Maybe the term valve float comes from the fact the valves are opening and closing so quickly they're thought to be floating open?

I always thought the violent open/close action on the valve itself is what did the damage. When it failed it felt into the motor. It would be pretty amazing to see the failure in slow motion cam.

 

At 4500 (cam) rpm the cams makes 75 revs per second.
In a 4-stroke engine, everything happens during a single stroke of the piston, this means half a crank turn which means a quarter cam turn.
Okay, in reality, valves may start to open a little before TDC and close slightly after BDC.
This is possible becase there is room between piston at TDC and valve.
At BDC there is, obviously, a lot of room......
In case of intake valves they do stay open after BDC to allow the flowing air/fuel mixture to flow in a little longer by itself, inertia of the air/fuel mixture mass at speed gives a little extra cylinder filling.
The same for the exhaust valve, it opens slightly before BDC to use the leftover pressure after combustion to start-up the flow.
Close to the end of a piston stroke the labour by expanding gasses is almost 0 anyway, why not use it to help the exhaust cycle.
Back to the cam.
So the complete exhaust cycle - opening and closing - takes a 1/4 cam turn in theory but a bit more in reality.
Look at a cam and see the lobe takes slightly more than a 1/4 of the circumference.
So in my first post I was wrong about 1/4 turn per opening and 1/4 per closing, it is certainly less than 1/2 turn in total for the exhaust cycle and I will correct it.
Simply because 1/2 a cam turn is 1 full crank turn.

For argument's sake I'll stick with 2/7.
This means opening the valve takes 1/7 cam turn and closing takes 1/7 cam turn - IF this is devided equal but for now I will use this.
This also means 5/7 of the cam turn nothing happens, that cam section is a cylinder, no lobe, IOW the valve has time to close.
And it does at 4500 rpm, there is no doubt about that.
75 times per second means 1 cam revolution takes 0.01333 second.
1/7th of this is 0.0019 sec, this is pretty quick.
How much quicker can a valve, retainer and spring move up?
Lets give it 5% safety margin.
So the quickest a valve assembly can close is 0.00181 sec, this is also 1/7th of a cam turn.
This results in a cam rpm of 4736, up to that rpm the valve stays in contact with the cam lobe and lands guided by it.
Also, this means 5 times that period, 0.009 second, the valve IS closed before the cam comes around again.
Obviously, another safety margin will give other rpms.

Anyway..
In this example what happens between 4736 and 5350?
The valve has reached its shortest closing time while the cam still speeds up.
It does NOT speed up enough though to cover 5/7 of a turn without lobe to come around before the valve has closed.
A cam making a full turn in the shortest time - including my guessed 5% safety - it takes a valve to close means the cam turns at .....
Well....?
So 5/7th of a turn has to happen in 0.00181 seconds.
1/7 of a turn then takes 0.00181 / 5 = 0.000362 seconds
A full turn takes 0.000362 * 7 = 0.002534 seconds.
In 1 second it then turns 394 times.
That is 23678 rpm.
For the cam.

I don't think so.

 

No surprices here......

Btw.. thanks for all you wonderfull advice on this forum.
It really is very kind of you - and it shows your ample knowledge - to advice people to check for damaged retainers by checking valve clearence.
Keep up the good work
And thanks for the contribution to this discussion, again, very usefull to share your knowledge.
You're a true asset to the forum.

 

?????

 

No no no.. not talking about you Billman
In a way I think that you know that I know that you know way better than that

Here's the post:
https://www.s2ki.com/...ap/page__st__25
Entry #34.

Oh well...


Btw..
Have anyone of you ever driven on corrugated roads?
I'm sure you have.
What really helps to stop - actually almost eliminate - the rattle and shaking is to speed up.
You surf the peaks of the mounds and the ride becomes smooth.
It takes a certain speed to ge there though.
To illustrate the difference in cam rpm to make valves float: suppose you have to drive on a corrugated road where the mounds are not 1 feet apart but 5 feet.
How fast would you have to drive to still surf the peaks?

 

I used to ride my dirtbike to work on the train tracks TO AVOID 1.5 HRS OF TRAFFIC. I got to work in 16 minutes VS 1.5 hrs So I have surely driven on corrugated surfaces and yes the faster the better anything under 45 mph would vibrate the hell out of me. Above that it was like being on the highway.

I know you were not taking about me....it just surprised me because he seems to have a grip on the mechanical stuff.

But make no mistake broken retainers will have 0 effect on valve clearance

 

Wow. Interesting! For the uninitiated, your argument is very compelling. FWIW - I don't have an opinion about who is "right." Like most, I'm mostly concerned with what rectifies the problem.

Based on this logic, though, how can you account for "real" valve float and P/V contact in a high over-rev situation? Following your explanation above it's only occurs at an essentially impossible RPM.

 

Someone else will have to comment, but wouldn't the boost pressure be added the the pressure on the valve seat? 10 PSI pushing up on the valve + the 64 lbs of spring pressure? I also don't think you could just add/subtract PSI from the spring pressure on the seat, since they are different units.

 

Hey, don't be an Asshat. I don't appreciate the sarcasm..... I never told people to check for cracked retainers by checking valve clearance. Never once did I even suggest that. So thanks for reaffirming that I am such an asset. I have provided countless hours of help to many people on here....

Bill,

I have seen a car where someone had done an adjustment on a car with cracked retainers (valve stems were sunken into the retainer). So rather than the feeler gauge contacting the tappet and the valve stem, it was contacting the tappet and the retainer. I definitely know what I'm talking about, and that's the instance I was referring to. Regardless, I know we agree an adjustment is just a good idea when doing retainers just for the sake of everything being ready to do so.

As for Mr. Spitfire's comment, I don't see why that was called for.

 

Im curious. So in that video linked above it shows the valve stem and spring wobbling quite a bit.

So there have been quite a few people with bent valves, is the bent valve due to actualy piston to valve contact?

Or is it due to excessive wobbling and heat just distorts the valve causing it to bend?

Would love to see a video of an F2x head spinning up and watching what happens above 9k.