Need technical explanation of Valve Float

[JohnnyWash1]

Ok, I already know that valve float occurs when the valve spring can no longer keep up with the rpms. What I do not know, however, is what will happen then. So...I would like a detailed discussion on valve float and it's dangers as well as remedies. Would this result in valve/piston collision? Can this be fixed with stiffer springs? Also, does this have to do with valve lash?


Jon

[KC Ron Carter]

the issue of valve float is usually caused by over reving an engine.

The manufacture knows and sets a max RPM.

Too strong a spring will cause the tulip of the valve to pop off.
Worse than valve float.

Most manufactures use a second valve spring to cancel resonate frequency in the spring.

Later,

[ivymike1031]

Originally Posted by JohnnyWash1

Ok, I already know that valve float occurs when the valve spring can no longer keep up with the rpms. What I do not know, however, is what will happen then. So...I would like a detailed discussion on valve float and it's dangers as well as remedies. Would this result in valve/piston collision? Can this be fixed with stiffer springs? Also, does this have to do with valve lash?
Jon

Quickie Vocab:
"Valve float" is the layman's term for loss of separation control within the valvetrain, usually over the nose of the cam. In other words, when separation occurs within the valvetrain while the valve is open, it's called valve float.
"Valve lash" is the clearance between valvetrain components, measured at the valve side of the rocker arm, when the system is at rest with the follower on the cam base circle and the valve closed.
A "Valve line" is the chain of components between a single cam lobe and its valve (inclusive).

There are a number of possible outcomes:
* If the separation is occuring on an exhaust valve line, then the piston will be on its way up while the valve is open. Usually the piston chases right behind the valve as the valve is closing, so if the valve stays open for too long, there could be an impact. Such an impact is not necessarily catastrophic; it's not altogether uncommon to tear down a hi-perf engine and find clear evidence of piston-valve contact. It would be preferred if this contact did not occur, and if the contact is too severe, it could definitely result in engine damage. In general, you need quite a bit of "float" before you'll have such impacts.
* When valves begin to "float," valve seating velocities increase substantially. This isn't good for the seat or the valve, and can cause damage over time.
* When valves begin to "float," it is often the case that this will be accompanied by an undesirable increase in the duration of the lift event, resulting in degraded engine performance. Sometimes a little bit of "float" is designed into a hi-perf valvetrain intentionally, and performance is actually improved (this is the exception).

Some Common Causes:
* As was mentioned previously, excessive engine speed leads to excessive valvetrain acceleration, and the valve springs may not be able to maintain contact between all the components. If you know the peak acceleration rate of your cam, and you know the force vs displacement profile for your valve spring, and you know the mass & inertia of your valvetrain components, you can calculate a kinematic separation cover factor that will tell you how fast you can run the engine without the mass x acceleration overcoming the spring force. This answer will always be wrong in real life, because...
* All of the valvetrain components, such as the pushrods, rockers, springs, etc., are flexible, and they will vibrate. Valvetrain vibration compounds the above, and significantly increases the required spring force to maintain contact between the valvetrain components.
* The valve springs themselves may resonate (vibrate quite a bit). This is called spring surge. Picture what happens if you hold a slinky outstreched, pinch some coils together, then release them. The compressional wave travels up and down the slinky. Spring surge is very similar. It increases stresses within the spring, and it reduces (periodically) the amount of force that the spring applies to the seat and retainer.
* Excessive valve lash compounds the above problems. Too little valve lash is worse, but for a different reason.

Some Remedies:
Valvetrain vibration:
* Lower mass valvetrain components
* Increased specific stiffness of valvetrain components
* Increased spring stiffness (add a helper spring if needed)
* Increased spring preload (add a helper spring if needed)
* Use a different cam profile (to excite less vibration)
* Reduce valve lash (to reduce impacts & excitation of vibes)
* Increase damping within the system
Spring surge:
* Replace the spring with a spring that has a higher surge frequency
* Use a different cam profile (to excite less vibration)
* Use concentric valve springs that rub against each other. The rubbing will remove energy and attenuate the spring surge. (this isn't the same thing as "cancelling the resonant frequency," because it reduces the vibratory magnitude at all frequencies vs the same spring pack without rubbing)
* Reduce the solid clearance of the valve spring, to increase coil clashing. Coil clashing removes energy and reduces the magnitude of the spring vibrations
* Add damping coils to the valve spring. Damping coils are coils that change the spring rate suddenly during a few mm of lift near the fitted length. Sometimes the damping coils are completely closed at the spring fitted length. The sudden change in spring rate breaks up the spring resonance as surge sets in.

[JohnnyWash1]

Awesome write-up. Thank you very much. However, I am a little sketchy on lash. Is it the distance between the rocker arm and the valve?

Jon

[ivymike1031]

Originally Posted by JohnnyWash1

Awesome write-up. Thank you very much. However, I am a little sketchy on lash. Is it the distance between the rocker arm and the valve?

If the rocker arm pushes directly on the valve, then yes. Valve lash is clearance within the valvetrain. You can think of it in terms of how you would measure it - it's the thickness of the thickest feeler gage that can fit (somewhat) smoothly between the valve and whatever pushes on it, when the piston for that cylinder is at TDC firing (valve closed, follower on base circle). Usually the specified lash is "cold lash," meaning you should measure it when the engine temp is at about room temp. Usually lash gets smaller as the engine warms up, although this is not always the case. With an overhead cam and an aluminum head, for example, the lash may grow as the engine warms up, because the head expands more than the valve does, lifting the valvetrain hardware away from the valve.

[jhillyer]

QUOTE:
Quickie Vocab:
"Valve float" is the layman's term for loss of separation control within the valvetrain, usually over the nose of the cam. In other words, when separation occurs within the valvetrain while the valve is open, it's called valve float.
"Valve lash" is the clearance between valvetrain components, measured at the valve side of the rocker arm, when the system is at rest with the follower on the cam base circle and the valve closed.
A "Valve line" is the chain of components between a single cam lobe and its valve (inclusive).
//


If "Valve float" is a layman's term, what is the engineer's term?