Hi.
At the start, sorry for my english. I hope you will understand what's my point.

I have 2 suggestions.
First one is to change TBO's engine characteristics.
The way they deliver power is inaccurate to type of car they are.
Turbo spool up is waaay to long for street car (as they are now)
They have max boost (0,7-0,8 bar) at around 5k rpm and delivers it to the rev limiter.
In normal street car this kind of boost can be delivered at arround 2,5-3k rpm till 5-6krpm when its dropping down because of the turbine efficiency.
What is the point to have the turbo pumping from 5k rpm till 7 when we need to change gear because car is no more accelerating?
It should be from 3k till 6 and dropping down, so at 7k rpm it should have 50 or 70% of the max boost, and after that rev limiter should hit at around 7500... but that second improvement....

Second thing is to lower the rev limiter in cars (not GTR's and formula's).
I know it has been suggested.
All cars should have rev limiter about 500rpm over the redline.
In the normal engine on too high rpm valves will hit pistons, and engine could blew up (or sth). They're not prepared for that kind of engine speed.
Try to downshift from fifth gear to second at 140km/h (86mph) in real car
At the moment in LFS there is some kind of damage when you rev up engine to much. I think it should kill engine more than now.
I would be happy if the engine could "turn off" after that

Implementing this things will be another step closer to reality
Cheers!

EDIT: changed title, rev limiters are added

I talked about this lousy turbo behavior and lack of sensible 7500rpm rev counters and realistic powerbands in TBO cars for a long time. Yes, it is and still remains one of the most blaringly obvious and neglected physics flaws in LFS, unfortunately.

The current clutch diffs with realistic behavior thanks to clutch preload had really fixed up the handling and cornering equation to a very large degree, Based on what Scawen said in some other post, these turbo engine problems will be fixed in some forseeable time, though it could be quite a few weeks or even some number of months before this actually happens.

I will bump my old thread because nothing except rev limiter was done with TBO's.
Made spool-up/boost chart:
http://img42.imageshack.us/img42/6618/preasure.jpg

As for now LFS is limited to "max boost" setting. Boost map is same for all cars, doesnt matter is it TBO or GTR or MRT, they all deliver boost/power in same way, and they shouldnt!
As you see on the chart FXR with "bigger turbo" deliver allmost 0.8bar@3k rpm, and at the same time FXO with small fast turbo deliver only 0.3bar of boost. Actual boost of FXO should look like i specifide with blue dots because its a road car, not GTR, which is driven only on high rpm.
If you think logicaly, why wouldnt we want to put in our road car big turbo from FXR which will pump max boost (0.8bar) before 3k rpm (instead of 4.5k).
TBO's should be fast even at 2.5k rpm. I dont know ANY road cars that have such a terrible spool up with so low boost.

I drove stock and modified Audis S2 2.2T, S4 2.7bi-turbo, RS4, and all were pulling like crazy at low RPM having at least 0.8bar of boost.

Scaven if you read this. Since you are working on "non-compatible" patch, maybe you could add to the code "boost map". After that TBO's should be more of street cars, and not race-like cars without ALS

Cheers.

I believe Scawen mentioned that the current whole engine simulation is mostly a substitute for what is to come at a later stage ("stage" not to be taken literally ). If I'm not mistaken the engine simulation - at least on the user end of things - didn't change since the days of the first demo. Thus I'm very sure that Scawen is well aware of how things should work. I base this statement on the days when Scawen discussed the sound system and said that it can't be improved until the engine simulation is sophisticated enough to simulate everything that makes a sound.

But that's not the point I was trying to make with this post.
I'd like to know wether you have any more information on exactly how engines start to choke above their red line. I realize that the valve springs not being able to pull back the valves in time before the cylinder smashes into them in a rather painfull manner is one good reason for the engine to very aprubtly lose power, but I am under the impression that this is not the only factor that influences things.
I'm especially interested in what manner and for which reasons the engine torque at large rpms diminishes rapidly. I assume the fuel pump and injectors should be able to hold up on most cars and being a gasoline engine I'm sure the time needed to burn the fuel cleanly shouldn't be an issue either. What other factors influence the loss of power beyond optimum rpm?
I know for example that my own car choked itself to death just few rpms above the redline while I know from some semi-professional racers that a stock Porsche 964 engine will easily go a thousand rpm above what Porsche specified as healthy for it. It won't deliver a lot of power, but it saves you two shifts between corners. I'd like to get a better understanding on the effects involved in this.
Thanks for any replies.

Vain

The loss in power after redline is caused by a number of factors, that can be different for each car.

The first factor is valve timing, there isn't one value for this that will give optimum power at all speeds. If the valve timing is advanced it gives you more power at high RPMs and potentialy higher RPMs. Retard valve timing and the reverse happens, you get more power at lower RPMs. This is of course limmited by how far you can go in each direction before you cut into the clearance or hamper either end to the point the engine won't run outside of the optimum speed range.

The next factor is spark timing, its basicaly the same thing, go too far either way and it will hurt the engine and its performance. However I do have some first hand experiance on what advancing it can do.

I advanced the spark timing on my dirt bike by a small amount, just enough so that it had a very slight change in idle performance, but nothing realy bad. The bike has a basicaly flat power curve from 4k to 10.5k RPM with the stock configuration. Now it has a noticable increase in power and the power starts realy comming at around 5.5k. It's like a completely different bike. However it is still limmited to a 10.5k redline by the first and last factor.

The intake/exhaust system is the last factor in the power drop off after redline. For the most part it won't cause a complete stop in unloaded acceleration, but it will drop the power enough to slow or even stop the acceleration of the engine when loaded. Both intake and exhaust have a limmit to how much air they can flow through. Once they start to hit that limmit the resistance will become enough to actualy cause a power drop off as the engine starts to run out of air.

Quote :

I'm especially interested in what manner and for which reasons the engine torque at large rpms diminishes rapidly.

The reason is simple. Stock cars are made for low/middle range rev torque

Car is comming from factory with turbo that pulls as low as possible, but its big enough to give about 70% of boost at max rpm. If you check factory claims, most turbo cars have biggest torque value at about 2-3k rpm.
Its because factory turbo has very fast reaction time (spool up), great for normal driving, low emission of CO2, great fuel consumptions etc.
Small turbo (in road car) can create a lot preasure at low rpm, and higher you go with rpm, then more small turbine is blocking the exhaust gasses.
Higher rpm equals higher requirements for air, and turbine cant pump more, and more air infinitly(?).
If you push "power button " at low rpm (lets say 2k rpm), than turbo is starting to pump more than you need, lets say 1.4 bar, you need only 0.8 bar, so you throw rest of it (0.6 bar) to air, or to exhaust, to slow down the turbine and not have such high overboost.
When we drive thru rpm's (5k) we have still 0.8 bar, but turbine is pushing max air as it can, and at bigger rpms turbo cant pump such big preasure, so preasure drops, and engine is not creating much torque

If you take bigger turbo than everything will be same, but moved later at rpms. So you would have max boost later, and boost drop would be later.
Its all depend from turbo. Stock cars have normal turbos, normal bearing, but when you tune your engine you can buy bigger turbo with ball-bearing. It will pump more at low, and wont block gases at high rpms, but you wont make with this turbo big distances (100k miles )

If you really want to know more read Corky Bell - Maximum boost
http://rapidshare.com/files/108424216/Maximum_boost.rar

Quote :

I assume the fuel pump and injectors should be able to hold up on most cars and being a gasoline engine

Injectors usualy are able to flow about 30% more than stock. Fuel pump same. If those 2 are not capable of filling cylinder with needed amount of fuel, than you will have poor AFR (less than 11), temperature of gasses will be extremly high, and you can burn your engine.

PS. sorry for lousy english, but when it comes to describe something technical im total noob

PS2. everything what DragonCommando is true. Good that he describe that.

A stock turbo will provide max rated boost right to redline.

0.8bar (11.6psi) is not stock for any car I know of. If that is being produced by a stock turbo it will not stay that high for the entire speed range of the engine.

My 6.5L turbo diesel runs at 9psi, it stays there from spool up all the way to redline. And the turbo is stock and reletively small. If I wanted more power I could raise the boost but it would drop off at higher RPMs.

I think the problem with the TBO cars is that the turbos are simulated as being high performance even on the stock cars, so they are able to maintain the boost at higher RPMs. So even though the car/engine is said to be stock, it realy isn't.

I think the boost on the XRT in particular should be dropped to 6.5psi which is the stock boost for the Mitsubishi Starion Turbo, the car it is based on.

Thanks a lot for the answers. I still have a few questions though:
DragonCommando: Does the spark timing on your bike vary with engine speed?
I know that usually spark timing is given in crank shaft degrees past top dead point of the piston. If this was constant for the whole range of engine speeds then engine speed should have no major impact on the efficiency of the combustion because, at least I assume, that the time needed for the combustion is largely smaller than the sort of durations we talk about regarding the movement of the piston and thus the rate of change of the combustion chamber.
I think I remember that spark timing normally moves away from top dead point with higher engine speeds. So if executed properly this shouldn't cause a significant loss of power with high rpms.

I'm thinking of another factor though. The spark plugs also need to be charged in time for the next ignition, which gets progressivily more difficult with high engine speeds due to the large currents involved. A too low voltage in the ignition system will cause no or bad ignition and can thus be a reason for a very large loss of power within a rather small range of engine speed.

@Simpson:
I assume you attribute the loss of turbo pressure at large rpms to the upper limit of turbine speed due to friction? High engine speed means large a amount of air is sucked in, but at high turbine speeds the turbo generates a lot of friction which may disable it from providing the engine with enough fresh air to maintain maximum boost pressure. Am I understanding this correctly?

Vain

Quote from DragonCommando :

My 6.5L turbo diesel runs at 9psi, it stays there from spool up all the way to redline. And the turbo is stock and reletively small. If I wanted more power I could raise the boost but it would drop off at higher RPMs.

Problem with diesel engine is that it has shorter rev range, and quite high boost (all VAG 1.4-3.0TDI), they also have Variable Turbo Geometry, so it pulls great at low, at not bad at high (4.5) rpms.
In petrol engines its almost same, but.. you have still 2-3k rpm range left.

Quote :

0.8bar (11.6psi) is not stock for any car I know of.

Earlier mentioned Audi S2 has 0.8-0.9 bar boost max (at 2.5k rpm), turbo K24-7000, dropping to ~0.6bar at 7k rpm. After reflash of ECU can pull 1.3 bar, dropping to 0.8-0.9 at 7k rpm.
S4 2.7 biturbo has similar boost at stock, but it dont remember what boost has at end of rev range. All i can say is that it pulls to the end with little bit boost drop.

Quote :

I think the problem with the TBO cars is that the turbos are simulated as being high performance even on the stock cars, so they are able to maintain the boost at higher RPMs. So even though the car/engine is said to be stock, it realy isn't.

Yeap, it looks like they have realy big hot side of the turbo, so the spool up is very weak and dont block exhaus gasses at all, and small cold side, so it cant flow such big air mass.

Quote from Vain :

@Simpson:
I assume you attribute the loss of turbo pressure at large rpms to the upper limit of turbine speed due to friction? High engine speed means large a amount of air is sucked in, but at high turbine speeds the turbo generates a lot of friction which may disable it from providing the engine with enough fresh air to maintain maximum boost pressure. Am I understanding this correctly?

Not quite that.
Turbo dont generate much friction, because it would killed itself quickly (its revs to 110-150k rpm!!)
The reason why preasure drops is because cold side of the turbo cant create such presure (push such high mass of air) engine would suck. If it could it would be almost "perpetum mobile".

As you asked about ignition timing, its depend is it electronic controlled or mechanicaly. In first one you can change ignition time because signal goes from ECU to spark plugs (thru) ignition coils. And in Mechanical controlled you have same ignition time as it goes thru distributor (here is link to wikipedia: http://en.wikipedia.org/wiki/Distributor )

Quote from Vain :

@Simpson:
I assume you attribute the loss of turbo pressure at large rpms to the upper limit of turbine speed due to friction? High engine speed means large a amount of air is sucked in, but at high turbine speeds the turbo generates a lot of friction which may disable it from providing the engine with enough fresh air to maintain maximum boost pressure. Am I understanding this correctly?
Vain

Quote :

Not quite that.
Turbo dont generate much friction, because it would killed itself quickly (its revs to 110-150k rpm!!)
The reason why preasure drops is because cold side of the turbo cant create such presure (push such high mass of air) engine would suck. If it could it would be almost "perpetum mobile".

Yeah this is right; but to answer that more in depth for Vain: turbines have a narrow efficiency range where they pump well. In essense they are really one-speed devices, and you need to match the efficiency range on the impeller with the demands of the engine. So the turbo that works well from idle to 4K typically "runs out of breath" because it's already past it's efficiency range while delivering enough boost to be usable lower in the engine's rev range. IE; the impeller like to be at certain RPM to pump well, and that particular rate cannot match (with old school systems anyway) every situation the engine encounters.

@BBT: What you likely mean is that the geometry of the impeller is optimized for a specific ratio of gas velocity to impeller speed so the impeller blades are hit by the exhaust gasses head-on for maximum efficiency. Is that right?
That implies that if I wanted to I could equip a turbocharger with an impeller that works best at 4k rpm (the actual design factor would be the volume of exhaust gasses per second), but beyond 5.5k rpm the impeller would mostly stop converting energy from the exhaust gasses to the propeller but rather act as a block in the exhaust system because it's geometry doesn't at all fit the current situation (in this situation it'd only move due to it's friction against the exhaust gasses, not by expanding the hot gas).
On the other hand I could use an impeller that works at a high efficiency at 7k rpm, but I wouldn't be able to get the turbocharger to rotate very much before 5k rpm.

Is that train of thought correct?

Vain

Quote from Simpson :

The reason is simple. Stock cars are made for low/middle range rev torque

Car is comming from factory with turbo that pulls as low as possible, but its big enough to give about 70% of boost at max rpm. If you check factory claims, most turbo cars have biggest torque value at about 2-3k rpm.
Its because factory turbo has very fast reaction time (spool up), great for normal driving, low emission of CO2, great fuel consumptions etc.
Small turbo (in road car) can create a lot preasure at low rpm, and higher you go with rpm, then more small turbine is blocking the exhaust gasses.
Higher rpm equals higher requirements for air, and turbine cant pump more, and more air infinitly(?).
If you push "power button " at low rpm (lets say 2k rpm), than turbo is starting to pump more than you need, lets say 1.4 bar, you need only 0.8 bar, so you throw rest of it (0.6 bar) to air, or to exhaust, to slow down the turbine and not have such high overboost.
When we drive thru rpm's (5k) we have still 0.8 bar, but turbine is pushing max air as it can, and at bigger rpms turbo cant pump such big preasure, so preasure drops, and engine is not creating much torque

If you take bigger turbo than everything will be same, but moved later at rpms. So you would have max boost later, and boost drop would be later.
Its all depend from turbo. Stock cars have normal turbos, normal bearing, but when you tune your engine you can buy bigger turbo with ball-bearing. It will pump more at low, and wont block gases at high rpms, but you wont make with this turbo big distances (100k miles )

Nice in theory, but irl not always true. Nearly all turbo-charged roadcars can be tuned to much higher performance levels. This can only be done because turbo' s are often oversized and overengineerd to make sure it is still reliable in case of poor maintenance by the owner.(although it will still last less long, but the engine/turbo will usually still be able to do over 200.000km). On road cars, it is really the ecu which determines how high a turbo must spool up at any given rpm and throttle position.

My current car it is in the extreme, because volvo used software to create a low power 2.5 diesel(2.4d) and a very nice higer powered 2.5 diesel(d5) with exaclty the same parts on the car to serve different market demands. So i tuned mine to the higher(d5) power version. but if the gearbox would be a lot stronger i could achieve the following by just software; : 130HP->198HP @4000rpm and 280HP -> 435NM@2200rpm. This is only possible because the fitted turbo is much better and overengineerd then needed to make it very reliable and durable. However, since the gearbox will fail within a year with so much power, i can' t make use of the extra headroom However i did run the the car with this tuning for a week, it was ehm very fast. No youtube movie of volvo d5(tuned or not) could beat my car in terms of accelaration. Since i don' t like buying gearboxes every two years, i had the ecu programmed back to D5 values.

So the powerdrop at higher rpm's with turbo-charged road cars is usually not caused by a turbo being too small but for other reasons. Generally, on road cars turbo' s are much better/bigger/stronger then needed and kept a bit down in terms of boost by the ecu. To get 20-30% extra power and torgue you usually don't need to fit a new turbo. The extra boost, if ecu is reprogrammed properly(very important, bad programming will break everything real fast) will not hurt drivability in anyway. The real challenge is, to get to know if all parts in the drive line are able to cope with the extra torgue of the engine.

In terms of reliability i was taking about ball-bearing turbos. They're not the same what you have in stock car.
As i said earlier, diesel engines have turbos with VTG, and quite short rev band. So creating flat torque curve is quite simple. How often did you see flat torque curve in petrol car? Ofcourse you can achieve that by changing boost at higher rpms, without doing same with lower rpms, but its not the point of reprogram the ECU.

The reason why factory is putting bigger turbo is to increase reliability, bigger turbo with lower boost = less material wear, colder air intake etc. In example, VAG 1.8T
150Hp = K03, 180Hp = K03s, 210Hp = K04, 225Hp = K04, on each turbo you can make additional +~30-40Hp, but that will be 100% of the efficiency turbo could give. Don't compare diesel cars to petrol cars in terms of turbocharging, Diesel turbo won't work that good in petrol and vice-versa.

It seems that all "production cars" in LFS are in fact slower race cars. Have you ever drived a "family car" with so little torque at low rpm ?

Quote from Keling :

It seems that all "production cars" in LFS are in fact slower race cars. Have you ever drived a "family car" with so little torque at low rpm ?

Hear, Hear.
Except for the engine in the FZ5, which can only be described as epic, torque wise.

Quote from Vain :

DragonCommando: Does the spark timing on your bike vary with engine speed?
I know that usually spark timing is given in crank shaft degrees past top dead point of the piston. If this was constant for the whole range of engine speeds then engine speed should have no major impact on the efficiency of the combustion because, at least I assume, that the time needed for the combustion is largely smaller than the sort of durations we talk about regarding the movement of the piston and thus the rate of change of the combustion chamber.
I think I remember that spark timing normally moves away from top dead point with higher engine speeds. So if executed properly this shouldn't cause a significant loss of power with high rpms.

I'm thinking of another factor though. The spark plugs also need to be charged in time for the next ignition, which gets progressivily more difficult with high engine speeds due to the large currents involved. A too low voltage in the ignition system will cause no or bad ignition and can thus be a reason for a very large loss of power within a rather small range of engine speed.
Vain

Charge time is rarely a problem on a healthy coil, unless it is damaged it will charge much faster than is needed.

My bike has mechanical timing advancement, the timing is adjusted above base advance by a spring loaded centerfugal advance system. This is most comonly seen on older bikes and cars with distributor caps.

I'm not sure exactly how to explain why timing advance can't simply keep the timing perfect for the whole range. But I'll give it a shot anyway.

It has to do with the actual burn rate of the fuel, in order to provide maximum power the fuel has to burn right from TDC to BDC. However, it has to be ignited before TDC so that it is combusting completely during that time.

The rule is that it always has to finnish burning as close to BDC as possible at idle, if it doesn't the engine won't run right since compression will be able to somewhat defeat the power stroke.

As engine speed increases the fuel will burn at the same speed but timing has to advance so that it always finnishes before BDC. this means it will start burning closer to TDC untill it gets to the point where it is spinning so fast that you can no longer advance the timing or it will burn before TDC.

When you hit the sweet spot, where the burning takes place directly between TDC and BDC, and burns almost the entire way through the stroke, you should be inside the power band. You simply do not need the timing to be perfect anywhere after that. And it can't be no mater how much you advance or retard it. The burn rate is simply too long at that point.

This is largely dependant on fuel, if you put high octane in a car, unless you set the timing for it, you will actualy not see any increase in performance, it simply burns faster and it will just run cleaner. Most modern cars with electronic ignition actualy have a seperate ignition timing map specificaly for high octane fuel. the engine can actualy detect what fuel is going into the engine based on it's burning characteristics and will switch to the appropreate ignition and injection maps for it.

This is why high reving engines need hotter and faster buring fuel, if you put E85 in a formula 1 car it simply would not run right, and may not even rev high like it should. It will simply lose to much power because the fuel won't burn quickly enough to finnish before BDC.

My bike comes from the factory with performance timing, but not aggressive timing. I advanced it to be slightly more aggressive and I lost performance down low, but gained performance up high. If I wanted I could re-spring the mechanical advance and get it to run aggressively over the entire range, but it would still need high octane to rev higher than 10.5k without damage, not to mention stronger internals.

Most stock factory cars will have the timing set so they run smooth over thier entire rev range right up to redline. This is not optimal because the engine isn't going to hit that perfect sweet spot at the power band because that would make it run aggressively, which is often seen as running rough by people who don't understand it.

I could also get into how the shape of the pistons effects how the engine revs and how high it can rev. but I would be writing way to much for one post.

Quote from Degats :

Hear, Hear.
Except for the engine in the FZ5, which can only be described as epic, torque wise.

Oh, I missed it.

FZ5 is a production sports car. ( Porsche 911 or Ferrari 550M ? ) Other production cars in the game are more likely to be "family cars".

At least, UF1/XFG/XRG should perform in the way of common family cars.

Do we need more "tuning feature" for the TBO Group ? The TBOs we have now look like normal production cars, but they just don't behave in that way. Will you feel better if they look like lightly tuned race cars so they seem to behave in the right way ? (still not completely right, but much closer)


There is a big gap in the LFS car list now. Factory stocks, then immediately professionally-tuned racers (GT-Rs) with slick tyres fitted, nothing in the middle. It's strange.

Quote from DragonCommando :

0.8bar (11.6psi) is not stock for any car I know of. If that is being produced by a stock turbo it will not stay that high for the entire speed range of the engine.

The Saab 9-5 2.3T has a max boost of 0.8bar stock

Quote from Vain :

@BBT: What you likely mean is that the geometry of the impeller is optimized for a specific ratio of gas velocity to impeller speed so the impeller blades are hit by the exhaust gasses head-on for maximum efficiency. Is that right?

Well, I was specifically talking about the impeller side not the turbine side. The turbine side works much less through kinetic transfer simply based on exhaust flow rate than it does allowing the exhaust to expand through the housing. That is primarily where the energy is dumped into the turbine; thus the pressure differential across the exhaust turbine is the primary determining factor of how much energy is put into it.

Quote :

but beyond 5.5k rpm the impeller (you mean turbine) would mostly stop converting energy from the exhaust gasses to the propeller but rather act as a block in the exhaust system because it's geometry doesn't at all fit the current situation (in this situation it'd only move due to it's friction against the exhaust gasses, not by expanding the hot gas).

Ah yes I see what you're saying now, sorry, that's basically right. But in my post I was speaking soley for the compressor (impeller) side of things. Obviously everything has to match properly - the flow characteristics of both sides of the engine have to match both sides of the turbocharger to acheive what you want out of it.

Quote :

On the other hand I could use an impeller that works at a high efficiency at 7k rpm, but I wouldn't be able to get the turbocharger to rotate very much before 5k rpm.

Is that train of thought correct?

As a generality yes, if I understand what you're saying.

Quote from Feffe85 :

The Saab 9-5 2.3T has a max boost of 0.8bar stock

Yeah, and the original Dodge SRT-4 was around the same. and would hold that pressure from around 2K to 4.8K, with a slight drop from there to redline.

All of that is realy interesting, and news to me.

I have never seen a stock turbo push more than 10psi and do it without dropoff. If auto makers are running the turbos to the point of drop off they can't be very well matched turbo chargers.

If they intend it to do 12psi and it doesn't stay there to redline there is something wrong. That turbo is running above its intended max speed.

The waste gate is controlled by manifold pressure, if the manifold pressure drops off the waste gate will close to make the turbo spin faster to maintain pressure. If the turbo simply can't maintain the pressure the waste gate will close and the turbo will spool up faster than intended.

At least that is the logical course of events, unless automakers have figured out a way of capping turbo speed without using the waste gate.

Smart is running the Smart Roadster with up to 1.0bar.

Remapping it, will allow it to run with up to 1.3-1.4 bar continously until redline. Maximum it can do is about 1.7bar (depending on weather conditions etc). The problem with small turbos is not that they are braking the exhaust gases that much. The bigger problem is that they are simply not able to deliver and compress more air at very high rpms. The extremly fast exhaust gases will cause the turbo to over-rev. Of course this only happens to engines with a ECU remap and of course they are running outside of their specs..

What many manufacturers are doing these days (on petrol cars) is to use small turbos on small engines (downsizing). Using big turbos is not really being done on "normal" street cars since big turbos cause big lags and modern small turbos are (combined with direct injection etc) very effective.

Not to mention that small turbos in use today can rev up to 250.000 - 300.000rpm!

//Edit:

Quote :

If they intend it to do 12psi and it doesn't stay there to redline there is something wrong. That turbo is running above its intended max speed.

That's the point.

The stock turbo for an engine might drop pressure, but only because the map is defined to do so (e.g. to keep a clean torque line). Everything else means that the turbo is running to fast or there could be another problem (like cracks in the exhaust manifold etc).

Quote from DragonCommando :

I have never seen a stock turbo push more than 10psi and do it without dropoff. If auto makers are running the turbos to the point of drop off they can't be very well matched turbo chargers.

It seems like you just said two different things?

I'll assume your first sentence was meant to actually convey that you've never seen some pressure loss at high RPMs on a stock vehicle, and also never seen a stock vehicle push more than 10lbs - correct?

Quote :

If they intend it to do 12psi and it doesn't stay there to redline there is something wrong. That turbo is running above its intended max speed.

Not really, it just means that the turbocharger they used has an efficiency range more suited to lower engine speeds for the engine it's mated to. I have no experience w/ the Saab, but the SRT-4 engine is just a torque beast. The torque curve is basically flat for a wide range of RPM and it has serious torque at low revs for a 4cyl. The compromise is that there is some pressure loss at the top of the rev range. They could, instead, allow higher pressure than they do and deal with a greater % of pressure loss as engine speed climbs. I'd have to double check, but I think that part of one of the OEM upgrade packages does just that as part of the package.

Don't forget that engine speed is climbing, not staying constant. It's possible that given enough load to stop the engine from increasing in speed, the compressor might flow enough air to fully pressurize the manifold - at least that's probably true at the point where it first starts to lose pressure. The impeller won't just continue to push more and more air at the same increasing rate indefinitely.

Quote from Ball Bearing Turbo :

It seems like you just said two different things?

I'll assume your first sentence was meant to actually convey that you've never seen some pressure loss at high RPMs on a stock vehicle, and also never seen a stock vehicle push more than 10lbs - correct?

That is correct.

By stock vehicle I mean no changes made. If the turbo is modified (ie waste gate respringing) I don't consider the vehicle stock anymore.

What I mean is that I have never seen a completely stock vehicle run at more than 10lbs before, and at stock boost I've never seen them drop off. But I have seen stock turbos pushed up before, and they ususaly don't handel it too well.

I don't work with turbocharged vehicles very often though, I have seen more superchargers than turbos. My knowlege about how they work is basicaly complete, but my knowlege of the application of them appears to be incomplete.