Define valve float......

[JOES66FURY]

Can someone define valve float, what causes it and how it affects engine performance.....


Thanks!


Joe

[miketblsr]

Re: Define valve float......

My tach and shift light. lmao ;D

[Ron Ward]

Re: Define valve float......

Ok, here it is in a nutshell:

All valvetrain components have mass. This includes valves, springs, pushrods, retainers, lifters, etc. The job of the valvespring is to keep the lifter firmly seated on the lobe of the cam and control all of the mass of the valve train. As the camshaft rotates, the ramp of the cam lobe lifts this mass. The inertia of the mass works against the spring as it tries to keep the lifter on the lobe. If the inertia of the mass exceeds the force of the valvespring (usually caused by excessive mass or speed) the lifter will actually be "launched" off the lobe of the cam. At this point, the spring has little or no control over the valve train and "valve float" will occur.

To control this "float," stiffer springs or lighter valvetrain components (or both) are utilized.

Make sense?


Ok, as for how it affects engine performance, consider this. In severe cases of valve float, the valve may actually remain open too long and allow cylinder pressure to drop, thus losing horsepower. It can also allow the valve to come into contact with the piston which can cause bent valves, pushrods, broken valve springs or worse yet, catastrophic engine failure.

Ron

[TheSilverBuick]

Re: Define valve float......

Performance drops off rapidly at that point too. My old Firebird had bad valve float at 5,000rpm, it was like a wall, the rpm's would just stop climbing and it was time to upshift. Car was totalled before I got a chance to build another engine.


Can I add to the question? What harm is there in valve float barring valve to piston contact?

[A/Fuel]

Re: Define valve float......

maybe a backfire,

[Ron Ward]

Re: Define valve float......

Can I add to the question? What harm is there in valve float barring valve to piston contact?

Consider this: Any time you have a gap in the valve train, bad stuff can happen. Pushrods can become unseated from the lifter or rocker arm, rocker arms can slip off the tip of the valve, etc. If the pushrod happens to get unseated and sticks off to the side of the lifter, things get ugly in a hurry. The cam lobe will be subjected to severe load, the pushrod gets loaded at a nasty angle and coil bind can occur on the valvespring. Valve float is not a good thing.... usually.

Occasionally, the guys who run in classes where valve lift is limited (read: NHRA/IHRA stock or NMCA Mean Street) will have camshafts ground with special lobes. They call these "launcher" or "lofter" profiles where additional lift is achieved by launching or lofting the lifter off the tip of the lobe. This is done ONLY with special camshaft grinds and I would in no way recommend it without first contacting your camshaft provider for all the necessary information pertaining to ALL of the related valvetrain components.

Ron

[TheSilverBuick]

Re: Define valve float......

Fair enough, Thanks

[DanStokes]

Re: Define valve float......

Valve float is a myth. I've put valves in water, brake fluid, motor oil, naptha, and even STP. None of them floated, although it did take a while for it to hit the bottom of the container with the STP. So I ain't buyin' any of it - these guys are just trying to sound Skyentifical.

Dan, telling the truth

[Dans 83]

Re: Define valve float......

Dan... Have you ever tried to float a set of sodium filled or hollow stems??? In lets say...Jello! ;D

[JOES66FURY]

Re: Define valve float......

Ok then....so let say an engine pulls hard from off idle to about 4800 rpm then seems to fall flat on its face unitll the 1-2 shift...rpms drop off and the engine pulls hard again unill it reaches the same rpm (It seems to hit a wall) till rpms drop off for the 2-3 shift...does this sound like what you would experience with float?


I have a feeling this is what I am experiencing with my car, I am wondering if I should attempt a spring change...This can be done without removing the head from the car right?

[CTX-SLPR]

Re: Define valve float......

You also have problems when the valve is returning to the seat slower than the valve train is going to base circle and instead of landing on the seat in a controled manner it bounces. Not only is this a bad thing for the valve mechanically but it also bleeds off cylinder pressure reducing power. I think that is more where the power loss is coming from than the lofting of the valve off of peak lift.

[TheSilverBuick]

Re: Define valve float......

Also make sure it isn't ignition coil or module break down, that'll cap rpm's too.

[JOES66FURY]

Re: Define valve float......

2 new coils, same issue....Pertronix units I am told either work or dont....so I dont feel thats the issue....Its pulls so strong till about 4.5-4.8K then it just hits a wall...till RPMs drop with the shift

[Freiburger]

Re: Define valve float......

You also have problems when the valve is returning to the seat slower than the valve train is going to base circle and instead of landing on the seat in a controled manner it bounces. Not only is this a bad thing for the valve mechanically but it also bleeds off cylinder pressure reducing power. I think that is more where the power loss is coming from than the lofting of the valve off of peak lift.

x2

[Groucho]

Re: Define valve float......

The valve's inability to follow the profile of the camshaft for various reasons, putting it off the seat at times when it should be on.