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While no stranger to engine building, I've often wondered what the differences are with valve springs and why you need one over another for different applications. My questions are:

  • Is there a correlation between seat pressure and valve lift? (I'm trying to find out if there is a linear chart which could be displayed.)
  • What are the differences between a spring used with a roller cam and a flat tappet cam?
  • Besides frictional losses, are there any other down sides to running a valve spring with a higher seat pressure than what you need?
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+50

Great question. Engine building is a bit over my head (for now), so this may be an incomplete answer, but I do have some insights that are waaay too long for the comments section.

Spring Design Stuff

The most important factors in designing a valve spring are going to be rate, free standing length (FL), coil bind (solid height), and stress. The amount of wire in the spring determines the rate (less wire = higher rate = higher stress), so wire Ø, coil count, and spring Ø are the dimensional factors that will effect rate.

Rate and FL, when combined with valve stem length, will allow you to calculate seat pressure. To increase seat pressure, you can increase FL or rate, both of which increase stress as the spring approaches solid height. Stress is a critically limiting factor in valve spring design, considering the extreme environment. For the sake of theoretical discussion, however, it does not matter, and will not be mentioned again. I will also ignore float, as that is fairly situational phenomenon.

Maximum valve lift is advised to be some number (say .050") over solid height of the spring so as not to break all of the things. Solid height is simply coil count * wire Ø.

Questions

1) Is there a correlation between seat pressure and valve lift? Yes and no, depending on your limitations. Given the above information, and keeping all other dimensional factors constant (FL, wireØ, coil count, springØ), there are only two individual dimensional changes to the spring that would effect both seat pressure AND valve lift:

  • Increasing wire Ø would increase spring rate and therefore seat pressure, but also increase solid height, limiting valve lift. Wire Ø is calculated to the fourth power in the spring rate equation, so seat pressure would increase exponentially as the valve lift decreased.
  • Decreasing the coil count would increase the spring rate, and again seat pressure, but would decrease the solid height, allowing more valve lift. Active coil has a linear relationship to rate, so seat pressure increases linearly with valve lift.

Note: All other individual changes to dimensional variables show no correlation. If you want to know the effect of changing multiple variables, it gets much more complicated, unless you have specific numbers to work with.

EDIT BASED ON COMMENTS I wrote this answer from a spring design point of view, as if you were shopping for springs for your engine build. As per Paulster2's comment, a common way to increase seat pressure for the springs you already have is to install a shim between the cam lobe and spring body. Doing so will increase your seat pressure by some number of lbs (spring rate * shim height), and will also DECREASE your valve lift, as your new installed spring height is lower, bringing the spring closer to its solid height. So, to re-answer the question, if you are using a shim to increase seat pressure, valve lift with decrease linearly as seat pressure increases. See example below.

Theoretical example: Say you have a 1.000" tall (unloaded FL) spring with a 100 lb/in rate, and your spring coil binds at .300". Right now, you install your spring at .800", which travels the spring .200 inches at 100 lbs/inch, for a seat pressure of 20 lbs. That means your remaining travel from installed height to solid height is .500", so in theory, your maximum valve lift is .499".

Now say you wanted to increase your seat pressure to 30 lbs. To do so, you install a .100" shim somewhere between the end of the spring and the cam lobe. This lowers the installed length of the spring to .700", giving you the 30 lb seat pressure you were looking for, but it also reduces the amount of travel between your installed length and solid height to .400", leaving you with a theoretical maximum valve lift of .399".


2) What are the differences between roller and tappet cam springs? I have no idea. I don't see why there would be a difference. I was thinking a roller would allow a higher seat pressure, but ideally you have the lowest seat pressure possible so as to more hastily allow air past the valve, so I don't see why you'd need different springs. Again, I could be wrong, please let me know if you have more specific questions.


3) What are the downsides of using a higher seat pressure than required? As you mentioned, frictional losses, which increase heat in an already toasty environment. As you know, as heat increases (beyond a certain material specific point), the torsional rigidity of the wire will decrease, possibly causing yield, definitely causing a drop in seat pressure/rate. From a spring standpoint, higher than necessary seat pressure has no benefit, and not insignificant detriments.

We can also get into the discussion of valvetrain weight (and spring material), which, like suspension systems, will effect how quickly the spring can react to changes. Regardless of whether the valve is open or closed, the center coils in a valve springs are constantly moving when the engine is running due to the weight of the material in the spring.


Hopefully this answered some of your question(s). Please let me know if I can clarify, add to, answer additional, or delete this post!

  • 1
    Next level answer. +1 – DucatiKiller Mar 3 '16 at 21:57
  • A couple of questions: In the para which starts, "Rate and RL" you state, "To increase seat pressure, you can increase FL or rate ..." Do you actually mean to decrease FL to increase seat pressure? A normal way to increase seat pressure is to lower spring height by adding shims. Just wanting to make sure I understand you. – Pᴀᴜʟsᴛᴇʀ2 Mar 3 '16 at 22:09
  • @ᴘᴀᴜʟsᴛᴇʀ2 by increasing Free Length (F L), you are increasing the displacement required to to travel from FL to load height, by adding shims you are doing the same. Say the FL is 1.0", and it's installed at .7", so .3" displacement. Adding a .1" shim to make the install length .6" is the same as changing FL to 1.1" and installing to the original .7", both require .4" displacement. The main difference is adding shims brings you closer to solid height, but then again FL is easy to change from a desk in the design stage, very hard to change once the spring is made. – MooseLucifer Mar 3 '16 at 22:21
  • I think I misinterpreted what FL is, then. On engine valve springs you don't usually worry about FL, but installed height. I thought that's what you were talking about. – Pᴀᴜʟsᴛᴇʀ2 Mar 3 '16 at 22:24
  • 1
    Coil bind is always a consideration. I don't remember what the recommended difference between coil bind and full open height of the valve is, but it is significant enough to allow for shimming the spring without issue. – Pᴀᴜʟsᴛᴇʀ2 Mar 4 '16 at 15:58
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Mainly what it will boil down to is homework. The application you are going after. Is it horsepower you want or torque? Is it an overhead cam or rolling rocker? Do you want them to last or last long enough? Are you getting quality manufactured springs or cheapest, and are you trying to match them all up? Is your cam a higher duration?

1) Too weak and the spring won't keep the valve closed. Engine manu specs will usually only list what the weakest it should be. Other than that someone else will have to enlighten me as well.

2) Spring compression. Rockers are usually used for longer stroke motors and lower RPM, they tend to need to compress a longer distance. They also have different designs available like the beehive style. Flat tappets will usually have 1 spring inside the other for added strength while avoiding coil bind with too much material. They can also be lighter.

3) If you are running a multi chain valvetrain(V style dohc), it is possible to advance or retard the timing as there is the possibility of slack(physical timing as opposed to ignition timing). Valve train tension could alter how the side of the v gets and releases its air. One other consideration: valve face, valve seat, and valve head quality. Are the materials of a high enough quality t that they can withstand the additional pressure? Enough of a face, or the seat cut right?

If I were building a Harley engine and went with a very aggressive cam. I would start looking at what recommended springs would be needed. Coil bind being the most important if the cam has a very high lift. Then finding the best spring that I can(best bang for the buck, weight, design and limitations of). After I order about 30 springs for intake and exhaust each, finding the ones that match. Meaning all intake springs are the same strength and coil bind, and exhaust are the same.

  • Manufacturers spec springs for a particular RPM redline. As the redline moves up with engine upgrades the strength of the spring also needs to move up to keep valves from floating. – vini_i Feb 16 '16 at 0:37
  • Goes back to homework. If you are having a performance shop do your build, ask how they plan to spec it. If you hear anything like "This is the way we have always done it..." . Time to start looking to see if there is someone else available. They should have access to all relevant specs and not just the marketing specs(what we see as consumers). Items like: material, free length, coil thickness, coil shape(wire, flat wound, twisted, ribbon)(beehive, single, double, linear, progressive). – Tobin S Feb 16 '16 at 15:21
  • I appreciate the answer, Tobin, but I'm looking for technical answers, not general answers. I already know the general reasons for the questions. – Pᴀᴜʟsᴛᴇʀ2 Feb 22 '16 at 0:45
  • I think you would be hard pressed to find a beehive spring for a flat tappet. It would require more torque to move the valves without much return. – Tobin S Feb 22 '16 at 0:55
  • Way way too much info to boil down to something a little more palatable. hotrod.com/how-to/engine/… – Tobin S Feb 22 '16 at 1:23

protected by Pᴀᴜʟsᴛᴇʀ2 Mar 5 '16 at 22:46

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