What makes your valves float, how can you tell your valves are floating at high RPM?
Phillip

 

Hey, wassup
Valves float from lack of spring pressure to get them closed at high revs. You can see it on a dyno as a premature drop in power output as you go through the rpm's. If you think you have it check your cam lift plus any lift from oversized rocker arms (1.8's) and see if the springs you have are rated for that lift....most aftermarket springs (single) should be good for .600......Zo6 (2002) are good till about .570 and Stock vette springs are .530 or so. If you need more get double springs or so.
Hope this helps.

 

Valve float is the term used when the cylinder head valve does not have sufficient time to close when the RPM exceeds a certain level.

The valves are shut by a spring and opened by a lever which is driven by a pushrod which rides on a lifter tappet which is driven by the lobe on the cam.

The spring must return the energy to the valvetrain as the high-point of the lobe recedes. If the mass of the valve train exceeds the ability of the spring to accelerate all those parts to a closed position, the valve will remain open slightly or "float".

There are two ways to address valvetrain float, decrease the mass of the valvetrain or increase the spring rate.

There are tradeoffs in both directions. Lighter parts are harder to fabricate and still make strong enough to do the job, heavier springs increase the load on the valve train and cam and also rob a little power lost in friction.

The LS6 uses hollow valve stems and stronger springs.

Overhead cam engines reduce the mass of the valve train by eliminating the pushrods altogether, which is why they can rev higher typically.

 

Wow, I forgot about those two options Phil. You can replace your valves with smaller lighter valves especially in the 396 cube motor-- perhaps 2"ers or you can just scrap the entire LS1 for a ford 4.6 liter over head cam set-up to get rid of the pushrods.....yes, I'm just being sarchastic. :jester :reddevil

 

<== 346cid

 

I thought you were getting a 396. Anyway, I was just pointing out that making the valvetrain lighter wasn't really an option. Springs are the best bet.
By the way, 11.3 with a 346 is awesome for just motor. Good job!
What gears are you using?

 

Valve float occurs when the cam follower is not kept on the nose of the cam.

In normal operation the follower remains in contact with the cam al all points in time. It is held there by the valve spring (and the tappet expansion under oil pressure).

When the cam follower leaves the cam lobe, the valve spring becomes part a a ballistic system, pulling up the valve and pushing down the pushrod as fast as its tension can. When the follower impacts the cam lobe, the pushrod stops accelrating, and a wave of energy is transmitted through the spring, bounces off the spring seat, back up the spring and bounces off the now stiff pushrod/rocker. This wave of energy heats the spring to the point of tension loss very quickly (see Lingenfelter: Building High performance Chevy Engines: chapter 2?) Vavle float ruins valve springs in quick order.

There are three causes:
A) too much valve train weight
B) too little valve spring tension
C) tappet compression

Notice that the pushrod and tappet weight is reduced by the rocker ratio when computing the valve train total weight.

At certain RPM levels, the force required to be transmitted through the tappet is greater than the force of oil holding the 'self adjusting' tapped at zero clearance. Oil get squished out, and clearacne increases, causing noise as red line is approached/exceeded. Once you loose the clearances, There is a great increase in force on the valve train as the valve is initiated into another cycle (lifted off the seat). This can also cause a wave of energy through the spring, and contribute to heating of that spring.

 

Wow - that was an informative response.. :cheers:

 

you will know if they are floating, as you can't go any faster.

 

Can valve float happen on a stock ls1 simply by raising the rev limiter and shift points??? Example, I drag race so when I do a burnout my car bangs off the rev limiter for some time. It does this somewhere between 6200 and 6400 rpms. Is this a no no???

Tony

 

I just Z-shaped a pushrod and wasted a couple of valvesprings on mine, just from banging the limiter a little too often before I got my shift points done. One spring was at 50 lbs. installed pressure :eek: and the rest varied from 75 - 90 lbs. They didn't break, but they do get very weak very fast after they've been floated. I'll be doing a leakdown test this week to see if I bent any valves.

 

When I had valve float on my LS1 engine it started at 5600 rpm and the Hp and Tq curve dropped off 35 hp. New Springs fixed the problem for me.

vetterdstr :cheers:

 

You can see it on the dyno graph.

 

Interesting point.

If I'm not mistaken the secondary or sometimes third spring is added as a damper spring, perhaps to dissipate the reflected spring energy from the main valve spring?

 

get a copy of the june 2003 issue of chevy highperformance and read all about valve float and how now the after market cam companies are copying chevys LS-1 and LS-6 behive shaped valve springs to cut down on valve float. even changing to a bigger rocker arm ratio will cause premature valve float:chevy


[Modified by clem zahrobsky, 9:59 PM 4/10/2003]