Could this be the REAL reason for vave float?

[TheInspiredFool]

Hey everyone. I have been sitting on this for a bit but figured it was time to make a thread about it to shake things up. As some of you know I am altering a stock Predator 212 Hemi on my other thread. My goals are to keep the internals stock and RPMs limited to valve float but try to improve HP and torque. I have had a nagging feeling as soon as I took off the head and saw how the exhaust valve was placed on seat. The exhaust valve was seated far up on the seat so much so that there was actually a lip of the back side of the valve overhanging the seat face about .040 of distance between the back side of the valve and the face of the seat. This was before I decided to alter the engine. So I put some blue machiner's liquid to see how much of a contact patch I had and where it was on the 45. The intake had an almost acceptable seat contact width but was too high off from the seat but the exhaust had just a tiny line of contact patch and was way high on the seat.


As soon as I saw the exhaust valve and how poorly it was seated I wondered if that was the REAL reason that these engines float the exhaust valve. If the pressure bbuilds up at high RPM it will get under the lip of the valve and force it open. And then at high rpm with such a small contact patch along with the lip I highly doubt it could stay closed at high pressures and high RPM. The more I think about it the more I am convinced that that is the main reason for valve float and not spring tension. I think the engineers built it into the engine for that specific reason for when we disable the governor.


I have heard of guys trying to get cheater springs to get more RPM but when you put them on a dyno they usually make less HP and only gain 200 RPM or so, while some even LOOSE RPM. I personally think that the stock 10.8# springs are plenty for over 6k RPM and that putting in heavy springs just cause loss of hp due to added mechanical work needed to be overcome by the engine. Stronger springs definitely have their place and are needed, but only on more radical applications. Since the valve is out of the seat by nearly an 1/8" it is preloading the valve spring and even with the valve preloading the spring it still floats the valve. And when you bump up to heavier springs it might only give you 200 RPM and a loss of power.



I am bringing this up for discussion because I am a bit concerned that my engine will end up revving much higher than I want. I really don't know how much above 6k rpm the stock flywheels are good for on average. I have heard of some guys turning then 7.5k without problems but I am not that brave, especially since my family will be riding it or nearby.


What are your thoughts on this? I think it could very well be why some can turn 5,800 while others struggle to turn 5k. It could be that one exhaust valve is a tab bit better seated than the other. I am pretty convinced that this is the primary factor in the valve float but want your opinions on this. Has anyone tried cutting the valve seat or lapping them in enough to make a difference to gain more rpm? Has anyone done it and it didn't make a difference?

 

[bob58o]

Im a bit confused. Are you saying the valve will seat better if the 45 on the valve extends to the edge of the valve and is sunken deeper into valve seat?

And what do you want? A better seal or a worse seal? Improved flow, or decreased flow? It seems like are identifying a problem that helps you achieve your goal. If you are correct, and you get a valve job done to improve the seating, isn't that going to increase rpms?

If you want to increase torque, get smaller tires.
If you want to modify an engine, modify it. Don't play games trying to stay safe. If you want safe, it was safest the way it came. Otherwise get a rod and flywheel.

Everybody trying to have their cake. Just eat it.

 

[TheInspiredFool]

Im a bit confused. Are you saying the valve will seat better if the 45 on the valve extends to the edge of the valve and is sunken deeper into valve seat?

And what do you want? A better seal or a worse seal? Improved flow, or decreased flow? It seems like are identifying a problem that helps you achieve your goal. If you are correct, and you get a valve job done to improve the seating, isn't that going to increase rpms?

If you want to increase torque, get smaller tires.
If you want to modify an engine, modify it. Don't play games trying to stay safe. If you want safe, it was safest the way it came. Otherwise get a rod and flywheel.

Yes. I am saying that if I cut the seals and get them to seal better that I am pretty sure that it will also allow more RPMs before valve float happens due to the springs not being able to close the valve fast enough against combustion chamber pressure and cam speed. It would be great as long as it doesn't get to the point where I have to spend about $200 in parts with shipping just to be sure that things didn't break or flywheel coming apart. I would just spend $25 to build an electrically controlled governor at that point rather than spending money on internals just to prevent things from happen at RPMs that the cart will rarely see. If we take it to another place that has more room I don't want to worry about it. I could only find one instance online that someone lapped in the valves on a Predator 212 Hemi and he said that he gained rpm. It turned about 5,800 rpm completely stock minus the governor. He just lapped the valves a little. Since I'm going to cut the seats they should set in the seat more and I am just concerned that it might make a huge difference and the rpm will increase to the limits of the springs while properly seated.



I just kept thinking about it and became more convinced that it could be the real reason behind valve float. I was counting on valve float to kick in on the rare instances that the rpm got above 5k. If this is the actual reason behind valve float then I will have to take that into account now before finding out after it's too late. So I figured that I would make a thread about it to get other people's opinion on how my thoughts on it weighs on others and to see if anyone else has seen actual results if they had lapped or cut the valves enough to seat them better. Just thinking ahead, sharing my thoughts and getting opinions and advice. Also just putting these thoughts out there for others because it could be a great little cheap way to gain a little more rpm for those who want it and a trick for box-stock class racers.

 

[landuse]

Valve float means the valve doesn't fully close at the proper time, because the return mechanism (usually a coil spring) isn't strong enough to close it. This usually happens at high RPM. Stronger springs are able to keep the valves functioning properly.

Thats how i understand it

Clone engines will have this poor quality work. Honda engines also have valve float at around the same RPM's, and i am pretty sure their valves seat a heck of a lot better than any clone

Just my 2c

 

[itsid]

that!


well there might be another very minor addition to that..
on high compression engines, a resonating frequency of the gasflow at the exhaust can be hit depending on the geometry and rpms..
then the valve might be pushed back ever so slightly by the resulting pulses..
(hence a non restrictive exhaust makes most sense )
and that might indeed help valve float to happen..
but ONLY
when the valve spring either gets hit by the lifter at it's resonating frequency,
OR (even more likely) the spring is too weak to exert enough force to return the valve quick enough.
since frankly that spring should be way tougher than a few cc of moving hot gasses

A suboptimal seated valve doesn't help building up compression and you get a
less than ideal combustion, but I too doubt that could introduce valve float at all.

So no.. as Paul said.. valve float is almost always the result of a weak spring
(too weak to close the valve in time for the next stroke)
resonating springs are badly treated usually (annealed by accident?)
and thus too weak anyways, other than that.. no..
No I really don' think a subpar valve seat would cause a floating valve at all.

'sid

 

[TheInspiredFool]

Valve float means the valve doesn't fully close at the proper time, because the return mechanism (usually a coil spring) isn't strong enough to close it. This usually happens at high RPM. Stronger springs are able to keep the valves functioning properly.

Thats how i understand it

Clone engines will have this poor quality work. Honda engines also have valve float at around the same RPM's, and i am pretty sure their valves seat a heck of a lot better than any clone

Just my 2c

True. But there can be a few things that affect the valve closing process that could introduce valve float prematurely. In all cases a stronger valve spring will reduce valve float. However there are other things that can help to delay or reduce valve float. For instance if a valve only seats at the top of a valve seat and there is valve overhang pressure can get behind the valve to start the valve opening early, reduce the valve seat pressure and if there is enough pressure behind the valve it can prevent the valve from closing fast enough at high rpm. Also, if a valve contact area is high on the seat it could cause valve bounce where it bounces off from the seat. If you combine those 2, at high rpm at max cylinder pressure the valve could bounce a tiny bit and the pressure behind the valve could cause enough of a cushion that the valve won't completely close. In addition to those you have to realize where the valve starts realitive to where it moves to within the seat pocket. Just the aerodynamics of how the combustion pressures move past the valve/seat curtain area in comparison to valve seating can also make a difference on how the valve behaves when moving back towards the seat.


The exhaust valve on my engine was extended into the combustion chamber enough to leave an overhang of the valve over the seat by about 1/16th of an inch and the amount of the valve that mated with the valve seat was only about the width of an arm hair. It wouldn't take much at all to unseat the valve from the valve seat with those issues. At high rpm with high combustion pressures it's hard to imagine that it wouldn't become unseated. Most of the pics online that I see of Hondas and clones look like the exhaust is set too high on the seat. Maybe people don't notice or don't think it's a problem. I think this is one area where it goes unnoticed and those who have figured it out keep it to themselves.



It would be very easy to check to see if it makes a difference or not. Test your max rpm at valve float then Just pull the head off and lap the valve until it sits further down in the seat and then see what your rpm is at valve float afterwards. Most people don't lap in valves or cut valve seats so it's probably not something people have noticed a gain with. I will grab a new engine and test it out for myself pretty soon but I figured I would put it out tgere now and get a conversation going on about it and maybe give people something easy to try out if they wanted.

that!


well there might be another very minor addition to that..
on high compression engines, a resonating frequency of the gasflow at the exhaust can be hit depending on the geometry and rpms..
then the valve might be pushed back ever so slightly by the resulting pulses..
(hence a non restrictive exhaust makes most sense )
and that might indeed help valve float to happen..
but ONLY
when the valve spring either gets hit by the lifter at it's resonating frequency,
OR (even more likely) the spring is too weak to exert enough force to return the valve quick enough.
since frankly that spring should be way tougher than a few cc of moving hot gasses

A suboptimal seated valve doesn't help building up compression and you get a
less than ideal combustion, but I too doubt that could introduce valve float at all.

So no.. as Paul said.. valve float is almost always the result of a weak spring
(too weak to close the valve in time for the next stroke)
resonating springs are badly treated usually (annealed by accident?)
and thus too weak anyways, other than that.. no..
No I really don' think a subpar valve seat would cause a floating valve at all.

'sid

Thanks for your input. Most people don't realize just how important resonate frequencies can be within not only the engine but transmission and throughout a vehicle.



I agree, as stated before. A stronger valve spring can always cure valve float, however, there can be other things that contribute to valve float... therefore, in some instances, there are things that you can work on to reduce the amount of time that valve float happens or delay how soon valve float starts to occur.


The pressures occuring under load at high rpm cannot be taken lightly. The rapid expansion of gasses and speed at which the super high pressure inside the combustion chamber is escaping to the much lower pressure of atmospheric pressure at the end of the exhaust is tremendous. To belittle the affects that such high pressures can impose on an improperly seated exhaust valve is impractical considering valve events, cylinder pressures and cycle times. It most definitely can effect an improperly seated valve from closing properly, especially with such a tiny valve to seat contact patch. Ofcorse a stronger spring would extend when valve float starts to occur... But it's the strength of the spring vs the pressures at play, time, etc that determines when or if valve float will occur... It's not only the speed of the cam vs valve spring rate, there are other factors that play into it.


It's a special condition where an exhaust valve sits so high on the seat that when fully seated pressure is allowed to get behind the valve edges. Add to the situation that the valve to seat contact area is very, very thin and I think that there is enough for a hypothesis to be drawn up that these specific conditions lends itself to early onset valve float and that given a proper seat contact area and position, it might delay valve float until further into to the rpm range. I atleast think there is enough to ponder whether it's enough to play a factor in how soon valve float occurs.

 

[bob58o]

I thought that valve "float" or "flutter" was really valve "bounce". Meaning the spring closes the valve shut, but it isn't strong enough to keep the valve shut. The back of the valve hits the seat and bounces off.

I imagine a screen door. Stronger springs can slam the door shut like Eddie Belfour and keep it shut. With a weak spring, the door may close, then bounce back open before closing again and coming to a stop. A stronger spring, if that is whats desired, might be able to slam the door shut and keep it there without a noticeable "bounce back".

Pressure would keep the valve closed. There's more surface area on the face of the valve. (OK, I can see how the force from the pressure might be slightly reduced with the overhanging lip.)
Escaping gas flow might help peel the valve open after a bounce, but the bounce has to be there to allow the gas to escape.

But I guess there has to be a point where the spring just cannot close the valve fast enough to keep up with the camshaft. It hurts me to think about it. Dropped valves, destroyed pistons and cylinders,...

Don't use valve float a safety measure.
And if looking for the 10mm socket...

 

[JTSpeedDemon]

Nope, valve float and valve bounce are different. Valve float just means the valve couldn't close in time, so the cylinder's compression literally goes out the hole. Valve bounce happens as you said, right after it strikes the seat.
I think I remember reading in a David Vizard book that too heavy a valve spring can make valve bounce worse, but I may have that backwards. IDK.

 

[bob58o]

Quickly researching, another cause of float is "loft". It is when the spring / lifter gets thrown from the nose of the cam lobe. The open spring pressure isn't great enough to keep the lobe, lifter, and spring in contact.
https://help.summitracing.com/app/answers/detail/a_id/4765/~/what-is-valve-float?

I guess this is the same as the valve not closing quickly enough.

 

[TheInspiredFool]

Yeah, there are different symptoms and causes. With the hemispherical heads the pressures can have more of an effect on the backside of the hangover. It would help to add to how soon the valve would start to float or keep it open just a bit longer on the bounce. Also, the additional distance that the valve face is from the valve guide along with the almost non-existent contact area of the valve and seat could prevent the valve from seating if the pressures caused just a tiny bit of runout and the valve did not seat where it was conical. I am just suggesting that this specific scenario where the valve back is above the seat and the valve to seat mating surface is extremely tiny might lead to the valves floating sooner or more-so (or both) than it would if the valve was seated properly. As soon as the valve has a proper seat then these specific ideas would not have a much less of an effect by way of altering or enhancing valve float.



These slight effects might increase the initial shock that the pushrod encounters between the cam and the rocker arm. These all in conjunction might produce a situation that could lead to the valves floating early or valve bounce that turns into valve float.


I could very well be wrong but that's the thought that I worked out in my head when I saw the overhang and crappy seat and it got worse once I saw just how little of a contact patch there was between the valve and seat. Just a tiny hairline contact patch plus valve overhang just cant lead to anything but a recipe for improper sealing and all kinds of things. I immediately built a head stand and lapped that sucker in! Lol (Well my wife, my 5yo daughter and I lapped them in lol)