What's valve float?

[sothpaw]

I was wondering what happens when an engine is over-reved.
I'm specifically interested in what parts of the engine control the
redline rpm--heads, cam, valve springs, or all the above?

I'm debating changing my LT1's cam or heads or both, and I would
do new valve springs. But I wonder what the safe rev limit really
is, after the changes.

Andy

 

[dmrodco]

Valve float happens when the cam spins faster than the springs can close the valves. I am not expert however I would think that the rpm limit would be tied to the strengh of your springs. A complete true roller setup would also help at high rpms. My 2 cents worth. good luck

 

[Mad-Mic]

valve float is when both valves are opened at the same time as dmrodco suggests it's the springs not keeping them closed. when this happens your only a few rpm away from head damage. you can bend pushrods fairly easy floating the valves too much.

btw happy b-day

 

["94 Sixspeed]

I agree

You certainly want to avoid this condition.. If you are floating valves and are within the normal operating range for the engine, you may have weak valve springs..

 

[sothpaw]

valve springs

So, nice, stiff LT4 valve springs would be good insurance against
valve float.

94,
I've never experienced valve float that I know of, but when I start
spinning the engine more with aftermarket heads or cam, I want
to know that I won't blow something up.

 

[vader86]

Its when youre revving so high that the springs arent tough enough to make the valve close and it stays open. This will cause warped springs, which will cause bent pushrods. So you have to find the right spring that will let them open, but be strong enough to close them to prevent floating.

 

[Mad-Mic]

if you stay with in the rpm range you'll be fine. trust me you'll know valve float when you hear it. it goes something like this....


spit sputter bang bang bugga bugga bang.... :L

since fuel air and exhaust try to come out of both intake and exhaust valves.

i also forgot to mention you can seriously bend or drop valves this way. in my 82 Z/28 i used to wing it to 6500+ rpm when it just started to float i'd shift since i didn't have a rev limiter in the car. then again i was 19 yrs old and wanted to blow it up but it never did. (had a 400 sb waiting for it)

 

[LD85]

Its when youre revving so high that the springs arent tough enough to make the valve close and it stays open. This will cause warped springs, which will cause bent pushrods. So you have to find the right spring that will let them open, but be strong enough to close them to prevent floating.

Which springs do you mean the valve springs or the spings in the lifters?

 

[dmrodco]

Valve springs..............

 

[Ken]

Valve float is a condition where the valves in the engine are forced back open before they have had a chance to seat; usually brought about by extremely high rpm. What happens is the valve lifters lose contact with the cam lobes because the valve springs are not strong enough to overcome the momentum of the various valvetrain components. The onset of valve float prevents higher-rpm operation. Extended periods of valve float will damage the valvetrain. It is also sometimes referred to as valve "bounce."

_ken

 

[sothpaw]

Any body know what rpm valve float occurs at with the stock LT1
springs? Will 1.6 roller rockers matter?

 

[Skant]

Valve float is not the only limitting factor for RPM range. Everything that moves in the engine has a maximum RPM it can handle. Including the pistons, main bearings, etc.

When an engine is upgraded to handle higher revs, it usually entails the addition of stronger versions of many of its components. The valve train is just usually the weakest link.

Some components may be able to tolerate the RPM... but suffer a shorter lifespan as a result.

I don't know that the valve train is the only thing that would need to be upgraded. I would expect other parts of the engines need to be upgraded, too. Or at least will wear out quick.

- Skant

 

[vigman]

Ok root beer valve float please

remember we are dealing with INERTIA here and MOMENTUM.

So you have a spring and it has a tension,
and you push the valve down SLOW and let it up SLOW no problem.

Vette redline on a stock Hydraluic lifter motor is at 5000 CRANK rpm and the ratio for crank to cam is 4/1 ( crank/cam ) so the cam is ONLY turning 1250 RPM@redline and the paticular valve is going up & down that amount.. which happens to be the mechanical limit of the spring tension vs time ( or ability to rebound).

How do you fix that?

Bigger springs/solid lifters ( roller style to reduce friction)/ roller rockers ( to reduce friction)/ cam locator button ( to keep the cam from walking)/ lighter valves ( that can take the heat of the higher combustion tempratures) Hardened seats. AND THE CORRECT CAM!

What do you get?

1000-1500 more RPM
BIG HOLE IN THE WALLET
reduced engine life

But did you also
Change the spark/ intake/exhaust & the fuel delivery systems to handle the increased demands?

Now what did you get
a BIGGER hole in the wallet
But you got TIRE SMOKIN POWER

So all these things HAVE to be considered when UPGRADING the engine or WHOOPS

Ask Ken or Chris the hole in wallet vs HP ratio.

I believe it $2,000 +2 weeks per 35 hp

Not including downtime for broken stuff that you THOUGHT would be OK!

Food for thought!


Vig!

 

[sothpaw]

ok, what is for stock LT1?

Vigman,

Ok, Ok, all that wallet denting stuff is what I'm trying to minimize.
So what rpm can the LT1 handle and have minimum loss of wear
life?

The stock rev limiter is 5850. I'm thinking about changing that
to 6200 tops and shifting at 6000 in a drag race situation. Not
everyday, maybe 8 times per year. Whatever I do with the heads/cam will not require shifting beyond 6k, I promise!

My tach only goes to 6 k anyway, and I'm not messing with that.

 

[Redbob]

Technically, valve float occurs when cam RPM hits the resonant frequency of the valve train.
Best way to prevent this is not to turn the engine that fast, and the cam manufacturer should be able to tell you how fast that is.

You can run stiffer springs, and/or spring dampers that act like shock absorbers in preventing valvetrain resonance.

 

[SPANISHVETTS]

1. There is valve float and lifter float. Most engines with hydraulic lifters will experience lifter float before valve float. This is when the lifter gives up and doesn’t push the push rod up high enough to fully open the valve. This has saved more engines than STP, Wynn’s, and Bardol combined. How many of you did just like me? Took out their Dad/Brother’s Camaro/Mustang, placed the Auto to N, pushed the left foot on the brake, the right foot nailed the gas to the floor, and then drop the shift into L. What saved those engines from blowing up? The hydraulic lifter, the invention that made the wheel viable.

2. You will be lucky to get 6000 RPM out of a set of Hydraulic lifters even with roller rockers. Solid lifters will see 7000+ if set up properly. Push rod engines just have too many reciprocating parts to rev high. Any thing over 7000RPM you should be looking at overhead cams. Then you have to think about balancing the complete rotating mass. GM balances their engines at the max recommended operating speed. Above their red line this just ain`t good enough.

3. Over here in Europe Engines are smaller because of the price of gas,+$4./gal., therefore they are forced to rev higher to produce horsepower. A good example is the BMW M3 Evolution. 3.5L @ 321 HP. If you went the same ratio with a 5.7L GM you would develop 522 HP off the show room floor and your Corvette would cost ½ a million dollars.
My top wrench drives a M3 and guesses what? He pays $80.each for his spark plugs and changes them every 5000Km. I still wax his a$$ every time we have a go. To make things worse, my Vette gets better gas mileage.
The 2 Audi TT`s in the neighborhood pretend to be VW Beetles. The owners wish for SNOW and assure me they would have me in ice conditions, but we do not get much snow in the south of Spain. Poor them.

4. The LT1 is as good as a factory engine gets. Why don’t you forget the RPM thing and spend that money to develop torque. Clean up the air flow, both in and out. Then look at a 3.73 rear end, Cam shaft, Mini Ram, injectors, etc., and get a chip custom programmed for your car. With a set of Craftsman tools and your VISA card you can go over 400 HP at 5000RPM and still have a car that is happy on $10. Spark plugs, 3000 mile oil changes, and pump gas.

Above all do not listen to some jerk like me. Knowledge is the cheapest commodity on this earth. The ability to organize and use knowledge is the most valuable.
:BOW

 

[Ken]

Sothpaw, did you ever expect so much feedback from so many different sources? I guess everybody has a version they like to air. My response came from an automotive dictionary. :L

_ken

 

[Mako]

I find some of the previous replies... interesting.

I have both an olde skool mouse w/solid lifters and a spiffed up LT1 running the stock hydraulic roller lifters on a cam that goes north of .600" lift.

My LT1's redline is limited by the amount of fuel I can throw at it from the 50#/Hr injectors, not the type of lifter.

A common LT1 valve spring upgrade for stock LT1's running more cam or 1.6:1 roller lifters is described in the cut 'n paste below from http://www.ba-f-body.org/index.htm

An LT1 hydraulic cammed engine will spin quite happily to ~6500RPM with this spring, assuming your cam and head flow numbers are up to it. My preference would be to have forged rods and ARP bolts on such an engine, but that's a conversation for another thread.

Higher rev's and your looking at better rods, rod bolts and dual coil springs, which is what I run on my LT1. Dual springs require machining of the spring seats on the head and opening up the push rod area.

Lifter float is a north of 6500RPM phenomena and/or requires a radical lobe profile. Solid rollers with a rev kit is the usual recipe on such an engine.

The LT1 engine can give you both excellent low end torque and high end horse power with a proper cam and head setup, provided you have also dialed in the exhaust and intake and have a good tune up in the computer.

CYa!
Allen

====
Running the Crane 99846 spring on an LT1/LT4 head

Got the answer folks!

Get 16 of GM part nos. 1018-5066. It is the L98 seat/shim. It has the same id as the LT1 seat/shim but has a much narrower guide on the id so that the Crane 99846 spring/damper combo will fit perfectly. The LT1 seat has a wider guide on the id since the stock springs are damperless. The part is 0.85$ each dealer cost. So for less than 16$ you are good to go with the trick 99846 Crane springs.

They bind at 1.100 inch and should be installed at 1.750 or so to get about 115#s of seat pressure. I am using the L98 seats which are 0.030 inches thicker than the LT1 seats and a 0.030 inch shim to get 1.740 inch installed ht.

Voila, no damper bind! And an easy (at 1.740 installed ht) 0.590 inch of lift at the valve available. That should work for most of the cams we use even with 1.6 rockers. (don't forget the valve guides need to be cut down a bit if you are using this kind of lift).

Perry Kincy
=====

 

[Vettelt193]

sothpaw... shifting at 6000 shouldn't be an issue with an LT1, with a 6200 red line.

Here is a quick run down of why I don't like to go above that though.

More RPM usually leads to more problems, AND a problem at high RPM is usually much worse than that same problem at a lower RPM. So, your chances of really screwing something up are greatly increased with higher RPM.

This doesn't mean that there aren't guys that spin close to stock LT1's up past 6500 with no problems, over and over... they have cars that are in tune (and are a bit lucky).

so... again, 6200 red line, with shifts at 6000 should be fine... you should not experience valve float at that RPM in your car.

 

[sothpaw]

what springs?

Everyone,
Thanks for all the replies!! Nice reading! Ken, with the title I wrote, I figured this might get popular.

Vettelt193,

I tend to agree with you, from what I've heard, 6000 rpm for that
engine is not at all unreasonable. I will try to cam it so that
the hp peak is 5800 for a shift at 6000.

BTW, I decided that what I will do is BOTH heads and cam. I'll do heads+52mm TB first this spring and then, when the optispark or water pump goes, I'll have the car towed and will have a cam selected to put in. I'll add in an LT4 timing chain (I'm told it's stronger than the LT1 chain) and maybe new lifters.

I'm assuming headers will screw up emmissions? I have to pass
on a dyno in MD!!

My springs will depend on the cam I pick (I'm inclined to pick the one you recommended, but dyno graphs would be nice so I see
that the combo will peak out at or around 5800) but right now I'll see what GM springs I can get. Maybe LT4 springs.