WicketMike

iTrader: (13)

Can you guys explain valve float,
how it happens, what happens, what it causes etc...

thanks

HumpinSS

iTrader: (1)

Weak springs from fatigue. Not enough seat pressure. Mechanically over-revving the engine (though I am not sure)

2001 Pewter WS6

iTrader: (43)

J-Rod just had a big write up on this...

BurnOut

iTrader: (1)

Valve float occurs when the lifter is "thrown" off the nose of the cam lobe and does not touch the back (closing) side of the lobe. Obviously, this has an adverse effect on power, as the timing of the closing of the valve is now significantly different from what it once was. Additionally, it is VERY harsh on valvetrain parts because, as you can imagine, the "landing" of the lifter on the lobe is not a gentle one. It's a hell of a good way to pop the face off a valve.

The cause, as HumpinSS mentioned, is valve springs that are too weak for the lobe shape and RPM level.

WicketMike

iTrader: (13)

thanks guys
im just going crazy trying to figure out how i lost 6+mph in the 1/4

i have my limiter set at 6800 and ive hit it alot so i am guessing maybe this is where my loss of power is coming from. should find out soon for sure when my heads/cam package gets here in a few weeks.

J-Rod

Valve float
A high-rpm engine condition in which 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. The valves will stay open and/or "bounce" on their seats. The clearance in the valve train created by valve float will also cause hydraulic lifters to "pumpup" as they try to eliminate the valve clearance. Extended periods of valve float will damage the valvetrain.

critter

iTrader: (9)

I've heard that state of the art NHRA type "stock" cams intentionally float the valves, launching them on the front side and catching them on the back, to get around lift rules. Any truth?

Camaroholic

iTrader: (1)

That's called "valve loft", and yeah, I've heard of it too. A controlled float. You measure lift while the motor is off, but you get more effective lift by lofting the valvetrain with a really aggressive ramp on the cam.

-Andrew

J-Rod

We currently use those cams in a class car that we run. NASCAR uses them too. Comp calls them a "launcher" cam. You find them in classes where you are lift limited. They alos use them in karting. Its controlled valve float. Its really crazy. All of the sudden @ 6200 the acceleration curve on the data computer starts this wicked climb since it gains a bunch more lift... They are VERY hard on cams and on all the other associated parts.

Old SStroker

Launching or lofting the valve requires very well matched valvetrain components, especially the springs. If it's done just right the "landing" on the closing flank won't be a disaster. If it'll live for a million revs in a Cup race, it can be done correctly.

It ain't cheap to do it right.

J-Rod

True dat...

Lets put it this way, We use Schubeck cool face lifters, 1.2 ratio break in rockers, and pull the inner spring out for break in. My buddy was in a hurry last week, and tried to cut corners and break the cam in with 1.7s. The cam went flat in 7 minutes. New motor is now junk...

Old SStroker

J-Rod:

"Haste makes..." You know the rest.

"cool face lifters" ? What are they?

Do you use welded steel cam lobes? It sounds like you might be in that enviornment.

180ls1

iTrader: (4)

bumping this old *** thread, didnt know about valve loft untill i ran across this a few months ago, it gets good around post #7.