Will someone describe/defeine valve float for me??

The way I understand it, Valve float happens when cylinder pressure and air velocity inside of the cylinder is high enough and fast enough to keep the valves from closing when they're supposed to. The pressure inside of the cylinder has the greatest impact of the face of the valve and would normally force that valve to close or seat (combined with spring pressure giving valve closing the mechanical advantage). At higher RPM the velocity and pressure affecting the back side of the valve overcomes the mechanical adavatage enough to keep that valve from closing on queue. Now that pressure and energy that is supposed to be pushing the piston "down" is going out the exhaust tube.

Personally, I think valve float at 5100 RPM is a bit hard to swallow but there's allot more folks out there that know allot more about cams and springs and heads etc. than I do who might be able to shed some light on the possibilty.

What I think he means is that normally with the valves there are intake and exhaust. when one is open, the other should be closed and vice versa. By float that means that one is opening before the other is closed all the way or not closing and the other one starts to open. Overlapping in other words. This obviously will hinder performance as the gasses mixing and the compression suffers as the overlap does not let thge cylinders build the same pressure as there is a time when both intake and exhaust valves are open at the same time. I hope this helps and I am correct with my description. If not someone fell free to correct me.

i have a slightly different take on it. valve float is when the sheer speed of the cam movements are too much for the valve spring to overcome. said another way: the spring take a certain amount of time to close the valve. now rev the snot out of a motor (remember how many times that valve is being open and shut in one minute) - the spring is still trying to close the valve when the cam is already opening it again. the result is a valve that never closes all the way.

each valvetrain component needs to be in 'tune' with the other parts. stiffer (and progressive, a la some beehive) springs can help with this.

old tech to help with this is desmo (desmodromic) valve train. Ducatti has used them forever, as have Mercedes and many others. there is a separate cam lobe to slam the valve shut (no spring). this complete mechanical actuation wards off valve float.

there are newer methods too (pneumatics, servos, etc) which are out there too.

HISSIN50 said:

there are newer methods too (pneumatics, servos, etc) which are out there too.

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Please elaborate on the servos, last I heard there wasnt one small enough that was fast enough... the possiblilities of no camshaft makes me drool

85GTlover said:

What I think he means is that normally with the valves there are intake and exhaust. when one is open, the other should be closed and vice versa. By float that means that one is opening before the other is closed all the way or not closing and the other one starts to open. Overlapping in other words. This obviously will hinder performance as the gasses mixing and the compression suffers as the overlap does not let thge cylinders build the same pressure as there is a time when both intake and exhaust valves are open at the same time. I hope this helps and I am correct with my description. If not someone fell free to correct me.

Click to expand...

What you are describing here is valve overlap. Which is somewhat desirable for a high performance motor. One aspect being the intake valve actually starts to open before the exhaust valve is closed, the theory(my simplified version) being the velocity of the air leaving the chamber via the exhaust valve will create a vaccuum for the air coming in the intake valve. This way you effectively start the intake stroke before the piston rounds TDC which should = better volumetric efficiency. But it doesnt really work until you hit higher rpms... at idle you exhaust nice intake air and raw fuel as well

Camman said:

the possiblilities of no camshaft makes me drool

Click to expand...

BMW has some production cars without cams. Don't ask me which models because I have no clue.

posted by "amossm" at the Corral

"Valve float" is really valve train separation. All of the various components of the valve train (lifters, pushrods, rockers, valves, etc) are designed to move as a single entity, following the cams profile with all the components "held together" by constant spring pressure.

To produce maximum power, it would be desirable to bang the valves wide open almost instantaneously at the right moment, and slam them shut equally quickly when required. In the real world that won't work, because the valve train components cannot withstand those physical stresses. To cushion these violent forces, there are "lash ramps" (mechanical cams) or "acceleration/deceleration ramps" (hydraulic cams) that begin the acceleration of the valve off the seat more gently, and also set it back down on the seat more gently. Once accelerating, the cam ramp speed increases in a progressive fashion to achieve a lot of movement in a little distance (lift under the curve).

As the cam follower approaches the nose of the cam, it must decelerate, stop, and start down the other side of the cam lobe. The valve spring pressure works against the inertia created by the speeding mass of all the valve train components. As long as valve spring pressure is greater than component inertia, the valve train components track the profile of the cam lobe, stopping, starting, accelerating and decelerating in proper fashion.

The problem comes when the speed of the valve train components becomes great enough that the inertia created is stronger than the valve spring pressure. Then, as the cam follower approaches the nose of the cam, the spring is unable to stop the motion in the opening direction, and the cam follower is "launched" right off the nose of the cam. Kinda' like flinging mud off of a stick.

With no cam pressure now pushing on the cam follower, the spring finally "catches up" and overcomes the opening inertia, and starts to close the valve train again. The problem is, the cam lobe has continued to turn and now the lifter will be accelerated violently closed by the valve spring until it slams back into the cam lobe at some point, causing severe shocks throughout the valve train, excessive wear, and often breakage. The valve usually bounces off the side of the lobe once or more, keeping it out of proper contact with the lobe. If the cam follower is "out of control" or bouncing in the air as the valve approaches the seat, the cam cannot set the valve gently back on the seat with the deceleration ramp. Instead, the valve slams hard into the seat, and bounces again.

So, you get valve train separation typically at two points...over the nose of the cam, and setting the valve back onto the seat. The valve spring SEAT pressure must be adequate to maintain integrity of valve train motion as it approaches the closed position (to prevent valve bounce off the seat). The valve spring OPEN pressure must be adequate to control the valve train deceleration over the nose of the cam. That's why it's important to use a spring with proper pressure specs at both points.

Valve train separation and the resulting "bouncing" introduces unnatural, high frequency undulations or harmonics into the valve springs, and will eventually cause breakage of springs, throwing of valve spring retainer keepers, or just plain allow a valve to crash into a piston....any of which spoils your day.

Spring pressure required to avoid valve train separation is dependent on several factors, including peak rpm required, weight of all the valve train components (and therefore inertia), acceleration and deceleration rates of the cam ramps, and even proper rocker arm geometry to avoid side loading and frictional resistance from the valve guides. Bottom line, listen to your cam manufacturer on spring recommendations. They get paid to do all those calculations, and they can do it better than we can by the seat of our pants.

We all know that unnecessarily heavy valve springs rob horsepower and increase wear; however, too weak springs almost always eventually result in breakage. Saved horsepower doesn't help when it won't run anymore. Don't let 'em "float!"

timmy/amossm have it correctly described.

There aren't any BMW's without cams. There are several that don't have throttle plates. They have the ability to infinitely vary the valve lift - so they control engine speed with infinitely variable valve lift.

We are ultimately headed to some sort of computer control of the valves with solenoid type devices. Don't be surprised if Bose leads the way in this field - they're about to unveil some amazing active suspension technology based on quick acting electromag coils (can you say speakers?). I believe this same technology may be brought to bear on operating the valves. And once it does - we'll have our cake and eat it too. Programmable engine control that allows infinitely variable valve timing/lift/open-closing rates. The ultimate VTEC - we can have a smooth idle, plenty of torque down low, stout midrange, screaming top end, no throttle plates, settings for max emissions or max performance or max fuel mileage - all at the flip of a switch.

The reason exhaust valve train bits are usually the first to bend during float - when the cam spins past the spring's ability to control the intake (peak intake opening) the piston is well down the cylinder - no harm no foul. When the cam spins past the spring's ability to control the exhaust valve - the valve hangs open a fraction longer while the piston is approaching top dead center at the end of the exhaust stroke. SMACK - piston hits exhaust valve. Now you know why more p to v clearance is always recommended on the exhaust valve than the intake.

Ok... now my head hurts. lol

"...now my head hurts..."

That's just what the piston said after the float....

This site explains the pnuematic valves pretty well. Still requires the use a cam but the pneumatic arrangement take the place of the valve springs - allows about 18k RPM.
http://www.indiacar.com/index2.asp?pagename=http://www.indiacar.com/nfs/technical/pvengine.htm

As far as the original question, I'm thinking you might be running out of fuel or spark or it could be valve spring coil bind (?) .