the spring surge issue needs addressed by using the tighter coil bind clearance? I've heard the valvetrain harmonics (fuss-points) aren't really much problem until the rpm is around 8k or higher. I realize the spring still needs to be sized (pressures) correctly for the application, but for typical SBC valvetrain components to 7000rpm, is running the springs real close to coil bind necessary?

Anybody see it on a spintron at that speed?

I ask because I had always ran them taller (farther from coilbind) than recommended (to improve spring life :lol:) and always had good spring life and no valvetrain problems. This was on 7500 rpm or less SBC's. Were my cam's just too whimpy or is this not a problem at lower engine speeds?

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Postby Keith Morganstein » Tue Jan 08, 2008 8:51 pm

Here are a couple good ones from Darin Morgan some time ago....



Valve train design and tuning is a science all its own and has a little touch of mysticism as well. The Kinematic motion of a valve train is extremely complex. Simply stating that a spring needs "more" or "less" pressure is overly simplistic. It overlooks the finer tuning aspects of the system and especially the spring. The spring can encounter many different problems; float being just one of them. The spring can encounter Resonance, Coil surge, Float or simple Fatigue due to excessive heat or use. Float is the term most often used to describe the spring not controlling the valve train and in most cases people are correct in using this term because Float is the predominant valve train problem in most racing engines operating at moderately high engine speeds in the 7500 to 8000rpm range. This is due to either the builder not using the correct spring to control the valve train or the springs being used have fatigued with time and or heat. This is where the valves can bounces off the seat as much as .020 bleeding off the rising cylinder pressure back up the intake tract effectively shutting the engine off or keeping it from accelerating any further. Having two or three valve bounces of about .005 at the valve is a normal but albeit unhealthy occurrence in high rpm racing engines. We would like to have zero bounce, but that just doesn’t happen. Coil surge is a scenario that takes place when the spring is set up to far away from coil bind giving the middle coils room to bounce back and forth from top to bottom as the spring is closing. This usually takes place with a highly energized spring (fast ramp rate) at high engine speeds. Having a spring with not enough rate can exacerbate the problem and having a spring with both to little rate and not enough seat/nose pressure in conjunction with being set up to far from coil bind is down right deadly. I have seen springs turn blue, shatter and literally melt due to this scenario. This will cause valve bounce similar to floating the system and letting the valve hit the seat. Resonance is a killer but I find it a rare occurrence with the Super Series engines or other engines that operate at or below 7500rpm. In the big Block Chevy engine there are two distinct points within the rpm range that present a problem. 7600to 7800 and 9800-10000. 7600-7800 is usually just before the cam goes into loft (not float) and the 9800+ range is where the loft goes Bozo for a lack of a better term. The 7600-7800rpm point has been shown to excite certain types of springs that are considered to be "better" because of there increased rate and pressures. I put a set of PSI 048 style springs on a truck puller engine with the Raptor heads. I mistakenly thought that the “better " spring would lend some protection to the valve train because this guy revved the engine to 9600rpm+ and at time free revved the engine to 9500rpm+. Well he called me the after a week and said that he had broken six valve springs and the rest where Jell-O. They had lost 100lbs of seat pressure. I instantly knew to ask him one question. Do you hold the engine at or around 7600-7800rpm for any length of time? He reported that that is exactly where the engine dropped back down to when the sled came in. BINGO, resonance! I sent him A set of Comp 048 springs and told him to change them when they dropped below 285lbs. He has over 100runs on the engine and going strong. He replaces the valve springs about every 25 runs. This is the point I was trying to make. Just because a spring has more pressure or rate does not mean that it is a good match for the valve train system you are using them on. On a Drag racing engine that goes to 7600-8000 or 8500rpm on the shift, you won’t generally encounter this problem because the spring goes in and out of resonance so fast its of no consequence. If you just go to the point of resonance or just past it and back down through it, you’re generally ok. It’s when you “hang" there for any length of time that the damage becomes evident.
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There is no pat answer for the proper procedure to set up valve springs. There are guide lines you should follow such as not taking the coils to bind and trying to set it up close to the manufactures recommendations. I always tell people that .050 away from coil bind is as tight as I would ever run a spring. I have never seen ( at least with the engines I have worked with) a spring that benefited from running it tighter than .050. Some springs like to be farther away from coil bind than others. On a Pro Stock engine they are about .120 away from coil bind. It depends on, the spring design, the camshafts lobe design, valve train weight and the amount of loft your are trying to control. The valve train weight and loft usually come into play with engine that operate at extreme engine speeds at or around 9000-10000rpm but Nextel Cup engines use these techniques as well.
To answer some of your questions, yes we do worry about both the open and closed pressures but the seat pressure much less than the open. In a perfect world the spring would have little pressure at close and about 1200 over the nose but this is not a perfect world so we have to set them up at about 425 on the seat to get that over the nose pressure and the proper rate to control the valve action. I wish I could get 200 on the seat and 1200 over the nose but they cant make a spring with that rate. An engine design is an exercise in compromise and that goes double for the valve train! The only way to learn what makes a particular engines valve train work is to get on a Spin Tron and the dyno and work it out over time. otherwise, you go off the camshaft and spring manufactures recommendations.

General rules of thumb,

(1) Don't get any closer than .050 to coil bind.
(2) As far as Seat pressure goes, A little too much will not hurt you but too little will DESTROY you!
(3) Follow the manufacture recommendations first before you venture out on your own. I say this because if you pick the wrong spring for a camshaft you will induce resonance into the valve train and hurt parts. Even if the spring has more pressure than what the manufacture wanted you to run! I cant count how many times people purchased some wiz bang HIGH pressure spring only to discover that its rate and coil design induced resonance some where in the power band and beat everything to death! Spring design still rates as a black art in my book.
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Postby SStrokerAce » Tue Jan 08, 2008 9:35 pm

SWR wrote:So,with the same valve weights etc and the same rates/pressures on springs,a smaller OD spring will hang in there a little bit further than an equvivalent spring of bigger dia??


The smaller OD spring will most likely be lighter.... that is unless you add a bunch of coils and drop the rate. Then the frequency will drop as well.

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Postby SStrokerAce » Tue Jan 08, 2008 9:37 pm

Rick360 wrote:Is there an engine speed where the spring surge issue needs addressed by using the tighter coil bind clearance? I've heard the valvetrain harmonics (fuss-points) aren't really much problem until the rpm is around 8k or higher. I realize the spring still needs to be sized (pressures) correctly for the application, but for typical SBC valvetrain components to 7000rpm, is running the springs real close to coil bind necessary?

Anybody see it on a spintron at that speed?

I ask because I had always ran them taller (farther from coilbind) than recommended (to improve spring life :lol:) and always had good spring life and no valvetrain problems. This was on 7500 rpm or less SBC's. Were my cam's just too whimpy or is this not a problem at lower engine speeds?

Rick


Yes below 8,000rpm this can happen as well. The more you push the envelope with everything reguarless of RPM range the more issues you can have. I've seen the difference in coil bind pick up a motor 15hp after HP peak and this was in the low 7,000rpm range. I've also seen it on some hyd applcations below 7,000rpm.

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Postby Old School » Tue Jan 08, 2008 9:38 pm

I know this is far to broad a subject to be this simple but is there some sort of guideline or formula that the rpm, weight of valve and retainer, rocker ratio, cam intensity, and maybe lift could be plugged in to find the minimum seat and open pressure?

Like I said I know it is not this simple, but a lot of us probably over spring in the name of safety. Reducing the spring pressure as long as the valve train stays on track could be beneficial. Some sort of starting point would be very helpful.

Following the cam companies recommendations is about the best we have without the ability to test with the spintron. Problem is a person can pick out a cam with nearly identical seat, .050 and .200 duration, lift, and intensity from 5 different companies and the spring requirements will vary as much as 50 lbs on the seat and 200 open.
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Pet peeves and nomenclature.

Postby OldSStroker » Tue Jan 08, 2008 9:39 pm

Sorry guys, but I just need to vent. No offense to anyone here on the board, but "once an engineer always an engineer".

"Load" and "pressure" are not interchangeable terms. Valve springs exert loads on the retainer (and of course on the spring seat). Loads are measured in units of force.

Pressure is load divided by area as in pounds per square inch (PSI). This is useful in calculating stress in the spring wire, but doesn't really apply to the loads exerted by the spring on the valve.

OK, I've got it off my chest. We all know what we all mean by spring "pressure" and I occasionally slip into the vernacular, so don't take all this the wrong way. Heck even a well known spring manufacturer uses PSI in their name. :)

FWIW, I don't think how close you set the spring to coil bind at valve open has a tremendous effect on valve bounce .600 or .800+ inches later.

Now that I've probably pissed off most of you, I think this is a great topic. I also think I'll have another single barrel bourbon!


Jon
Last edited by OldSStroker on Tue Jan 08, 2008 9:49 pm, edited 2 times in total.
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Postby SStrokerAce » Tue Jan 08, 2008 9:42 pm

Old School wrote:I know this is far to broad a subject to be this simple but is there some sort of guideline or formula that the rpm, weight of valve and retainer, rocker ratio, cam intensity, and maybe lift could be plugged in to find the minimum seat and open pressure?

Like I said I know it is not this simple, but a lot of us probably over spring in the name of safety. Reducing the spring pressure as long as the valve train stays on track could be beneficial. Some sort of starting point would be very helpful.

Following the cam companies recommendations is about the best we have without the ability to test with the spintron. Problem is a person can pick out a cam with nearly identical seat, .050 and .200 duration, lift, and intensity from 5 different companies and the spring requirements will vary as much as 50 lbs on the seat and 200 open.


No I don't have a easy formula for you. Most of it is either very complex math or experience or BOTH if you don't have a Spintron.
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