Laurence Gibbs

lighter valve train = quicker response. lighter retainers (keepers) i believe are a popular fittment. Titanium being stronger and lighter.

Geoffrey

You will definately need EITHER the stock valve installed height reduced to increase the spring pressure provided there is enough clearance to coil bind OR performance valve springs. I do both depending on the application.

To rev past 7000rpm, you need to address connecting rods and pistons.

Mike Murphy

We used to call this "floating the valves" in Chevy engine talk, and it was a great way to get flames to come out of the exhaust at high RPMs. When drag racing a Chevy V8 designed to produce power no higher than 7500, many would simply hold the accelerator down between shifts, floating the valves and running at or near 8500rpms. Then when the clutch engages, you get a little extra force from the momentum of an engine running at 8500 versus 7500. But at the end of the day, you're looking at having to replace the valvesprings at best or a head rebuild at worst.

In the Chevy world, one would not think of running stock valve springs if any component of the valvetrain were changed. Double & triple springs are not uncommon on these types of engines.

Indycam

"In this case I think that bind implies the coil can't overcome the pressure exerted on it to close or more accurately close quickly enough ! It can't react in time to the cams rotation."
Thats float not bind .

If you look above , you will see that he has accurately defined coil bind , "the spring rate becomes infinite."

My question is still , how does excess rpm lead to coil bind ?
My question is not , how does excess rpm lead to valve float ?

Geoffrey

Indy, they are essentially the same. In simple terms, you have 2 forces, the spring pressure, and the force from the acceleration of the valvetrain mass from the cam (F=MA). So, if the valve acceleration force > valve spring force, then the valves will float, and in the process, the springs will compress further, until coil bind. When this occurs, something mechanical will be damaged and eventually break.

Indycam

So the cam opens the valve to X amount , the normal , the coil is not bound , the valve keeps opening up the gap between the valve and the seat , even though the can is no longer in contact , and the coil binds up stopping the valve from opening any more ?

"and the force from the acceleration of the valvetrain mass from the cam "
So are you saying the cam is launching the valve ?
Its gone past float and gone to launch ?
The cam rotation is causing the acceleration of the valvetrain mass to go past its normal max lift and right into such a large amount of lift that the coil binds ?
Just for grins , at what lift , valve off seat , does the coil bind ?

Geoffrey

Two things can occur when the spring pressure is not sufficient. First, the cam can accelerate the rocker arm and overcome the spring force to keep the rocker arm on the cam. When this occurs, the valve is lifted more than the specified cam lift and this is when the spring can go into coil bind. The second situation occurs when the cam is decelerating the valve back onto the seat and that rate is faster than the force of the spring so the valve bounces on the seat. This is also bad, but does not affect coil bind since the spring is at or near its maximum length.

The stock valve spring coil bind can be measured by using a valve spring tester. I've done it, but cannot remember the values. It will also vary on which retainer you are running and at what installed height you shim the spring to. It is also possible to have the retainer hit the valve seal which is also bad. You really need to be checking these things when assembling the heads and you should know what spring forces and at what installed height you assemble the heads at. This is why you cannot just pick an off the shelf cam and mate it with an off the shelf spring and expect it to work properly without checking all of the clearances.

Indycam

"When this occurs, the valve is lifted more than the specified cam lift and this is when the spring can go into coil bind. "
The valve is launched off of the end of the open ramp and it keeps on going until the spring says no ?
That leads to my other question , how far does the valve have to be launched before the coil binds ?
If I had a spare cylinder head , I'd measure it .


"but does not affect coil bind since the spring is at or near its maximum length."
I believe that when the valve slams into the seat under the force of the spring , some of that shocking contact does go back to the spring , how much I do not know . It would be like the shock you feel when hitting an anvil with a hammer , but that a different subject . Its sufficient to say , not having the valve placed gently back onto the seat , is less than ideal .

Steve Weiner-Rennsport Systems

Geoffrey has raised some excellent points here,.....

I'd add a few things in the context of AutoX, DE and racing,

1) Stock valve springs slowly lose seat pressure (fatigue) over time and that lowers the threshold of valve float. In short, that RPM value is lower at 80K miles than when it was all new. Springs should be checked when any head work is done and if they don't pass muster, replace them.

2) Stock valve spring retainers are sintered metal and while accurately made, they are brittle and can fracture when the valves are repeatedly floated; think AutoX. When these break, the valve spring assembly come loose and the valve falls into the cylinder to meet the piston. Its not pretty and VERY expensive. Engines that are used for DE or lots of AutoX should consider Titanium retainers (and better springs).

3) Performance springs are not all the same and while some help prevent valve flioat, their seat pressures put a of of strain and wear on cam chains, sprockets and intermediate shaft gears. Talk to a professional/successful engine builder and ask all the questions since you will bear the cost of a bad decision.

4) Software. Make sure any chip you buy retains the stock rev limiter unless the engine's internals have been upgraded for the anticipated RPM limits. That can mean anything from better springs & Ti retainers, all the way to ARP rod bolts, or aftermarket stronger rods (Pauter, Arrow or Carrillo).

Great thread,....

Geoffrey

As I mentinoned before, you'd have to measure it as it will depend on the height of the valve guide, valve seal, shape and thickness of the retainer, length of the valve, and the profile of the camshaft. Stock springs are surprisingly good durable springs and depending on the application, don't necessarily need to be replaced by aftermarket springs. In some cases simply adjusting the installed height will give you the spring pressure required.


And this point is where the oil issues get raised. The newer multi valve engines simply don't require the same amount of wear protection that a 2 valve racing engine with stiffer springs requires. I've had experience with a number of aftermarket springs, and some of them are simply way too stiff as Steve mentions. This is one place where bigger/stronger isn't necessarily better. Getting the correct spring pressure is key.

Mike Murphy

Also, if the spring is too stiff, it will cost horsepower due to increased frictional losses. I have seen a change in valvesprings result in a 2 HP loss.

Geoffrey

yes, that is very true and we are very conscious about that when building engines.

Cupcar

Another thing, if I recall correctly, the 964 Carrera Cup cars had some issues with these failures using the stock springs and there was a factory fix changing the valve springs to a different specification without removing the engine from the car.

And these problems occurred with the stock 6800 RPM limit...so "racing" with stock springs is dicey anyway.

Sire

Is there any combination you would advice Jeff?

Geoffrey

I could not recommend anything without knowing an application and doing the measurements. Sorry.