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Valve spring pressure and boost

Valve spring pressure and boost

(OP)
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?

RE: Valve spring pressure and boost

First of all, on a turbocharged engine with conventional valve timing, the pressure in the cylinder will never be appreciably less than the boost pressure, because the lowest it can get during the exhaust event is the pre-turbine pressure, which is typically similar to or higher than the boost pressure.

With early Miller intake valve timing, the cylinder pressure will indeed drop below the intake manifold pressure by several psi, depending on the severity of the Miller timing. Of course, with OEM engines engineered for early Miller timing, it can be taken for granted that the valvetrain has been validated to behave itself under all foreseen conditions. Also, obviously, this delta P across the intake valve will occur regardless of the intake manifold pressure, so in this case it doesn't matter if engine is boosted or not.

On the other hand, a mechanically supercharged engine, assuming its exhaust system is worth a damn, will see significant delta P across the intake valve toward the end of the exhaust event. However, this is about the time that the intake valve should be opening anyway, so if it should happen to "blow" open, who would notice? At any rate, I have difficulty envisaging a delta P large enough to overcome the spring force.

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz

RE: Valve spring pressure and boost

(OP)
Thanks for the input hemi that's exactly how I have been looking at it. Even with a supercharged motor like you say just before valve opening is the only time that that delta P should have any chance of doing anything and like you say at best maybe it would just crack the valve open just before the cam did. Even this is a long shot on all but the most heavily built motors. Fixed displacement superchargers don't like high pressure ratios and very few street driven supercharged cars would come close to the kind of pressures required to overcome valve spring force.
On intake valve closing one would hope that the pressure in the cylinder was very close to the pressure in the runner.

So the question remains though is there any possible truth to this myth and any way that stiffer valve springs could have any way of improving performance related to how much boost you are running?
People have reported the stiffer springs fixing whatever problem they were having.
Some of my theories.
Placebo effect. It has been pretty much proven that if you throw time and money at a car you will see, feel or find an improvement.
Something blamed on boost but maybe actually another issue. Maybe they raised their redline at the same time as boost and started floating valves which stiffer springs then fixed?

Any other possibilities?

RE: Valve spring pressure and boost

Other possiblities?

Dynamic lift closer to kinematic lift? Not sure why that's more important for a boosted engine though. Just one more thing, possibly, if you're already in the eeking out more power mode.

- Steve

RE: Valve spring pressure and boost

(OP)
Here is another question.
I have briefly thought about this before but never gave it a ton of my time but one of my customers goes to a technical institute and brought this up to me. His teachers are teaching that boost will cause valve float and that heavier springs are needed for more boost.
This bothered me because it means that either my theory is completely wrong or they are teaching bad theory.

So what are peoples thoughts on this being taught in a technical school?
Is this an old wives tale that rather educated engine builders can believe even though it can't be true?
Or am I still missing a very big piece that could make it bear truth?

My thought is that if it's possible for intake pressure to hold the valve open then the problem that really needs to be addressed is cams or cam timing to reduce or eliminate the delta P. Stiffer valve springs is not addressing the problem but just covering it up.

RE: Valve spring pressure and boost

(OP)
Does anyone have any good combustion chamber pressure graphs?
I just spent a while searching and most only include the combustion cycle. Others that include 720 deg of rotation don't have enough resolution to really see what kind of pressures you would expect during the exhaust stroke.

RE: Valve spring pressure and boost

Measure the seat pressure of your valves.

Measure the diameter where the valve is seated, where it actually contacts the valve seat.

Calculate the area of that circle.

Calculate the pressure difference that would be required to open that valve.

Ask yourself if that is a plausible situation and whether there is any foreseeable condition inside the engine which could cause that.

Supercharging (not turbocharging) trying to open the intake valve late in the exhaust stroke, just before the valve starts opening anyhow, is one foreseeable situation.

On a turbo engine, excessive exhaust manifold pressure during the intake stroke is another one.

RE: Valve spring pressure and boost

(OP)
Still interested to see anything that anyone else has especially if there are graphs of combustion pressure on turbocharged engines.
I did just remember that I have a copy of Scientific Design of Exhaust and Intake systems and remembered that a good chunk of the book does focus on this very subject.
I snapped a few pictures of some of the graphs and thought I'd post them up here.
[img https://fbcdn-sphotos-b-a.akamaihd.net/hphotos-ak-...]

[img https://fbcdn-sphotos-d-a.akamaihd.net/hphotos-ak-...]



If we were to imagine a worst case scenario which would be a supercharged motor with an optimized exhaust we could guess that the intake and exhaust strokes would be similar to what they are here.
On a turbo the low pressure would be much closer to the pre turbine backpressure which is almost always higher than the boost pressure.


With a supercharger running extremely high boost on a motor with big valves and low seat pressure it does seem plausible that the valve could be pushed open by the boost pressure.

The 4AGE that I am used to dealing with has a 30.5mm intake valve or 1.2". Stock valve springs are about 35lb seat pressure so for boost to unseat the static valve it would require a delta of 30.9 PSI.
Granted these graphs are different motors and at much lower RPM than the 4AGE operates but all of those graphs show positive gauge pressure at the point the valve would be opening meaning your boost gauge pressure would need to be well over that 30.9 PSI to overcome the spring.

On the other hand if you had a big two valve V8 with a 2" valve and 35 lb seat pressure it would only take about 11 PSI delta to open the valve so in a supercharged motor with a high enough surface area to spring pressure ratio I can see this being plausible.

The same goes for the valve closing event. The stock 4AGE IVC is 44 deg ABDC.
The last graph shows the expected behavior of the pressure taking longer (IN reference to crank rotation) to hit atmospheric pressure. If the cam is sized anywhere close to proper the valve should be closing as the pressure in the plenum and pressure in the cylinder are the same. Unfortunately I believe these graphs are in reference to atmospheric pressure instead of port pressure but it starts to give us an idea.

Since these graphs are relative to gauge pressure I think it wold be pretty safe to say that cylinder pressure would never get lower than about 6 PSI below pre turbine backpressure.
If you have the perfect turbo with a 1:1 boost to PTBP ratio and are running 40 PSIG boost this means that cylinder pressure shouldn't go much below 34 PSIG leaving a 6 PSI delta between the port and the combustion chamber. This is at the point where the valve should be wide open. The delta would be much less at any other point.





RE: Valve spring pressure and boost

Valve spring force is needed to control accelerations and keep the lifter in contact with the cam. Any pressure differential across the valve (times effective valve area) is subtracted from the spring force and reduce the force available to maintain that contact. For most engines there will be moments during the intake cycle (and possibly the exhaust) where this effect will reduce the rpm required for valve float to occur.

je suis charlie

RE: Valve spring pressure and boost

(OP)
Thanks BrianPetersen
I was in the middle of typing the above doing just that when you replied.

On a turbo motor I supposed I could see the same issue on a motor with large valves and very light seat pressure.
I am less familiar with two valve motors but in a quick search I see seat pressure numbers over 100 lbs.
If you had a 35 lb spring on a 2" valve it would be quite possible for a small turbo to push an exhaust valve open.

Going back to the 4AGE that I know with a 25.5mm std valve and 35 lbs seat pressure you would need a 45 PSI delta to crack open the static valve or even stand a chance of stopping a closing valve.
The thing is that with a turbo motor the pressure in the chamber is going to be somewhere between intake pressure and pre turbine back pressure. It seems unlikely to me that you could ever achieve anything close to that 45 PSI delta even in the highest of boost applications.

RE: Valve spring pressure and boost

(OP)
gruntguru
While this is true it is going to depend greatly on the pressure curve in relation to the valve travel.

It seems like we have narrowed down the likely points of float to be on the intake valve with supercharged motors and exhaust valve with turbo motors.


I am a little weak on fluid dynamics so my first question is whether the force being applied to the valve is directly proportional to the delta P when the valve is open and air is flowing past it.
As the air accelerates past the valve would it not create a low pressure zone around it?

On a supercharged motor if there is a 30 PSI delta between the port and the cyl as the valve is coming to a stop and then returning back downward while the valve is open and air is flowing past it is there 30 lbs of additional force being applied to the valve spring?



On a turbocharged motor on the exhaust valve there is the pressure wave drawing the exhaust pulse out and the intake pulse in. If your cam is timed ideally the exhaust valve will close as that pulse is moving toward the turbo or at least before it has started to move backward.
If that pressure wave has enough time to come to a stop and revert and start applying any significant pressure on the exhaust valve filling the cylinder with exhaust wouldn't it be safe to say that the problem that needs to be addressed is cam timing and not valve spring pressure? This should be long after the valve has hit max velocity meaning that even when the RPM is slightly off and a small degree of reversion hits the valve at the end of it's closing cycle it would not only be facing the spring pressure but the valve inertia.

This would take us back to my original idea that on a turbocharged engine valve float would be a symptom of improper valve timing and not insufficient spring pressure right?




RE: Valve spring pressure and boost

Valve timing is optimised for somewhere in the region between peak torque and peak power - not redline and beyond which is where valve float begins.

je suis charlie

RE: Valve spring pressure and boost

(OP)
Just sticking to the topic of the turbo and exhaust valve.
As the motor spins faster it will have less time for a reversion pulse.
At low RPM there will be more time and it will be more likely for the pressure wave to leave the exhaust and for a returning pressure wave to have time to bounce back in through the open valve. As revs get higher it will be more likely that the gasses will still be flowing out the exhaust valve as it closes. Therefore the higher the RPM the less force holding the valve open. Or if the gasses are still flowing out then it would help force the valve closed.


With supercharging and on the intake valve it would take longer in terms of crank rotation to fill the cylinders so it would make sense that the intake valve would start closing with lower pressure in the cylinder creating a larger delta P. The question is how much? It seems like if you are maintaining a reasonable VE that there should still be a pretty decent amount of pressure in the cylinder.
I wish I could find some better cyl pressure graphs for a motor and RPM more similar to the motors I am used to dealing with.

Does anyone have any further thought on how the pressure would behave with air flowing past an open valve. Like I said I'm not great with fluid dynamics but it seems to me that when the valve is open and when the air is accelerating past the valve that the pressure being applied to the valve would be less than the pressure in the port. Is there any truth to this?

I think it's pretty safe to say that specifically in regards to a turbo motor it is extremely unlikely that pressures will have any significant effect on forces applied to the valvetrain. Does anyone disagree with this?

RE: Valve spring pressure and boost

Valve held open will significantly reduce any pressure difference across it, because the flow would simply try to equalize. Obviously there would still be some pressure across it because of the flow losses and the pressure wave effects - but the only valve in which the normal flow direction is in the direction of opening the valve is the intake valve, and with anything like normal event timing, there is no plausible way for there ever to be any meaningful pressure forcing the valve open at the end of the intake stroke. If there is ... you need a better camshaft; it would be better to simply leave the valve open longer and use that pressure to continue filling the cylinder.

RE: Valve spring pressure and boost

A couple of points.
1. Valve float does not occur near the end of the valve-closing event. During this phase, the valve is approaching the camshaft and decelerating i.e. the valve is accelerating away from the camshaft - no spring force is required at all. Valve float occurs when the valve is accelerating towards the camshaft which occurs during most of the "middle" phase of the valve cycle. The cam profile is designed to take advantage of the available spring force as the spring length changes thus the peak acceleration towards the cam occurs at peak valve lift. (One advantage of pneumatic valve springs is the average acceleration can be much closer to the peak) So valve float is equally likely at any point where the acceleration (blue) curve is negative. 20 lbs of extra (gas pressure) force acting on the valve means 20 lbs extra spring force required to avoid valve float.

[img ]

2. Valve float occurs near the rpm limit (usually beyond) not the point of maximum VE where the camshaft is optimised. At the rpm limit it is very unlikely the valve has remained open long enough for pressures to equalise.

je suis charlie

RE: Valve spring pressure and boost

(OP)
It will not have completely equalized but it should be close. That is my point. There will still be a differential but it should not be big. I feel like you would need to run a massive amount of boost on a supercharged motor to get to a 20 PSI delta as the valve is trying to close. If we are talking Top Fuel levels of boost and performance then yeah at 60 PSI there may be a notable force from pressure acting on the valve.
Then as BrianPetersen said and as I believe, the localized differential between either side of the valve face would be much lower and the overall forces acting on the valve faces should be lower than the differential as measured just a cm away.

RE: Valve spring pressure and boost

What if i said "5 lbs of extra (gas pressure) force acting on the valve means 5 lbs extra spring force required to avoid valve float"?

As an example a small block Chev intake valve is approximately 3 sq in area, so 1.7 psi differential gives 5 lb force. Relevant differential would be the delta of total (stagnation) pressures measured either side of the valve.

je suis charlie

RE: Valve spring pressure and boost

I think Hugh Macinnes in his book "Turbochargers" pretty much dismissed the theory of boost requiring increased valve spring pressure, and offered at least one alternative explanation for perceived valve float.

RE: Valve spring pressure and boost

I have no firsthand experience, but I've read that the symptom of valve float on an engine with hydraulic lash adjusters, i.e. adjuster "pump-up", can just as easily occur with valve "bounce", i.e. when the valve does not seat cleanly upon closing. It is obvious that this behaviour is promoted at high rpms, when acceleration and/or any of the higher derivatives of the lobe profile might exceed the capability of mechanical system to control valve motion.
It is also obvious that a marginal system might be "pushed over the edge", either by excessive rpm, or excessive delta P, or both in some combination. Based on the above discussion in this thread, a mechanically supercharged engine might experience a significant delta P on the intake valve at IVC, and thus be vulnerable to this failure mode. By the same token, the exhaust valve of a turbocharged engine with an adverse exhaust to intake delta P would also be vulnerable to this failure mode. Interestingly, the adverse intake to exhaust delta P is (generally) a positive function of rpm (at least as redline is approached).
From a symptomatic point of view, I doubt if you could tell the difference between intake and exhaust hydraulic adjuster pump-up.

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz

RE: Valve spring pressure and boost

After a little thought I have decided there is probably no effect.

On the same engine with and without boost, there is essentially no difference in pressure drops throughout the system. This is because volumetric flow through the system is unchanged. Mass flow increases with boost but this is due to density increase. The same applies to throttling where VE and volumetric flow rate is constant at any given rpm - what changes is density.

je suis charlie

RE: Valve spring pressure and boost

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.

RE: Valve spring pressure and boost

Hi Crerus,

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

RE: Valve spring pressure and boost

(OP)
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.

RE: Valve spring pressure and boost

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

RE: Valve spring pressure and boost

(OP)
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.

RE: Valve spring pressure and boost

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

RE: Valve spring pressure and boost

(OP)
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.

RE: Valve spring pressure and boost

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/1406-how-to-maint...



RE: Valve spring pressure and boost

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

RE: Valve spring pressure and boost

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

RE: Valve spring pressure and boost

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

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

RE: Valve spring pressure and boost

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.

RE: Valve spring pressure and boost

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

RE: Valve spring pressure and boost

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.

RE: Valve spring pressure and boost

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

RE: Valve spring pressure and boost

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?

RE: Valve spring pressure and boost

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

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

RE: Valve spring pressure and boost

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.

RE: Valve spring pressure and boost

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.

RE: Valve spring pressure and boost

Valve train forces are primarily inertial - it is highly unlikely you will have issues below 3,500 rpm. The worst case would be a small reduction in the engine speed at which valve float or bounce begins.

What is the point of opening the supercharger bypass? Airflow through the bypass will be away from the engine - recirculating through the blower and adding heat. In addition, the turbo compressor has to run at a higher PR and exhaust BP will increase as a result. Reduction in beneficial scavenge follows.

Even if you don't agree, do yourself a favour and be prepared to compare operation on the dyno with and without bypass open.

Lots of info on this forum in the "twincharging" threads.

je suis charlie

RE: Valve spring pressure and boost

So in other words you don't think that under any circumstances boost pressures high enough to mathematically (due to valve area, neglecting to consider what pressure may or may not be in the chamber) lift my valves off the seat would blow my valves open, but I will eventually at some speed earlier than I would otherwise encounter, run up against the high rpm valve float more in the 6-7000rpm+ range, correct?

As far as my setup goes and the logic on that, please correct me where I'm wrong because I am here to learn but here is my line of thinking/how I understand it. The purpose of opening the bypass is to reduce or eliminate the pressure differential across the supercharger, and therefore the work it has to perform and the associated drive losses. It would easily take over 100hp to spin that thing up top...not only can the 8-rib belt I currently have not handle that without slip even if I wanted to run full compound, but my supercharger causes more intake charge heating than the turbocharger does...especially at higher pressure ratios, and frankly, I don't need the extra boost up top anyway so why would I depend on my less efficient power adder there? My turbo will push 55psi if I want it to and my engine should only need around 35psi up top to max out the turbo as well as my fuel system. The recirculated air from the supercharger is not heated other than through contact with the warm supercharger rotors because it has not been pressurized by the supercharger, so bypassing it should not be an issue.

I do have refrigerated air/water intercooling after both the turbo and supercharger as well as direct port water/methanol injection so even if there is some heating during the bypass operation I can handle it, and I would certainly think it would be substantially less heating than taking that already compressed turbo air and compressing it again by a 2.4 pressure ratio with the supercharger. As far as scavenging goes, I have log manifolds going into a turbo and as stated before am nowhere near having any valve overlap...so I'm at a loss as to what kind of scavenging could I possibly hope to achieve anyway? Certainly none from having a higher intake pressure vs exhaust pressure since I am 44.5 degrees from having any valve overlap...and as far as exhaust scavenging goes, I just don't see that happening or having a significant effect either way with logs and a turbo, particularly with such relatively short valve duration.

As far as why I went twincharged and do use it down low in full compound is that it makes the car significantly more predictable and driveable. In turbo only mode it takes a lot higher throttle position to get the car accelerating at a decent rate and when the thing starts to spool it comes on so hard and fast that accidental wheelspin is inevitable. In twincharged mode I can accelerate at a much lower throttle position with a more linear and predictable power delivery...and if I want to go fast, the response is instant and satisfying. The transition when the bypass opens is actually fairly seamless, you can hear the tone of the engine change but that's about it. The loss in boost is mostly made up for by the corresponding loss of the cost of driving the supercharger and the turbo is spooling hard and replaces the lost boost before it's missed.

Every dyno I have seen from twincharged cars it seems they have done the comparison turbo only, supercharger only, and full compound and from what I have seen turbo only always makes the most peak power. I assume this is mostly due to the inherent problems with trying to drive a supercharger on a street car with that kind of power requirement. Now that may change if you needed the extra boost pressure to get enough flow through your engine (say you had a really small displacement relative to your power goals), but trying to get 1000hp out of 3.8L isn't exactly the stuff of legend. That's not to say I don't intend to find out for myself and what works on my setup, but my opinion is that if your most efficient power adder can make the goal power on its own then that's the one you should use. Can I make more peak power with both in full compound? Provided that I keep the charge cool and keep the belt on the supercharger and don't demolish any engine parts then I would say the answer is obvious, yes I will make more power with more pressure available at the intake valves. But frankly, if I wanted more peak power I would run a bigger turbo...

The purpose of the supercharger for me is simply to provide me with an instantaneous 2.4 PR of boost to enable me to spool a very large turbo very quickly, as well as having a driveable powerband when I am below the boost threshold of that turbo...and it does both of those things extremely well. The purpose of my question in this thread was really that I am intending to ramp up the duration and pressure of the compound boost portion of my car and I was curious how that might affect my valve spring requirements.

RE: Valve spring pressure and boost

Most high-performance valve springs are designed with little concern for long fatigue life. Increasing the seat force of your existing springs by reducing the installed height means they will be operating at much higher stress levels. And this will reduce their fatigue life.

I would agree with gruntguru that valvetrain inertias are the primary consideration. So if your existing valve spring setup is performing OK, leave it alone. A failed valve spring can result in a very expensive repair bill.

RE: Valve spring pressure and boost

Thanks. They have been fine up until this point but I was intending to raise the boost significantly when I get the new engine in. I will leave it alone unless I run into problems.

RE: Valve spring pressure and boost

Zeph. Thanks for the well-reasoned response regarding bypassing. A few points.

Drive torque is mostly a function of delta P (supercharger) so shouldn't increse much further as rpm increases. Of course belt slip is also a function of speed (centrifugal tension unloading the pulley-wrap area) but this is less pronounced on multi-rib vee belts. Your drive must be pretty marginal at low revs if it slips at higher revs.

If your bypass reduces supercharger drive power by say 100 hp, the increased exhaust back pressure will cost some power in pumping losses alone. The big loss however, is combustion quality due to reduced scavenge (or increased passive EGR).

A positive delta P (engine) is highly beneficial (especially in boosted engines). "Scavenge" should be considered as a variable that exists on all engines and ranges from negative values (passive EGR) through to positive. Any change in delta P will change the "scavenge" value.

Note: Not advocating increased boost here - all the above assume same boost - with and without bypass.

je suis charlie

RE: Valve spring pressure and boost

Good point. I had never thought of scavenge in terms of relative pressure and had always seen it referenced in terms of the exhaust pulses literally pulling vacuum on adjacent emptying cylinders, but thinking about it in terms of the residual exhaust and chamber pressure vs intake pressure it makes sense even without valve overlap.

I am not sure I quite agree with your drive torque, the equation I have seen is (.00467hp * cfm airflow * psi boost)/compressor efficiency %. That makes airflow through the supercharger and pressure ratio equal influences on the drive requirements, and since for all practical purposes at a given boost compressor flow on a roots supercharger is linear with RPM, the increase in drive requirement is also very linear with RPM. To answer your question, yes my setup is pretty marginal since the supercharger I am using has a 10-rib drive from the factory and I am making due on an 8-rib as well as significantly higher pressure across it (smaller displacement engine). I have been strongly considering adding auxiliary iders or tensioners near the snout to improve belt wrap.

With that said, I was also planning on setting up a more progressive means to control the crossover from supercharger to turbo, so if I can come up with a valve that has a much more variable lift to recirculate the supercharger boost I could possibly experiment with different combinations of supercharger and turbo boost up top to see what has the best results. I would like to do this with a large external wastegate, but am having trouble finding one that can be configured to operate on direct pressure inputs that high (they all seem to require boost controllers above about 25psi).

Part of my difficulty with this build is that I want the car to remain a daily driver, as well as continue to be as inconspicuous as possible. Suffice it to say, it's a sleeper, so that's been my big hesitation to get a cam, or do any number of other mods that would make life easier but give away the car's secrets. I love the fact I can start the car up on a freezing morning and it sounds and idles 100% stock smooth.

RE: Valve spring pressure and boost

"0.00467hp * cfm airflow * psi boost)/compressor efficiency %."

Looks more like a formula for power than torque.

je suis charlie

RE: Valve spring pressure and boost

You are 100% correct, it is a power figure based on blower RPM (or in this case CFM which is directly tied to RPM) and torque requirements are the same across the board. However, I was under the impression it was this HP demand that causes belt slip, not the torque. Am I incorrect that HP is a better representation of work being done whereas torque lacks the context of RPM and thus what amount of work can be done by/gearing advantage applied to that instantaneous torque? Obviously my gearing on the supercharger is fixed during a particular run and the limitation becomes what amount of force the belt can apply to the supercharger pulley without slipping. The question becomes is that dependent on the torque requirement, or the HP requirement? I am operating on the (possibly false) assumption it was based on HP requirement...and have come to that conclusion simply due to the fact that SC belt slip seems to occur at higher RPMs rather than across the board with a certain pulley size.

Or am I mistaken and is it simply the added inertial loads from accelerating the supercharger that is causing the slip at higher rpms, not necessarily the higher HP draw? If that were the case it may not be present at a fixed load even at the higher RPM? I'm not trying to be argumentative, I am just trying to understand.

RE: Valve spring pressure and boost

Belt slippage will occur at a given torque not power. Speed also has an effect due to centrifugal force lifting the belt away from the pulleys (as mentioned above).

je suis charlie

RE: Valve spring pressure and boost

Good to know.

RE: Valve spring pressure and boost

This reminds me of the perennial "which is more important - torque or horsepower?" question. At a certain fixed RPM I would think that the higher the HP - the higher the torque. So you could say just as easily that slippage is power-dependent?

RE: Valve spring pressure and boost

The discussion was more to do with the influence of speed on slippage. OP says the belt doesn't slip at high boost/low speed but he is concerned about slippage at same boost, higher speed (ie higher blower power).

If boost is proportional to blower torque (true if efficiency is constant), blower torque will remain constant as speed increases. Slippage is a function of torque (not power) so the tendency to slip will remain constant (neglecting centrifugal tension loss).

je suis charlie

RE: Valve spring pressure and boost

I wasn't actually disputing that torque was the controlling factor in determining slippage - just that almost invariably in torque versus power arguments it is power that is shown to be the controlling factor - not torque. Clearly if you did a static test of slippage in a pulley/belt system by just increasing the loading of the driving wheel (without the pulleys turning) it could only involve torque. I presume that in a similar way testing a chain/sprocket system statically by increasing the torque until the chain broke also demonstrates that a chain system's ultimate failure is dependent on torque not power. Similarly it would follow that with a pair of gears it is not the amount of power transmitted but the torque that determines the failure point of the gear teeth.

RE: Valve spring pressure and boost

Just read back thru some of the recent posts. Is ZephTheChief serious about installing a turbo-supercharged V6 engine using over 2atm boost in a 4-door Buick sedan? And using it as a "daily driver"?

RE: Valve spring pressure and boost

Pretty darn serious. I've been slowly upgrading the car over the last 4 years. Cuz why not? It's been twincharged for about a year now, fighting one hurdle after another. I had finally gotten all the major bugs worked out and was tuning and turning the boost up when it popped a rod at around 30psi. I had installed a stock LeSabre engine as a temporary fix after an intercooler leak filled my engine with water while I was on vacation for a week. It was a higher compression (9.4:1 vs 8.5:1), as well as has super wimpy connecting rods by comparison. I knew it was only a matter of time. I already have the supercharged bottom end freshened up and ready to swap back in as soon as I get the heads finished. It should be back on the road by the first of the year so I can continue turning it up until something else breaks.

RE: Valve spring pressure and boost

Sounds very cool but popping rods and stuff breaking is not what most peeps equate with a "daily driver". wink

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

RE: Valve spring pressure and boost

Well I would tend to agree with that. But it's not always going to be breaking stuff. The L32 bottom ends have seen 800+whp, so I don't think it's a stretch at all to expect some reliability out of it once I get the supercharged bottom end back in. It's really not like I will be able to push it much harder than I already have on the street anyway...traction is slim anyway. It was my daily for over a month while it was running this time, and will be again once the new engine is in.

RE: Valve spring pressure and boost

Isn't that car a front wheel driver? What sort of transmission will you use to handle 800+hp?

RE: Valve spring pressure and boost

Yes, it is. I've already swapped in a 4t80e from a Northstar-equipped Caddy, with a limited-slip differential and bumped shift pressures. It would still destroy that trans if I had traction and was launching...but when would I ever have standing traction with that amount of power? I don't really have plans to go to the drag strip. There aren't a lot of guys running those transmissions yet (it's a bit of an involved swap) but from the couple big players that have blown them up, they seem to be good for at least 600+...and I can buy replacement 4t80e transmissions around here for $2-300 all day long so no huge deal if I do wreck a transmission. Plenty of Caddys rotting in salvage yards with bad engines and good trans.

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