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?

 

[gruntguru]

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

 

[ZephTheChef]

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.

 

[tbuelna]

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.

 

[ZephTheChef]

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.

 

[gruntguru]

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

 

[ZephTheChef]

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.

 

[gruntguru]

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

Looks more like a formula for power than torque.

je suis charlie

 

[ZephTheChef]

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.

 

[gruntguru]

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

 

[ZephTheChef]

Good to know.

 

[BigClive]

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?

 

[gruntguru]

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

 

[BigClive]

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.

 

[tbuelna]

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"?

 

[ZephTheChef]

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.

 

[Lou Scannon]

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

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

 

[ZephTheChef]

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.

 

[tbuelna]

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

 

[ZephTheChef]

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.