Valve spring pressure and boost 1

[yoshimitsuspeed]

Many highly respected engine builder/tuners will tell you that you need to increase spring pressure for high boost. There are even reports of fixing floating valves related to boost (However they determined that IDK) or performance issues by going to a stronger valve spring.

Now I don't believe this theory for one second. If the pressure differential is that great between the cylinder and the plenum then you are seriously liimiting the amount of air going into your engine to a point that I would find hard to believe. I mean if you had a 20 PSI pressure differential between the runner and the cylinder I would still expect a valve to close just fine unless the spring was already so precariously sized for the cam. On the other hand if you had a 20 PSI pressure differential between the two the amount of power that you are leaving on the table would be insane.

I'm confident enough in this that I'm not really even looking for confirmation although if I am missing something huge please tell me.
I am more here to talk about the stories of stiffer valve springs actually curing these issues allegedly related to the boost pushing the valve open.
What else could change with a stiffer spring? What would a logical explanation be that valve springs would actually fix the problem?
Or is it possible that they are running such an improper intake cam that they are actually preventing a massive amount of air to fill the cylinder?
In which case a properly sized cam would be the solution whereas stiffer springs are just going to help hide the problem.

Thoughts?

 

[crerus75]

Could this old wives' tale simply be a misinterpretation of observed phenomena? Perhaps something like this occurred:

1- Supercharger of some kind is bolted to an engine, perhaps one that already has marginal spring pressure for the conditions seen at redline
2- Engine pulls harder, as it is supposed to do, especially near redline.
3- Driver repeatedly zings it past redline and into valve float due to engine's greater power. Before the days of rev limiters, this was not difficult to do.
4- Analysis ensues. "Ever since we put that blower on, the valves float. We need stiffer valve springs on a blown engine."

I could see this happening in the heydey of, for example, privateer NHRA competition, when most guys were learning the hard way.

 

[Tmoose]

Hi Crerus,

I think that was Hugh Macinnes' suggestion in his book "Turbochargers".

 

[yoshimitsuspeed]

crerus75

This is exactly why I come to engtips to discuss theories like this and not boards like SpeedTalk.
There are a lot of very smart and extremely capable tuners and builders on boards like that but there is also a lot of bad science and old wives tales that exist because the answer may have solved a problem and so therefore must be correct.
I definitely suspect that may be the case here.
I also believe it would be much more likely in something like a supercharged dragster with 30 to 60 PSI boost pressure on the intake and atmospheric pressure on the exhaust.
This extreme scenario trickles down to a generalized understanding that any boosted motor needs more spring pressure.

 

[Lou Scannon]

Case in point, my modified Ford 2.3L turbocharged engine runs 20psi of boost, with considerably higher exhaust pressure (my 30psi gauge pegs) at WOT redline (7500rpm). She pulls hard to the rev limiter, with ~100 lb. valvespring pressure on the seat.

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz

 

[yoshimitsuspeed]

hemi what diameter are your valve springs? The 4AGE only has about 35 PSI seat pressure stock and spins to 7600 from the factory.
Some of the heavier valve springs I sell are around 60 seat. 100 seems like a lot in the world I am familiar with.

 

[Lou Scannon]

The Ford 2.3L is an old fashioned 2 valve design from the late 60s. Valve diameters are similar to a small valve Chevy smallblock. I'm running Comp Cams 990 valve springs, 1.437" OD. In my world, 100# on the seat is definitely heavier than stock, but not heavy at all for a modified valvetrain.

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz

 

[yoshimitsuspeed]

Good info.
I'll bet the mass on those valves is also quite a bit more than the four and five valve heads I am used to playing with.

 

[KevinJohnson]

Well, some people might have gotten the idea that increased valve spring pressures were needed on a turbo because they read SAE 840252.

http://papers.sae.org/840252/

which mentions that valve spring pressures and valve materials were changed. So perhaps without knowing why for certain they tried to follow sound engineering practice by emulating the factory.

Recently, Katech did laser valve bounce measurements on various combinations of valves and springs of the LS7. While this was for a normally aspirated engine, I am sure that all aftermarket turbo valve train components and their permutations are also rigorously tested in this way so as to remove the possible experimental confound of valve bounce versus valve float due to pressure deltas.

http://www.superchevy.com/how-to/14...in-stability-ls7-exhaust-valves/spintron.html

 

[Panther140]

Iff the pressure in the intake runner and intake port are constant in both the FI and NA setups, and the air can flow freely to the cylinder for the entire intake stroke, the pressures should equalize and the valves should willingly close. Also: Iff the pressure in the exhaust port and in the chamber are the same at the end of the exhaust stroke, that levels it out also.

I think its a question of whether or not cylinder fill correlates 100% with the boost level in the plenum or charge pipe on the other side of the intake valve, port, and runner. Also, is the exhaust port and pipe evacuating it as well as it would N/A? How is flow friction in the ports being changed? How much more momentum and mass does the intake air have before it hits the (closing) intake valve?

"Formal education is a weapon, whose effect depends on who holds it in his hands and at whom it is aimed." ~ Joseph Stalin

 

[Martin445]

Hi Guys, i found this thread and was interested, i have an issue with a tuned Golf GTI k04 turbo, on certain years they had weaker valve springs and at high rpm resulted in valve float and misfires on a single cylinder. The tuning companies have resolved this by reducing power output at high rpms.
I did read somewhere that reducing backpressure can reduce valve spring float. So my question is, will adding a bigger downpipe with high flow cat or no cat at all help?
Also this generally happens only in 1st gear, it loses traction so fast the revs hit redline and thats when misfire happens. Would more traction also help? Changing springs is out of the question because its ridiculously expensive. Thanks guys

 

[Lou Scannon]

Rev-limiter, i.e. fuel cutoff = "misfire", right?
What's the issue?

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

 

[dplecko]

Depending upon your existing valve train (pushrod vs. direct acting, stiff vs. not so stiff, hydraulic vs. solid and not least the amount of reserve spring pressure capacity) you may or may not notice any valve control issues as a result of added boost and just because you don't notice, doesn't mean it does not exist. Any valve train separation allows hydraulic lifter pump up, exactly the task they are designed for, however if the (momentary) extra clearance is not 'permanent' i.e. due to unequal thermal expansion or wear, then the valve will be held completely off it's seat when the cam tries to close the valve as the hydraulic lifter has now expanded to compensate for that temporary clearance.

During certain portions of the valve lift curve the valve open area enjoys sonic flow, implying a rather large pressure differential across the valve at that particular portion of the lift curve. Sonic flow is more prevalent closer to redline, after peak torque and peak power. *Any* pressure differential across either the intake or exhaust valve will add to (or subtract from) existing forces on the valve. Operating the engine in a 2 atmosphere environment (i.e. boosted), compared with operating the engine at 1 atmosphere, will result in approximately (for the sake of this thought experiment) twice the pressure differential across the valve at any point in the lift curve.

Opening the exhaust valve when there is more stuff in the cylinder due to boosting is obviously more difficult compared to NA and flexes everything not made from unobtainium which, if it doesn't break, 'unflexes' when the exhaust valve does eventually open and the pressure differential across the exhaust valve falls to controllable levels. This 'unflexing' brings us to valve train dynamics 101 and all sorts of instabilities. If you have pushrods think pole vault.

At the exhaust valve closing event the pressure in the exhaust port can be very high, particularly at RPM's higher than any exhaust header tuning range. If the exhaust valve cannot close as the cam closing ramp dictates due to the high exhaust port pressure and decreasing cylinder pressure due to the descending piston on the intake stroke (or re-opens simply due to the high exhaust port pressure) it will then crash into the valve seat and the exhaust valve will bounce off it's seat and probably seriously dilute the new cylinder charge with hot exhaust gas.

Earlier than designed intake opening due to boost is probably very positive on engine output as a valve timing event, however if the early opening results in a valve lift greater than the gentle opening ramp describes and the lifter/cam follower slams into the cam profile then please refer to valve train dynamics 101.

Intake valve closing may be the most important valve event. Any intake runner tuning is magnified by boost, i.e. any pressure wave in a 2 bar system will be twice that of a 1 bar NA system. The spring seat pressure is required to stabilise the valve on the seat, at higher than optimum RPM, cylinder pressure will be lower than port pressure at inlet valve closing. If the spring seat pressure subtract the cylinder port pressure differential is not enough to stabilise the valve on the seat then by definition the valve will lift off the seat as the piston continues to rise (possibly a couple of times, eventually pushing charge back up the inlet port).

The highest dynamic pressure differential across the valve needs to be added to all other valve spring pressure requirements.

 

[gruntguru]

Nice post dplecko, although you can't have it both ways re inlet valve bouncing. If the pressure differential across the valve is contributing to the valve lifting off the seat, the charge will be pushing towards the cylinder not the port.

je suis charlie

 

[dplecko]

After the inlet closing event, the valve may bounce/lift of the seat helped by the pressure differential across the valve (due to boost and inlet ram too late due to high RPM). At this point in time the piston is rising and the cylinder volume decreasing, either the inlet valve is happily seated or in a mild case charge continues to flow into cylinder until pressures equalise (one might not detect anything wrong) however in a severe case the valve can bounce several times even when the pressures have somewhat equalized or even reveresed. A keen eye will witness the tell tail marks on the cam lobe in extreme cases.

 

[Tmoose]

Maybe nobody has suffered the problem since 1958, but Chevy claimed the non-boosted 348's valves were bouncing off the seat multiple times due to spring surge at a mere 4500 rpm. Some form of spring damper helped a lot

 

[Lou Scannon]

Not sure when damper coils became commonplace in OEM engines, but my 1955 Olds had them (and 90 lb. on the seat). FWIW, at the time, the Olds Rocket engines of the mid-late fifties featured fairly mild duration, combined with rather sporting valve lift, in the + 0.400 range, all with an edge-of-the-envelope rocker ratio of 1.8:1. Peak HP was in the 4000-4600 rpm range, and they were not known for rev potential, in factory configuration, although they held their own in NASCAR, through most of the fifties.

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

 

[ICEaddict]

I am currently implementing a GM LDK block with LHU ECM and aftermarket turbocharger compounded with a high pressure roots blower with dual intercoolers. This thread relates to my current issue of determining correct spring pressure. Test RPM will be at 3000, but boost pressures will be well over 4 bar utilizing direct injection with water for charge cooling in the cylinder. Having the compounded compressors there will be a delta across the intake during the end of the exhaust event. I feel we have a good handle on the peak delta issue, but the situation dplecko describes is the main reason I've come here seeking some input. I am concerned about valve bounce after intake closing. The pressure waves through our intake system will be dramatic to say the least, and the concern isn't loss of power at intake closing, it's the long term effects of the repeated hammering of the valve seat when exposed to extremely high operating temperatures. Also the affect the induced pressure wave will have on neighboring cylinder's intake valves creating an unpredictable delta. Another consideration is the exhaust valve opening of a neighboring cylinder which will create an impressive yet short lived spike in exhaust manifold pressure that may create an unpredictable delta across a companion cylinder's exhaust valve near the end of it's intake stroke. Does anyone have any logP logV plots that recorded any type of valve bounce? Are there any thoughts on this attempt to re-write the limits of pump gas and water injection?

 

[Lou Scannon]

You're describing territory where OEMs with deep pockets and corresponding resources go cautiously. Good luck!

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

 

[ZephTheChef]

I am interested in the answers to these questions as well...I am running a similar compound boost setup (large turbo feeding an eaton TVS2300 blower) on a Buick 3800, which is a 2-valve engine and has stupid large valve diameters as a result. I am capable of running tremendous boost numbers at lower rpms and am trying to figure out if I really need more spring pressure or if I should really mostly be looking at supercharger boost only when I am calculating unseating forces. I really don't know what the pressure delta looks like in an engine with both a supercharger and a turbo at lower RPMs. It is an absolutely stock cam with a significant gap between exhaust closing and intake opening (like 44.5 degrees if I am correct 180 intake 191 exhaust, 115 LSA). I am primarily concerned with operations below 3500rpms, because the supercharger's bypass door is opened after the turbo has built boost. I am only running compound in the first half of the RPM range.

 

[ZephTheChef]

I should add that I am currently running 105# seat pressure springs, I believe 293# open pressure. Due to still having stock lift, I can shim these springs up to as much as .189" before getting to the manufacturer's recommended minimum open height. Looking for input on whether I should leave them alone or shim them. I would like to see as much boost as possible down low, but am realistically expecting to end up in the 50-60psi range.