How to design a turbo for high altitudes?

yank yank yank on the boundless chain (that is for sure, this is one tough low wattage lightbulb to get lit )......and my two hyperabused VNT-15's have been working flawlessly everyday for the last four years too!

OK, I have a tendency to agree, due to the light weight aluminum being easily ejected, ether it chewed the motor or did not.

Since I do all my own work, I would of course pop the head, which is really not that hard, (turbo and intake are already off.

Good luck and yes flush the tubes, if you see nada past the intercooler, run with it....

Originally posted by GoFaster:
... at sea level!

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<font size="2" face="Verdana, Helvetica, sans-serif">GoFaster, maybe you're not familiar with where SP is from.

Down there, the land is naturally flat. But, they have produced the largest man-made mountains!!! Also known as landfills!!! The dump truck operators & dozer operators are issued oxygen packs due to the altitude of these mountains. Then there are the yahoos like SkyPup that try to drive up these things with their cars!!!! That's where SP gets his high altitude experience.

I remember that the title of this thread was, "How to Design a Turbo for High Altitudes."

One thing is for sure, if I were designing any kind of turbocompressor for operation at any elevation, including high altitudes, Boundless is probably close to next to the last person I would hire for the job after VelvetFoot.

Originally posted by SkyPup:
I remember that the title of this thread was, "How to Design a Turbo for High Altitudes."

One thing is for sure, if I were designing any kind of turbocompressor for operation at any elevation, including high altitudes, Boundless is probably close to next to the last person I would hire for the job after VelvetFoot.

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<font size="2" face="Verdana, Helvetica, sans-serif">Another thing for sure is you're gonna have to get another engineering consultant to read all those documents you claim to have, and explain them to you. The present consultant isn't very successful or you are uneducate-able.

Since I have them all, and they form the basis of your conclusions, you better hope so, otherwise all your efforts are more than totally useless...

I called a Manager of Engineering at a pretty good turbocompressor company.

I explained what happened to these high altitude turbos, and here is what he explained:

Overspeed:
He says we are most definitely going into over speed. When the compressor goes into surge, it unloads. He said there is not periodic or excessive torque peaks during surge, as I had thought. The wheel just unloads and air simply blows though the compressor wheel in an unstable mode. Without the normal steady state compressor load and the turbine still being driven, it overspeeds. He says the wheel may then grow, deform, deflect and contact the housing. When that happens, all heck breaks loose. Also, since overspeed is what's happening, at least according to him, throwing a blade may also happen on its own or after contact with the housing due to overspeed deformation. He said when larger wheels go into surge, they overspeed in 0.1 sec. He said our little turbo is definitely overspeeding if it is going into the surge regime. Once it unloads, it soars. What happens when you cover the inlet to a vacuum? RPMs go up instantly!! Our little turbo has much less rotational inertia than a vacuum cleaner.

Cause:
It is probably not "one thing" that did it all. One thing might have started it, but it then cascaded other other problems. Still, what came first?!?!? Looks like overspeed. (What caused over speed? Thin air & insufficient altitude derating.)

Surge Effects:
When a compressor surges, the wheel and air gets very hot very quickly. This exacerbates the surge, a la positive feedback.

Loads:
Surge produces severe unbalanced loads in the axial direction on the wheel due to the imbalanced and unstable pressure loads caused by the surge airflow.

Surge Speed:
Surge results in overspeed which causes the wheel to deform & grow. This consumes already close tolerances and interference results.

Snapped Shafts:
When I told him that we had a couple of snapped shafts, he said that the overspeed is probably deforming the wheel and then the wheel contacts the housing causing all sorts of other damage that locks or binds the wheel and the shaft is then snapped in torque.

Just a reminder, from the VW tech doc:

"To ensure that the air mass supplied to the engine stays almost constant, the charge pressure specified map is corrected in dependence on the air pressure using the information supplied by the altitude sender F96. The charge pressure is reduced above an altitude of approx. 1500 m to prevent the turbocharger overspeeding in excessively thin air ."

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<font size="2" face="Verdana, Helvetica, sans-serif">If at altitude, probably above 4,000 ft, you might want to consider these:

</font><ul type="square">[*]<font size="2" face="Verdana, Helvetica, sans-serif"> Lower temp air for given barometric conditions is better to evade surge. Note that A4s with VNTs get cold air from the grill..... Hmmmm..... A3s with GT15s get air from a void in the engine compartment/fender well. Do cold air induction. </font>[*]<font size="2" face="Verdana, Helvetica, sans-serif">Don't chip </font>[*]<font size="2" face="Verdana, Helvetica, sans-serif"> Don't accelerate hard under 2000 RPM </font>[*]<font size="2" face="Verdana, Helvetica, sans-serif"> Don't do anything to increase boost above stock </font>[*]<font size="2" face="Verdana, Helvetica, sans-serif"> Adjust the VNT mech. stop screws to enlarge the minimum nozzle opening to reduce boost in the max guide vane position </font>[/list]<font size="2" face="Verdana, Helvetica, sans-serif">

If you are really as seriously concerned as Boundless is about all of the turbochargers detonating at altitudes over 500 feet, you should pull the damn thing out now and install a replacement to keep your TDI operating at top notch. There are a variety of ultra simple, super easy ways to replace your defective turbocharger before it becomes defective, that way you never give it the first chace to blow up on you and leave your TDI stranded at chilly dizzying high heights.

How about simply removing the defective Garrett VNT15 before it explodes and replacing it with a much more reliable device for rarified air operations at heights above 15,000 feet when touring through Tibet, Western Afganistan, or the Peruvian Andes?

A permanent 12-volt Panasonic Automobile hair dryer that plugs into the cigarette lighter can easily replace the VNT15 for high altitude airflow and you can put it on a rheostat to adjust the voltage to change the speed of the air flow to automatically match the fueling from the MAF. Possibly, for a high performance version, you could run two of the Panasonic hair dryers for double the boost without having to install any electrical relays since the amperage draw is relatively low with these high quality units.

Most Panasonics are warranted for one year, so you may want to see if any other brands give more lengthy warranty to cover the cost of the original investment. I am sure some other good replacement brands can be shopped for at Wally World, Target, or your local hairdresser.

Originally posted by Boundless:
Also, since overspeed is what's happening, at least according to him, throwing a blade may also happen on its own or after contact with the housing due to overspeed deformation. He said when larger wheels go into surge, they overspeed in 0.1 sec. He said our little turbo is definitely overspeeding if it is going into the surge regime. Once it unloads, it soars. What happens when you cover the inlet to a vacuum? RPMs go up instantly!! Our little turbo has much less rotational inertia than a vacuum cleaner.

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<font size="2" face="Verdana, Helvetica, sans-serif">So this makes me believe that a larger wheel would spin slower, and reduce the risk of overspeed - right?

It's also interesting to hear the suggestions for keeping it from happening again. I have had a habit of driving 95% of the time between 1200-2200 RPMS. While accelerating moderately, I shift out at around 2200 RPMs, so that in the next gear, I'll land at around 1600 RPMS, just enough to maintain full boost. I shift quick to keep boost pressure from falling during the transition too. So the turbo is probably constantly surging and trying to negotiate this difficult RPM range where the airflow is unstable.

We have several hills nearby that I climb at 45 MPH in 5th gear, at nearly 100% throttle and 16 PSI. Is that too much lugging? The car didn't seem to mind at the time

If I'm not accelerating, I'm afraid I cruise at 1200 RPMs all the time. There are lots of 35 MPH stretches around town that I saunter through in 5th gear.

I'm not always a slow poke - y'all know I shift at 4500 every now and again. I've chirped my tires in third gear! But it sounds like that's the lesser abuse my car has endured, compared to my usual driving style.

Hopefully a bigger turbo will let me keep my low -rpm lifestyle without this problem.

No skyPup a car should be able to be driven like that without blowing up. I do not drive like that but this is not a case of driver error!

The stuff above from the "major turbo manufacturer" was right on - very good. Why this turbo broke is one of two reasons or a combination of the two: the car was chip-tuned. the VNT turbo is at the edge of its design performance even with stock chip.

We cannot tell for sure what the reason for failure was, but chipping certainly didn't help. Nor did those "high-flow" airfilters! LOL.

It's quite simple really, the turbocharger is powered by exhaust energy and gives that energy to the intake charge. When the exhaust energy greatly exceeds the ability of the turbo to release that energy, you have a big problem. Surge, then overspeed, then it flies apart.

Okay, with that in mind, if you increase the exhaust energy beyond a certain amount or do so too quickly, the turbo is toast.

What increases exhaust energy? Primarily the accelerator. Then the full-load stop, or max fueling limit. (Raised by chiptuners, especially at the worst possible point.)

Then there's ignition timing. If you use low-cetane fuel, the combustion event is shifted later in the power stroke and your exhaust-gas temps go way up. Whenever you have smoke you have high EGTs, all other things being equal. Remember that the smoke level you see out the window is artificially low due the small cat, which burns soot to some degree.

Now, if you combine an aggressive fuel map designed to get boost up really fast and combine it with a higher boost target, AND don't make any compensation to the altitude de-tuning feature of the ECU, you are asking for problems!

aaron's post makes this situation much more clear: those driving habits combined with request for too much boost at too low revs killed the turbo. Low-rpm high-load is bad for all sorts of other stuff besides the turbo, too: Main bearings, rod bearings, transmission ...

You need to keep the revs up. I use 3rd until about 55 km/h (34 mph??) and 4th until about 80 km/h (50 mph), any lower feels like lugging (to me). And that's only at constant speed on level ground, and that's at this altitude. If any acceleration or hill climbing is involved, I let it go higher. Never full load below 2000 rpm.

80 km/h in 5th gear is about 1700 rpm, that's still too low to take full load. If I have my trailer behind, I use 4th until 100 km/h on hills. Revs (within redline) won't kill these engines. LACK of revs will!

Your owner's manual, if it's like mine, says to shift to the next lower gear when the revs go below 1500 rpm; it's safer to add a couple hundred to that, especially at high altitude.

By the way, a BIGGER turbo is NOT the thing to solve this problem. If anything, you need a SMALLER turbo. I know mickey's has stood up, but his driving style is not like this from what I've heard. The important part of the mickey / Forced Performance turbo modifications in your case may not be the different compressor, but rather the clipped turbine.

Stock turbo with VNT mechanism opened up should be OK ... but KEEP THE REVS UP! More than just the turbo is suffering at low revs under load.

Sorry Ted, your nonesense and the Boundless nonsense rants maybe easy for a novice to swallow, and yes the truth always hurts, but you are totally and completely wrong and have lost whatever respect that I ever extended your way, you have no knowledge about TDI turbocharger operation at all.

Skypup, I can understand that this driving may have helped toast my turbo. Nevertheless, I attest that a car should not be designed such that operating within it's design parameters creates a catastrophic failure. I've lugged all of my cars (my past Toyota Supras, both of them, and my present toyota cressida) past 200,000 miles, even on the original clutch!

Yes, my driving style maybe sped up a dying turbo - live and learn right? But I sure feel like you're kicking me while I'm down. The car should have been up to the task without blowing the turbo, like my turbo supra did. In my opinion anyway . . .

Aaron

Sorry arron, I am not kicking you while you are down, you are kicking yourself.

The truth of the matter is this is a case of pure and simple driver ignorance.

Keep driving your refurbished TDI like a Mack truck and you will keep having the same problem.

Talk about all you want with Ted, Boundless and the gang, you'll still have the exact same problem over and over and over again.

A perfect plan for failure at any altitude or elevation in your exact words:

"I have had a habit of driving 95% of the time between 1200-2200 RPMS. While accelerating moderately, I shift out at around 2200 RPMs, so that in the next gear, I'll land at around 1600 RPMS, just enough to maintain full boost. I shift quick to keep boost pressure from falling during the transition too. So the turbo is probably constantly surging and trying to negotiate this difficult RPM range where the airflow is unstable."

" We have several hills nearby that I climb at 45 MPH in 5th gear, at nearly 100% throttle and 16 PSI. Is that too much lugging? The car didn't seem to mind at the time."

"If I'm not accelerating, I'm afraid I cruise at 1200 RPMs all the time. There are lots of 35 MPH stretches around town that I saunter through in 5th gear."

The only way I know of to insure faster failure of your turbocharger is to make sure the air cleaner is dirty.

GoFaster, I beg to differ with your assessment of Aaron's driving style as a significant factor in this failure.

Look at the way the engine was designed, specifically the torque curve. The torque peaks at around 1800-1900 RPM, much lower than a similar displacement gasser. I'm sure VW designed the engine to support that torque, ie: crank & rod bearings and transmission at RPMs reasonably below 1800 RPM. If not, there is a major engine design flaw, and that I doubt. A diesel engine is and always has been a source of low RPM torque, plain & simple. We have to stop letting our thinking in the TDI domain from being contaminated from our knowledge and experiences in the gasser domain.

Here are some tidbits from my VW Owners Manual:

"Drive in 5th gear for optimum fuel economy when cruising. However, if more acceleration is required (when passing, for example), shift down."

and

" Upshifting early saves fuel and reduces engine noise.
Shift to the next lowest gear when engine RPM's drops below 1,500 RPM."

and

"Avoid full throttle.
Accelerating gently reduces fuel consumption, engine wear, and does not disturb the environment."

and

"Upshift as early as possible - do not drive at unnecessarily high engine speeds.
The highest fuel economy and the least disturbance to the environment are attained when you drive at low engine speeds and in the highest possible gear.

... Therefore, drive as often and as long as possible in the highest gear."

and

" Downshift only when the engine is no longer running smoothly.

Depending on the transmission you have, you can normally drive in the highest gear at 25 to 37 MPH on flat land and still be able to accelerate."

and from the trailer towing section,

"The cooling effect of the radiator fan cannot be increased by downshifting because fan speed is independent of engine speed. Therefore, when driving uphill, you can always drive in the highest possible gear with low engine speed. "

The altitude makes this turbo problem happen, and apparently the VNT has some altitude sickness. HA! I just crack myself up!!!!

Turbo Hypoxia!!!!


Here is a partial summation of all that has gone on regarding this altitude induced turbo failure situation:

</font><ul type="square">[*]<font size="2" face="Verdana, Helvetica, sans-serif">At altitude, the PR is greater for a given mass flow rate. At lower engine RPM, this puts the compressor operating point closer to or further into the surge regime.</font>[*]<font size="2" face="Verdana, Helvetica, sans-serif">Greater turbo RPMs are required. This disturbs the charge air flow on the low pressure side of the compressor blades causing premature flow/boundary layer separation, which leads to surge.</font>[*]<font size="2" face="Verdana, Helvetica, sans-serif">Turbo overspeed, which the control system has been designed to attempt to avoid, is far more likely if not guaranteed at altitude if the compressor goes into surge. The compressor will already be at much higher speed for a given boost level at altitude than at sea level. If the compressor goes into surge, it unloads since it is still being driven hard by the turbine. The turboshaft will increase in RPM at an incredibly fast rate, overspeed, and experience abnormally high stresses. Hoop stresses for one. The compressor can deform and contact the housing. Overspeed also imposes stresses in the wheel in certain directions that could be in excess of the fatigue limit stresses. The wheel eventually blows the next time it overspeeds, or the repeated overstressing fatigues the wheel allowing it to deform more than before and it contacts the housing. These stresses are non-linear, that is they increase very fast with small increase in turbo RPM, and even greater at higher RPM levels. </font>[/list]<font size="2" face="Verdana, Helvetica, sans-serif">As far as bigger or smaller wheel is concerned, the folks at altitude need a wheel that can spin, hold off pressure at high PR with low charge air flow, and make the boost. The turbo RPM duty cycle at altitude will be, make that is, far more severe than at sea level.
If driving style were that important, GT15's should be failing similarly, but they aren't or at least we haven't been made aware of it.

The VNT appears to have hypoxia. If I were at altitude, I would be adjusting the VNT so that the max. boost VNT mech position yielded nozzles that were a bit bigger to reduce the boost at low engine RPM.

The hypoxia results in overspeed. The wheel is then overstressed resulting in wheel fatigue. It eventually goes, taking out a bunch of other parts in the process.

That's my story, and I'm sticking with it, until somebody comes up with something better.

[ March 29, 2002, 18:08: Message edited by: Boundless ]

Ho Hum, just another in a long series of turbo dribble by the master himself. What a waste of time and energy your vain post is.

Yawn.........

Let us know when the dream is over, although apparently it is endless......

As to "loosing respect" for someone,

SP, does the phrase 'Shut the f*ck up' mean anything to you?

Here is a pic of the screw to open up the minimum nozzle size...

Originally posted by weedeater:
As to "loosing respect" for someone,

SP, does the phrase 'Shut the f*ck up' mean anything to you?

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<font size="2" face="Verdana, Helvetica, sans-serif">No it doesn't mean anything to me, but it sure does for you.

Boundless, which direction do you advocate turning the screw you have indicated to "open up the nozzle" as you stated for the effect you have implied?

Originally posted by SkyPup:
Boundless, which direction do you advocate turning the screw you have indicated to "open up the nozzle" as you stated for the effect you have implied?

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<font size="2" face="Verdana, Helvetica, sans-serif">It is obvious from the picture SkyPup. You tell us, by simple observation.

What position is the VNT mech in in the picture: Max. or Min. boost? You've got a 50/50 chance, SkyPup. The pressure, the pressure...

If you knew how the system was designed and how the maps work, you would know. Again, by simple observation.

Good luck.

That's what I thought, you have no idea what you are talking about again.

Don't go messing with your VNT - just don't take a chip-tuned car to altitude, and if you do, make sure to only apply low loads below 2,500 rpm, and to increase load gently.

Better yet, get the chip-tuners to comment on and perhaps change the two problems that may contribute to this problem.

And Thom - while your posts used to be pretty helpful, I think we can all agree that posting a photo of a foot sans big toe qualifies as "nonsense dribble" moreso than some good turbo discussion, whether or not you agree with it.

Of course only you know how it works and how to drive it. This is what makes you smarter than everyone on the forum - and at the factory. That's fine, but it doesn't change the fact that you are a rude son of a ***** who wastes the time and bandwidth of this community. May a pox descend on your two TDIs mit WETT, UP, or otherwise dicked with. Or better yet may your hard disc crash and the telecommunications company go under so we can have some peace and quiet around here.

At least when you were campaigning on the cetane ticket you had a good cause. Now we just have your high-sulfur ego, and man does it stink.

SkyPup: Seriously...do you have to be so nasty all the time? Do you have any idea how much time I spend answering emails from people who say "I'd ask on the forums but I don't want to get flamed"?

Flame ME, if you like. It doesn't bother me.

-mickey

Boundless: Interesting pic. I'd forgotten how miniscule the VNT-15 compressor wheel is.

I knew my T-27 "hybrid" wheel's inducer was even bigger than my VNT-25 wheel, but recently I got a look at the exducer for the first time. HOLY CRAP! I've ordered large pizzas that were smaller! It's an enormous wheel.

-mickey

The "overspeed" question is moot. You cannot "overspeed" a stock turbo, no matter how you chip it or fool with the VNT. The ECU simply won't allow it. It knows your altitude. It knows your manifold pressure. The turbo is "mapped", just like the fuelling. It simply cannot be made to "overspeed."

I would agree with SkyPup. (Only more politely.) Heavy engine loads at low engine speeds are hard on the turbo...and even harder at high altitudes.

-mickey

Hell's bells! What an absurd way to drive a car!

A few more bits'll be getting knackered or falling off it before long if that's your regular driving style, to say nothing of the likelihood of bore glazing!

But then, I do happen to have noticed quite a general lack of knowledge about how to drive cars properly and safely since I've been over here .....

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<font size="2" face="Verdana, Helvetica, sans-serif">Richard, I consider myself quite mechanically inclined. Like I said, I've put well over 200,000 miles on several cars without incident. I do most of my own maintenance, including clutches, CV joints, timing belts, etc. I've driven a lot of cars this way. After 200,000 miles of my driving on a 7MGE toyota cressida i6 motor, my cylinder compression is still between 180 and 185 on all cylinders. Just like bran new. My oil pressure is dead on. My bottom end is fine. It runs like a bat out of hell. And it's a gasser, which should disapprove even more of low RPMS than our TDI which was made to use them.

Heck, my Infiniti Q45 has a redline of 7500, but rarely sees over 2000 RPMS during the usual drive to work. It pulls like new too. I've replaced injectors, crank angle sensors, a myriad of hoses and connectors, but the motor is in great shape in spite of my driving it.

I've also taken several trips to bonneville raceway with my turbo supra to have fun blowing away the 5.0 mustangs. I blew a head gasket in the process, but that's the price for racing, right? In my TDI, I don't see the need to "race" to the grocery store on a sunday afternoon.

Please tell me what it is about my driving style that is unsafe, or suggests a lack of knowledge about HOW to drive. And please tell me how my driving will cause bits to be knackered off. I've heard the lugging argument, and I'm probably guilty of lugging more than I should. But I still insist that the motor should not be designed to operate in a condition that would cause catestrophic failure. If you'll notice, I'm AGREEING that my driving style may have stressed the turbo. I'm surprised, and I'm disappointed in VW for designing a turbo that is evidentally as delicate as it is (chipping aside). But I'm also going to insist that it's reasonable to expect a production vehicle to function in a mechanically safe mode when driving in a manner that I'd consider more-or-less normal, albiet kind of slow most of the time.

So I've been accused of driving too slow - but I've also been told to slow down with my "too much wheelspin" post. Sometimes I think folks post just because they want their own little daytime talk/fight show. Now let's keep this productive eh?

Aaron

[ March 29, 2002, 21:22: Message edited by: aaron ]