Thanks
Jim

 

It's the point on the rpm chart where the valves don't open and close precisely like they do from idle to sane engine speeds. I've usually heard it in reference to the quality of the springs, though I guess it could be cause by any component in the valvetrain not being up to snuff...Sorry if you knew this already, don't mean be so basic....

On an L98, that's about 5500 rpm or so, I learned that firsthand :cry
But my experience isn't as technical as others might be, so surely someone will lay it out cold for you...


[Modified by TrueBlue ChevyDude, 11:32 PM 6/24/2002]

 

It is my understanding that it relates to the strength of a valve spring and the RPM's. A stronger spring returns the valve back to its seat quicker than a weaker spring so therefore one can increase the RPM and still have the valve seat fully before the cam lobe bumps it back out. If a valve cannot return to its seat fully before the cam comes around again, one will experience a burned valve. It is necessary for the valve to seat fully in order to transfer combustion exhaust heat to the head. :crazy:

 

When the valve is pushed down by the cam/pushrod/rocker assembly, it takes time for the valve to return to its closed position. Valve float occurs when the valve is pushed down by the cam before it has had time to return to the closed position. This happens at high RPM's. This is a very bad condition (I assume you knew that). Results of valve float include backfiring through the intake manifold and bent pushrods.

There are several ways to raise the RPM where this occurs. The first fix is a stiffer valve spring. There is a limit to which you can raise the valve spring stiffness. Stiff valve springs can collapse hydraulic lifters. They also increase the friction, resulting in a HP loss. The other way to accomplish this is by reducing the weight of the valve assembly. Titanium spring retainers are popular. Light weight valves are also used. Hollow stem or ceramic valves are possible options. Lighter weight springs are a last resort to get the last little bit out. I assume these would have to be custom made.

The reason the RPM limit of the LT1 is low is not due to weak pushrods. It is due to weak valve springs. I have a friend who used stock LT1 rods in a 7000 RPM engine with no ill effects.

 

Hammer:
"Valve float" refers to loss of contact between the components in the valve train. Think in terms of one cam lobe and one valve and all the stuff in between....
Operating normally, the bottom of the lifter is in contact with the cam ... the top of the lifter with the bottom of the push rod ... the top of the push rod with the rocker arm ... and the other end of the rocker arm with the valve stem.
When you get everthing going too fast the momentum of one or more parts in that train can overcome the ability of the valve spring to keep all of that moving mass in contact. When that happens, you've got valve float. It can destroy cam lobes, lifters, pushrods and break / bend valves. You want to avoid this one like the plague. Hope this encyclical helps! Cire96

 

The thing that really limits a stock LT1 is the bottom end. It tends to come apart at about 6400rpm.

 

I did not know that. I do know that an L98 will live for years with regular twists up to 7000 RPM with a rev kit and double valve springs. ( Provided you have a cam/intake/head combo that will make power that high ) I can't see the LT-1 being any weaker.

I do know that valve float will destroy a block if the valve ever contacts the piston...maybe this is what you are referring too?


[Modified by BBA, 5:31 PM 6/25/2002]

 

Mercedes Benz (among others) had a positive method of dealing with this problem. The 300 SLR had two cam lobes for each valve, one to push it down and another to pull it back up. No valve springs were needed. The rocker arm had a fork in the end which fit in a groove in the valve stem.

Push down, pull up. 10,500 rpm redline. Slicker than an eel in a bowl of snot. BTW, valve float is the reason for overhead camshafts. You don't have the inertia of lifters or pushrods - sometimes even rocker arms. The 300 SLR was an inline six with dual overhead cams and a desmodromic valve train. (Desmodromic = push-pull)

Rich

 

Some of you have it. Let me try to explain it a little differently. E=MV The energy of the valve train and most specifically the valve and retainer increases as the rpms (V=velocity) increases. The mass (M) doesn't change in a given running engine. When the energy of the receprocating valve train exceeds the energy of the spring, the lifter is "light", or floats off of the cam profile. The spring eventially DOES close the valve, but in a manner uncontrolled by the cam profile. The valve hits the seat violently and can actually bounce. All manner of engine damage can result from this unconrolled valve closing. One of the more common problems is a broken valve. It doesn't take too vivid an immagination to picture the damage a wayward valve head can do bouncing around inside a cylinder of an engine turning 7000 rpms.

Most forum members have engines with hydraulic lifters. Either flat tappet or roller tappet. Engines with hydraulic lifters very seldom, if ever, experience "valve float". With hydraulic lifters, what happens, is that as the rpms approach "float" conditions, the plunger spring inside the lifter, combined with oil pressure, over power the reduced force of the valve spring. This allows the plunger in the lifter to move up in the lifter body. Since oil won't compress the valve train is lengthened. Removing this "clearance" or preload, prevents the valve from seating. This is "lifter pump up". Valve float never occures because when the valves don't seat, the rpms can't advance to "float" conditions. Similarly, damage can occure from lifter pump up, but often it is less severe than the damage caused by valve float.

This is the reason you hear the topic debated as to how far the rocker nut should be turned down from "zero lash". The more preload in the lifter the more it has to give up. Run at zero lash, the lifter won't pump up. But at slightly higher rpms, you'll experience Valve float. Not to mention the noise you will have to endure at all running speeds. Solid lifter cams are quieter, will tolerate more spring pressure, and can have better acceleration ramps. Rather than zero lash your hydraulic cam, buy a solid. They're cheaper, at least the lifters are, and have more performance for a given set of valve events.

Valve train 'Energy" can be reduced by a lighter valve train or be tolerated by a stronger controlling spring. And of course, some of both. These steps won't prevent the possibility of float or pump up , but the "E" of the valve train will be reduced as the mass is lowered. A stronger spring has a higher "E" to be overcome. At SOME speed, all valves will fail to be controlled. Hopefully that speed will by beyond the needs or desires of your power curve.

Desmodromics are a totally diferent animal. I am dicussing OUR engines, here.

Class dismissed.

 

Valve float is very bad on springs also. They overheat and deteriorate.

Valve float is possible with hydraulic lifters. I have evidence of that in my engine that is apart at the moment.

 

Ummm...youve learned just enough physics to make you dangerous. J/K :D

BUT: It would have been better stated as: Valve spring force is exceeded by the movement of the valve train components in the valve opening direction. At higher rpm, this movement builds enough momentum to greatly exceed the force exerted on the components by the valve spring, resulting in the inability of the valve spring to control valvetrain movement in the closing direction, allowing, among other things, the lifter to loose contact with the cam lobe, and the valve to loose seating pressure. This condition is known as valve float.

Valve float can and does result in loss of high rpm power, as well as physical damage to the components of the valve train, and can even result in piston to valve interference that will physically destroy the engine.

Ok...I don't even know where I should begin...but that statement is factually flawed. First, I'll start with the noise...solid lifters ARE LOUD AND NOISEY. Second, they wear faster than hydraulic lifters because they are always set with mechanical clearance ( lash ) allowing physical impact damage every time the cam lobe opening ramp aproaches the lifter.

As for being cheaper...yes. As for tolerating higher spring pressure, yes. As for better performance...well, thats all in the design.

( BOY.....I never knew this topic could bring so much confusion. )



[Modified by BBA, 12:04 PM 6/26/2002]

 

BBA:
Amen on the confusion / misinformation comment. Sometimes posts here remind me of a quip I once heard about two windy types discusssing something technical (with less than full knowledge. It went ..."they wern't having a discussion, they were exchanging inaccuracies".
Despite the much appreciated and really sound information I've gotten here, I think that some of the not so good technical advise passed out here ought to be appended with the old lawyer's admonition that, "solitious advise is often worth the price paid for it". Cire96 :cheers:

 

In any conversation, it is up to the listener to sort the chafe from the wheat. Thta can often be difficult to determine if you are clueless as to the discussion topic.

I would wager a bet that most who have followed this thread have a pretty good idea and visual impression of what is being discussed. In every thread of this nature, I tend to learn a little more than I knew before I started. Most likely, I would not have bothered to read this if I was clueless from the start.

I have been with this forum since it began, and I have noticed a significant difference in how debates are handled today vs. 3 years ago. I appreciate the fact that, as a group, we tend to build upon replies vs. tearing them apart. Keep up the good work guys.

~ Purp

 

You said a mouthfull there Purp ! ! ! Love these great threads, not only educational, but entertaining as hell....... I have a problem here though, what dictionary do YOU find DESMODROMIC in ? ? ? ? ?

All in fun, Eddie in Tucson :jester :flag :cheers:

 

Ed........
Look in the glossary of Dr. Doolittle
Desmodromic, n. Latin name for the Pushmepullyou :troll
:cheers:

Rich

 

This forum could be better served by constructive criiticism, and corrections rather than name calling.

I fail to see where any of your "correction" disputes any of my statements. Yes, I could have been more wordy, and maybe it would have been best if I had pointed out that the lack of valve train control exisists during the closing cycle. I included the simple, "E=", formula to help make it clear that both the weight of the parts and their speed contribute to the condition. If you think the words, "movement" and "momentum" are more accurate and descriptive, than the ones I chose to use, then so be it. To call me "dangerous" is simply, counter productive. If I am wrong, point out where, and explain it to me and the rest of the forum. Don't let any of remain misinformed.

This MAY be subjective. We are just going to have to disagree on the noise issue. A solid cam has a lash take up ramp. Properly adjusted a solid set up does make some noise, where as a hydraulic system is essentially noise free. But, in MY opinion, nowhere near the noise that a zero lashed hydraulic cam does. The zero lashing of a hydraulic set up defeats IT'S built in lash absorbers. "factually flawed."? Please present your facts.

If there is any confusion to this topic, it is perpetuated by unsubstantiated statements. If you feel that I am confused or confusing, please set us straight.

 

You couldn't tell in the first part I was just razzing you a bit...nothing offensive intended, as shown by the "J/K :D "

Now, as for hydraulics, they are always adjusted at zero lash and slightly preloaded so that the hydraulic plunger in the lifter will be able to correct for changes in needed lash due to temperature and wear.

Now...I don't care who you are trying to prove a point to...solid lifters are ALWAYS noisey. If you think solid lifters are not loud and noisey...just post a poll of those of us that have and had used them in our own engines, I'm sure we will all tell you how noisey solids are.

Ok, I'm done.


[Modified by BBA, 9:43 PM 6/30/2002]