How to design a turbo for high altitudes?

Hi all,

So please lend me some advice, fellow TDI-enthusiasts, about turbos at high altitudes.

As many know, my turbo blew chunks last month. Literally - when I dropped the engine cover, the wheel fell on my head along with hunks of metal that were destroyed beyond recognition.

We can speculate why, but I'm mostly concerned with putting something reliable back together again. I see a couple of options:

1. Buy a new stock turbo for $1K and remove the Upsolute programming - just to be safe. Cross fingers! But most of the other blown turbos in Utah weren't chipped either! So am I any safer?

2. Buy a bigger turbo wheel for $1.5K and leave the upsolute programming in place. Cross fingers! There is much less data to draw upon when considering the reliability of this option.

I'd like input from you fellow TDI enthusiasts as to which you believe would be more reliable in the long run. If I could understand the cause for failure in my turbo, I'd better understand how to avoid it next time.

Was the EGT too high? I doubt EGT since my wheel failed while at near-idle, after a very lazy trip on the freeway. Would a bigger wheel reduce EGT at all? Probably increase it, right?

Was it excessive wheel RPMs? Of course a bigger wheel would spin slower - so that would be safer, right? Or is the bigger wheel heavier anyway, negating this advantage?

Was there a computer (ECU) malfunction that caused a sudden catastrophic failure? Can the turbo physically be instruced to overspin to oblivion as just a freak accident?

If I had a bigger wheel, wouldn't the ECU continue to adjust the boost pressure regardless of the turbo's size and capabilities? I mean, if all else is identical, I'd generate the same boost pressure at the same altitudes I always have - just at a lower wheel RPM, right? So this would be safe at sea level too, right? Any opinions on this assumption?

Input along these lines is appreciated. Thanks for the support.

Aaron

Not to be facetious, (sp?), but maybe Upsolute or Wett could come up with a program that derates the turbo at altitude more than the stock programming. Isn't there some kind of device (eg, by Dawes) where you can dial in your max boost (maybe for A3's pressure-activated solenoids only). Or maybe the MAP sensor could be modified so that it translates what it sees to higher pressure. Or maybe there is an undiscovered adaptation where the turbo could be derated using the vag-com.

All this is pending a possible VW change which will, of course, require flashing the memory. VW seems less reluctant to do this now.

Aaron,

Sorry to hear about your ride.

Your opening post seems to automatically "Blame it on the altitude". Is there anything else? Or are you sure it's the altitude?

If parts fell out when you pulled the belly cover, and the wheel fell out, then it sounds like a shaft snapped? Any details would be helpful.

You wouldn't happen to have any boost data, VAG or otherwise? Not just peak, but sustained at some steady speed. Maybe dig out some old VAG files....

BTW, "wheel" seems to be used for both the compressor & turbine? Can we be more specific here? Thanks. I get confused real easy.

If it over-RPMed, it might have thrown a blade that threw the wheel out of balance and snapped the shaft. Or if the fragment was big enough, just jambed the wheel and broke the shaft.

Since you were just coming off idle, and very gently at that, there probably wasn't enough spent gasses to over-rev the turbo. The initial damage apparently began well before the final failure.

What was the condition of the turbine & compressor? Both trashed?

How was the shaft? Intact? Blue? Wear? How was the lube path? Clean, gunky, charred?

Total miles on the car, duty cycle (hwy/town), history...?

Stuff like that....

At altitude, you're probably looking at excessive turbo RPMs, so what can you do to get the boost you need at lower RPMs? Your closing paragraph sounds good to me,

If I had a bigger wheel, wouldn't the ECU continue to adjust the boost pressure regardless of the turbo's size and capabilities? I mean, if all else is identical, I'd generate the same boost pressure at the same altitudes I always have - just at a lower wheel RPM, right? So this would be safe at sea level too, right? Any opinions on this assumption?

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<font size="2" face="Verdana, Helvetica, sans-serif">The ECU (stock) should control the boost to the same levels with a more potent turbo as with a stock turbo. There might be a bit more lag with a bigger wheel though, if that is important to you. The ECU should not be bothered by wheel size. At sealevel, the more potent turbo would only be using a smaller percentage of its capablity.

If your opening post was all the info there is to offer, then assuming it is altitude related, building a better turbo for altitude seems reasonable.

I'm looking forward to you getting back on the road so we can resume your VAG-COM reports. I just love how you get involved in a thread by "Well, this morning on the way to work I used VAG-Com to read this, that and the other thing, and here it is. Gotta love that VAG-COM!"

With the T-27 wheel the turbo will make much bigger boost than stock, yet do so at considerably lower shaft RPMs than a stock turbo turns.

Robert clips the turbine blades to reduce backpressure and keep the shaft from fatiguing and snapping.

I've been pushing my hybrid HARD for 25,000 miles, at much bigger boost levels than the stock one could ever make, and it's still going.

-mickey

The shaft snapped because of the altitude, boundless, along with about a dozen others. At high altitudes, the ratio of boost-to-backpressure gets higher. The shafts in these turbos are VERY thin and fragile. They fatigue and snap. It's happened to both chipped and stock turbos. The common factor is that they are VNT-15 turbos, and that they are operated at high altitude.

Aaron's turbo failure was an absolutely classic high altitude failure.

-mickey

Simply follow Mickey's advice and go with Forced Performance.

Boundless, if you take note of the boost which the A4 engine gets at 1500 engine rpm (or thereabouts) and full load, and you figure out how much air the engine is drawing in, and you plug this into the compressor map, you'll find yourself on the wrong side of the surge line. Now increase the pressure ratio (because the air is less dense at high altitude to begin with but the ECU is still requesting the same boost) and you'll find yourself even further on the wrong side of the surge line. (Remember, altitude de-rating does not begin until over 1500 m altitude, by which time the air density is already something like 15% below that at sea level.)

The only way to get on the right side of the surge line is to reduce boost at low engine speed, and that means either modifying the turbine or adjusting the VNT linkage so that it cannot close the vanes as far.

Those of us with the old style wastegate turbo (be it GT15 or KO3) don't need to worry about this, partly because they simply won't generate as much boost at low engine speed, partly because the shaft is larger diameter. I've NEVER heard of a GT15 or a K03 installed on a TDI that failed in this manner.

If compressor surge at roughly 1500 rpm engine speed is what is killing high-altitude VNT turbochargers - and every case I know of has involved driving at LOW speed or accelerating away from a stop - then chipping is only a secondary issue. The vanes are closed under the operating conditions in question, and the chip cannot change that, the only solutions involve physical modifications to the turbo ...

Aaron,

Pics of the shaft, particularly both segments of the break, could be informative.

The pattern of the break could tell what the failure mode was. There are several possibilities. It would be interesting and helpful to have an idea what mode it failed in.

It might simply be duty cycle. At altitude, a turbo with 50,000 miles may have spun the equivalent of 100,000 miles at sea level. Under certain conditions, failure is unavoidable. A closer inspection of the failed parts might help.

Since it failed at low load (just off idle) is also interesting. Very interesting.

Now that I think about it a little more, I would like to see the compressor, turbine and shaft (parts thereof) and the other parts. If you would like, you can send the parts to me, I'll ship them right back after I look at 'em. If interested, email me for more info.

From mickey:

The shaft snapped because of the altitude, boundless, along with about a dozen others. At high altitudes, the ratio of boost-to-backpressure gets higher. The shafts in these turbos are VERY thin and fragile. They fatigue and snap. It's happened to both chipped and stock turbos. The common factor is that they are VNT-15 turbos, and that they are operated at high altitude.

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<font size="2" face="Verdana, Helvetica, sans-serif">mickey, the shaft can fatigue and fail in a number of different modes. Some mode(s) more likely than others. Different modes indicate different load conditions. Which was it?, the mode of failure for the dozen or so high altitude turbo failures?

The mode of failure is the effect and can help us determine & understand the cause. High altitude is not a cause per se, but an influencing factor that increases the possibility of certain failure modes occurring.

Thanks

sorry to hear of your misfortune- some companys have gone to the more expensive bi-turbo route! I'm not saying its the way to go, but it's a expensive option!

Since it failed at low load (just off idle) is also interesting. Very interesting

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<font size="2" face="Verdana, Helvetica, sans-serif">So did mine. So did a stock turbo, within 1 block of the tech session at Cutrubus Motors, after driving all the way across the continent from North Carolina. You take off from a stoplight, and put the most possible torque on the turbo shaft. SNAP! Game over. This type of failure would be expected to happen at fairly low rpms.

But, think what you want. Most of us know what happened.

-mickey

GoFaster,

Is surge the cause of the failure here?

So, if the failure mode is at low speeds, then GF is right, the stress is likely due to being near or across the surge line.

If one wanted to keep the stock turbo, couldn't the ECU map be modified to request less boost at lower RPMs? I.e., shift the full-load point down in the low-airflow parts of the compressor map. This would effect performance at <2000 RPM, but wouldn't effect the high-end. And if you're in the chipping camp, then you're concerned with improving the high-end anyway, right? This is certainly cheaper than buying a new turbo.

Do chips alter the boost map at low RPMs? If the stock system is on the bleeding edge of surge as it is, increasing requested boost and adding high altitude into the equation is asking for trouble.

It really makes sense that VW would try to get close to the surge line. After all, most diesel owners are used to shifting at 2000-2200 RPM, so they want to get performance out of the turbo at low RPMs.

-davin

GoFaster,

Couldn't different control of the turbo (the nozzles) help?

Not as closed at low RPM to prevent the surge, but wait a bit until the turbo is better able to handle fully closed nozzles?

Also, could it just be too much acceleration of the turboshaft? It might be a rate of change (boost or shaftspeed) that exacerbates things here.

ONE word WASTEGATE, control max surge to 20 PSI at sea level and maybe 18.5 PSI at altidude.

Second word, pop off valve, contol flow to the above and at the same time bleed away a little flow so the turbo remains within surge lines, yes the VNT-15 surge line is MUCH higher at higher flow levels.

If this is a bucks no problem build, I'd do an A3 manifold and a VNT-17, GF or that guy from Canada or Germany? with the pics would know if a PD manifold would fit (along with the VNT-17), I have no idea. TDI-RS of course has the real manifold!

Guys,

Here is the VW description of altitude compensation:

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">Overspeed does bad things.

Could we be consistent in our use of the word 'surge'. Surge is when the flow in a centrifigal compressor goes batshXt. We also use surge to describe the normal boost overshoot.

oldman, the idea to bleed charge air is excellent!!! At low RPM, the engine is not inhaling the air at a great rate and the compressor wheel has to hold off the pressure. It is more difficult for a centrifugal compressor to hold off pressure at lower flow rates. ala more likely to surge.

The low RPM boost can be most deadly since the engine is not spinning fast to inhale the air at a great rate and the turbo has to hold off the boost overshoot pressure with a low charge air flow rate. Hold off flow to your shop vac and notice the change in RPM?????

Ya gotta control that boost overshoot, actually what leads up to the overshoot. We all want that mega boost at low RPM for the torque rush, but it's a fine line. Dumping flow overboard during the rise will definitely help.

"Surge" is most definitely NOT the cause of Aaron's failure...or mine. Or any of the myriad others. If it were the cause, then installing bigger wheels would make the situation worse. Bigger wheels are even MORE prone to low-speed surge than small ones. Surge happens during high boost/low airflow situations. I've put my T-27 hybrid through far worse torture than the stock unit ever experienced for nearly 30,000 miles so far. No problems. If it were even possible to force a stock turbo to make the boost I'm getting (which it isn't) then it would fail almost instantly.

These failures are all a result of the shafts snapping. Why on Earth would you argue the point with somebody who has seen a bunch of them? The shafts fatigue, and then snap. It happens at relatively slow speeds, when accelerating from a stop, becaused that's when there is the maximum torque on the turbo shaft.

-mickey

I live at about 6600 feet above sea level and I occasionally drive in the mountains at about 10,000 feet above sea level. So, is it a foregone conclusion that I will experience early turbo demise because of "classic high altitude turbo failure"? That sucks. What were they thinking?

mickey, shafts fatigue and snap because something is putting them under cyclic stress. There is more to it than just "altitude". WHY???? do the shafts snap at high altitude? And why doesn't yours?

You have a different compressor wheel and a different turbine wheel in yours. If I recall correctly, you have a larger compressor (more inertia) and a trimmed turbine (less inertia - remember that rotational inertia depends VERY strongly on the diameter - I believe it's diameter to the fourth power).

Hypothesis: It is possible that there is some complex relationship between compressor surge and the amount of torque fluctuations that get fed back through the shaft. Increasing the inertia on the compressor side would be expected to reduce the magnitude of the cyclic torsional vibrations that happen due to compressor surge. Reducing the inertia on the turbine side would reduce the amount of torque transmitted through the shaft for a given amount of torsional vibration. The two effects compound each other.

Your compressor wheel may well be further into the surge region than stock ... but the physical changes may improve the ability of the mechanical components to deal with it!

"Surge" is different from "spike" ... the things oldman is talking about deal with "spike" but not "surge". "Surge" can happen at boost pressures LOWER than the maximum specified by the computer. I don't think changing the specified boost level at low revs will accomplish much, the vacuum-controlled system does not react fast enough. Mechanically limiting the vanes so they can't close as far should work ...

I live at about 6600 feet above sea level and I occasionally drive in the mountains at about 10,000 feet above sea level. So, is it a foregone conclusion that I will experience early turbo demise because of "classic high altitude turbo failure"? That sucks. What were they thinking?

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<font size="2" face="Verdana, Helvetica, sans-serif">I wouldn't say it's a foregone conclusion, but it's certainly a risk. Don't chip the engine, unless you want to be replacing the turbo! Or, if you want to chip it, do a pre-emptive strike and have your turbo modified by Forced Performance.

I think the biggest reason the hybrids are more reliable is that they simply don't have to spin nearly as fast. And the clipped blades help, too, by reducing backpressure. The obvious tradeoff is that you have increased "lag". Any attempt to force the hybrid to spool up as quickly as a stock unit is asking for trouble. Something has to be compromised, somewhere.

-mickey

Originally posted by mickey:
I've been pushing my hybrid HARD for 25,000 miles, at much bigger boost levels than the stock one could ever make, and it's still going.

-mickey

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<font size="2" face="Verdana, Helvetica, sans-serif">Wow!! You've already put 25K on the new engine? How long (time-wise) has it been?

Hope it runs a good long time for you...

I am chipped and running at the bleeding edge of pump timing at 70/110. Would shifting at 2800/3000 rpm help to prolong the life of my turbo, or, should I just de-chip and/or do the Forced Performance adjustment? From what I'm reading here, running a TDI in the Rocky's can be kinda tricky.

Aaron, I would like to review your broken turbo. Please let me know if you would allow this review.

High altitude results in thinner (less dense) air. This results in turbocharger pumping power being made more so by turboshaft RPMs than turboshaft torque, as compared to sea level turbo pumping power.

Surge is when the charge air flow through the compressor goes unstable. Wild flow fluctuations and flutter occurs. This causes many load problems with the compressor ass’y and its parts.

The so-called surge line is not a line. It is a regime. Even close to the surge line, approaching from the right, is risky. On the left side is scary. The closer one gets as approaching from the right, the more likely some level of surge is to occur. Surge is not an all-or-nothing affair. There may be low level flow instability occurring (as in near the surge line), but is cumulative and consumes the fatigue life of the compressor assembly. The regime between stable and fully unstable flow is not a precise line of demarcation.

At altitude, for a given boost, the pressure ratio is greater, putting the turbo in greater danger, closer to or deeper into the surge regime.

There are a number of scenarios to consider in high altitude turbo failures:

Out of Balance

At altitude, a turbo will spin much more than at sea level. The rotational balance of a turbo under these conditions is critical. An imbalance can cause a bending moment in the shaft and wheel, which could then result in a vibration, which will rapidly consume the fatigue life of the affected parts. Balance is critical at altitude due to the turbo spending more of its life at higher RPMs. The unstable frequency (RPM), and other resonant modes, may be at a low RPM that the turbo must pass through repeatedly to get to higher RPM levels. This is all bad for the compressor ass’y.

Surge

When surge occurs, the charge flow exerts wild pressure fluctuations on the compressor wheel. The compressor wheel simply can’t hold off the pressure and backflow occurs. The pressure differential across the wheel drops and the compressor resumes pumping till it can’t hold the pressure, and a vicious cycle occurs. This kills compressors.

The compressor wheel is like a plate spinning on a stick, like the trick magicians do. The plates and sticks wobble. Well, the compressor wheel and shaft ass’y is not perfectly rigid, they are elastic. (Yes, I know the plates spin on the sticks and the compressor wheel is fixed to the shaft.) They are susceptible to bending and vibration. Surge will result in extreme pressure fluctuations and gradients across the face of the compressor wheel. This will result in the wheel being used as a lever to bend the shaft. This can result in eccentric loading, vibration, resonance, over stressing and rapid fatigue life consumption of the shaft and/or wheel.

Like I said in an earlier post, the fact that the failures are occurring at the low end (RPM) is very interesting. This seems to be a consistent factor in these high altitude failures. This would indicate that the loading that is causing the failures is taking place at the low end (RPM-wise, like taking off from a stop light). The failure also would therefore be cumulative, AKA fatigue. Since the car takes off from stop lights repeatedly, it is not a single catastrophic take-off event that is intolerable by the compressor ass’y. The pattern of high altitude turbo failures demonstrates cumulative fatigue stressing, leading to failure.

Pressure Distribution Across Compressor Wheel Face

Even under normal operating conditions, there is a bending moment present on the shaft. The normal pressure distribution on the wheel is not uniform and there is a resulting bending moment on the shaft from the wheel acting as a lever. This bending moment is a cyclic loading that leads to consumption of the fatigue life of the shaft. Since a turbo that operates at altitude sees a much higher RPM duty cycle, the fatigue life can be consumed much faster if the stress levels are exceeding the fatigue limit, Se.

Apparently, the stress loads being imposed on the compressor ass’y (particularly the shaft, which is a rotating beam) are greater than expected. The fatigue limit ( Se ) stress level is being exceeded. A well designed device that under goes cyclic loading (as a rotating beam does) can be designed to survive without fail by margins to stay away from certain load levels. That ain’t happening with these high altitude failures. Something is causing excessive loads (forces), as explained above, and this is compounded by the fact that a turbo operating at high altitude has to spin a lot more than at sea level.

Surge leads to excessive bending loads on the rotating beam (shaft), from possibly several modes as explained above, which exceeds the fatigue limit stress level, coupled with higher duty cycle (RPMs). This all leads to rapid consumption of the fatigue life. This is how, in my opinion, high altitude leads to premature turbo failures. And full rack (AKA WOT) at low RPM doesn’t help either...

Surge is the predominant factor in these high altitude turbo failures. Surge appears to be more likely to occur as altitude increases.

See this cool thread on turbos, surge & altitude.

Boundless: FYI, it's impossible to "over-speed" a turbo at high altitude, unless you fool with the MAP sensor signal. The ECU contains a barometric pressure sensor. It reduces boost at high altitude to prevent over-revving. No matter your altitude, the wheel will be operating in the "fat" part of the map. Unless, as I said, you fool with something to force it to spin faster.

Chipping the engine does increase boost by about 2 psi, but that's not nearly enough to over-rev the turbo. If the ECU sees that manifold pressure is exceeding a safe level for the altitude, it releases all the vacuum from the VNT hose and dumps ALL the boost. It also cuts fuelling, just to make doubly sure. It's simply not possible to get the stock turbo to over-rev, even with a chip and the VNT mechanism cranked to the mechanical limit.

The failures are happening at low engine rpms because that's when there is the most torque on the shaft.

Most of your theories are valid enough, but not applicable to the real world. These failures happen because the shafts fatigue. The stock VNT-15 turbo has a pathetically thin little shaft, and it's prone to failure under torque. That's all there is to it.

-mickey

Right Mickey, Boundless is just out of bounds here again.

The altitude maps keep the compressor from overspinning through the MAP sensor directly and the temperature correction factor keeps the boost down directly as well.

High altitude doesn't contribute to surge as boundless states, surge occurs when the air is too thick, and unfortunatley for Boundless's theories, the air is NOT too thick at higher elevations.

One day maybe all his hypothetical theories will actually add up to something, but sadly, today is not one of them. He did have at least a 50% chance of guessing though....

LOL

Interesting thread. Here's what I'm sure of so far:

The turbos on the A4 TDIs are weak, or at least weaker than the non-VNT versions on previous TDI engines. This is certain because no matter what you did to the regular turbos, they were fine; the VNTs break, sometimes even with stock engine management. The cause of the weakness is very low inertia, which combined with the variable nozzle allows the sucker to get on boost very quickly.

The turbos are failing in transients, i.e. not at 4,000 rpm full load on some damn mountain. They break when asked to go from idle to full boost.

That's for certain. The following is opinion - take what you wish:

The common chip-tuners do a couple of things that make a bad situation worse. They increase the maximum boost, presumably across the board, and they do not raise the altitude-compensation limit to protect the turbocharger.

One chip-tuner in particular seems to aggressively increase fueling in the 1,500-2,000 rpm range, in an effort to get the turbo on boost quickly for maximum torque. This is often accompanied by a black sooty "burp." In my opinion if you drive in the mountains long enough that black burp will be followed by a blue-white burp signaling turbo failure. It is just a matter of time.

Another broken turbo story...

My stock turbo shaft snapped at about 21PSI when driving wide open in high RPM's.

An experienced shop said the shaft broke from torque. Plain and simple. Basicaly the torque requred for high boost twisted off my shaft. The guy also said he wouldn't put a bigger wheel in this turbo because it would only put more torque on the shaft.

I asked another shop and they said the turbo shaft snapped because the oil was fried out of it at one time. In other words turned off fast when hot. The tech said the oil burns and turns into grinding ash.

From looking at the shaft it does look like it twisted off. It's skinny then a clean break. The nut on the end also came off which I was told is common with shaft breakage.

Originally posted by Ding:
Another broken turbo story...

From looking at the shaft it does look like it twisted off. It's skinny then a clean break. The nut on the end also came off which I was told is common with shaft breakage.

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<font size="2" face="Verdana, Helvetica, sans-serif">Ding,

Could you send the parts to me so I can see them? I'll send them right back.

The fatigue and overtorque failure modes are very different and it shows up in the broken parts. I'd like to see the parts to determine, or at least rule out, failure mode(s).

If interested, email me for more info.

From what you posted, "clean break", it doesn't seem like overtorque.

Thanks.

Originally posted by SkyPup:
Right Mickey, Boundless is just out of bounds here again.

The altitude maps keep the compressor from overspinning through the MAP sensor directly and the temperature correction factor keeps the boost down directly as well.

High altitude doesn't contribute to surge as boundless states, surge occurs when the air is too thick, and unfortunatley for Boundless's theories, the air is NOT too thick at higher elevations.

One day maybe all his hypothetical theories will actually add up to something, but sadly, today is not one of them. He did have at least a 50% chance of guessing though....

LOL

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<font size="2" face="Verdana, Helvetica, sans-serif">Saved for future reference.

I've got to back up boundless on this one... A clean break is not usually idicative of an excessive torsional load, rather a bending or stretching type of load, although stretching seems to be a bit unlikely. If someone (Boundless) could post the pics, I'd also be interested in seeing them, or Boundless, you could send them to me and I'll send them back. At any rate, it seems to me that if the shaft got "thinner" it was either a stretching condition, (like when you over tighten a cheap bolt) or a wear condition, which would weaken the shaft and make it more prone to vertical breakage. Just my opinion.

You should immediately contact Garret and VWAG directly and let them know that their turbos are going ballistic everywhere.

Perhaps a Class Action Suit, with Boundless as the Plantiff is in due order since he has all the photos and documented evidence to make 'em pay.