Vacuum actuator settings at high altitude

Nevada_TDI · Oct 24, 2009

I reset my vacuum actuator from the factory setting because my car just seemed to lack the "quick" throttle response I wanted, (in spite of a great tune and nozzles) so I shortened the rod, and wow, what a difference! This is both good and bad; good because the boost comes in fast, but bad because my VNT-15 was spiking to 21psi. (I thought) This normally wouldn't be a big deal to occasionally spike to 2400 milibar at sea level, but 2400 milibar at 4000ft is 23psi, not 21, and due to the pressure differential between atmosphere and the manifold, the turbo is really overloaded at altitude. My thanks to VWMikel for pointing this out to me as my turbo is less than 5000 miles old, and I am not ready to replace it again due to my own personal stupidity or laziness. $$$ My decision (at the suggestion of VWMikel) is to buy a boost valve to potentially save my turbo. This way I can have the throttle characteristics that I desire and not have reset my actuator when I am driving at lower altitudes for any length of time. I am also sure when I climb out of the valley I live in, I won't have to worry about overboost, limp mode, or a broken turbo.
Do you live at high altitude? If so what did you do to fix your spiking problems? Thanks in advance.

milehighassassin · Oct 24, 2009

First off, what do you consider high altitude? I am dealing with this now. I don't really go below 5000', and many times I go over 11,000 feet.

I would think a good tune can correct it and I am counting on that.

Thread I have which brings up elevation:
http://forums.tdiclub.com/showthread.php?t=261249

Edit, I just say you mention 4000 feet. I wouldn't call that high altitude, but I suppose many would.

Farfromovin · Oct 24, 2009

I feel for you guys at high altitude. I hate high altitude (from an engines standpoint at least). I think my car likes sea level better. My car behaves quite differently at sea level and above 5k ft'

Nevada_TDI · Oct 24, 2009

milehighassassin said:
First off, what do you consider high altitude? I am dealing with this now. I don't really go below 5000', and many times I go over 11,000 feet.

I would think a good tune can correct it and I am counting on that.

Thread I have which brings up elevation:
http://forums.tdiclub.com/showthread.php?t=261249

Edit, I just say you mention 4000 feet. I wouldn't call that high altitude, but I suppose many would.

Well, compared to sea level and standard pressure readings it is high altitude.

Actually I do have a good tune from VWMikel... the change in response is something I did, as it didn't have big spike issues before I messed with it.
I read your thread and saw you were getting 27 psi spikes. I do wonder: is the gauge you have calibrated for your altitude? If not, your spikes are way over 30psi at least. I like the way she runs now even better than before, and I am willing to spend $50.00 and an hour or so to prevent catastophic failure of my turbo because I chose to shorten my actuator rod and have full boost 1000 rpm sooner.

milehighassassin · Oct 24, 2009

Guage shouldn't need calibrated. It reads zero when it is not running. I have another gauge that has vac as well and it also reads zero.

I only had the 27 PSI spike once, now I watch for the climb and back off before it has a chance to spike. I have an MBC which should be here Monday.

I think you can adjust your VNT mechanism for the elevation, but instead of just dialing it in by butt dyno, you should do it right and adjust for when it open and when it reaches max. If you do it at your elevation used the OEM adjustment procedure it should be set for elevation.

Granted, weather changes and with that so does pressure, so all of this is ballpark.

vwmikel · Oct 24, 2009

The stock ALH software actually has the altitude compensation disabled. I guess they felt that there was sufficient margin. It can be instituted again if need be, but boost wasn't set amazingly high before (~17.5 psi). After his timing belt broke his timing ended up being slightly advanced when they set it up again. Predictably, this seems to have dropped EGT's a bit and caused a slight amount of lag (noticeable nonetheless). All that the altitude compensation really does is limit manifold pressure based on atmospheric pressure...so it detunes with decreases in atmospheric pressure. I can put in the altitude compensation if it would make you feel better

.

KERMA · Oct 24, 2009

N75 has its own altitude compensation maps
And for the record, baro compesation is not totally disabled in an ALH, just the 10x10 boost limiter is a flatline, there other maps that adjust the boost limit according to altitude (both 3d and 2d)

milehighassassin · Oct 24, 2009

Which is why when you VAG-com Data log, you can read atmospheric pressure, correct? I assumed the ECU knew that, but I will say at elevation it seems the OEM tune could use some work.

Nevada_TDI · Oct 24, 2009

vwmikel said:
The stock ALH software actually has the altitude compensation disabled. I guess they felt that there was sufficient margin. It can be instituted again if need be, but boost wasn't set amazingly high before (~17.5 psi). After his timing belt broke his timing ended up being slightly advanced when they set it up again. Predictably, this seems to have dropped EGT's a bit and caused a slight amount of lag (noticeable nonetheless). All that the altitude compensation really does is limit manifold pressure based on atmospheric pressure...so it detunes with decreases in atmospheric pressure. I can put in the altitude compensation if it would make you feel better .

Mike,
Thanks for the offer, but I don't know if it's necessary and maybe you can clear this up for me... I realize that my current altitude causes the pressure in milibar reading to be "incorrect", but let's say I compensate and set the boostvalve to 18-19psi at my current (and usual) altitude. If I go to sea level will have I less boost, and if I go across the Sierra mountain pass at 8000' will I have too much, and will it even matter, really? I guess the real point is: since I like where the vacuum actuator is set right now, would having you reset the altitude compensation fix my current spike issue?

vwmikel · Oct 24, 2009

KERMA said:
N75 has its own altitude compensation maps
And for the record, baro compesation is not totally disabled in an ALH, just the 10x10 boost limiter is a flatline, there other maps that adjust the boost limit according to altitude (both 3d and 2d)

Yes, there are other factors that play into it, but that 10x10 map is what would normally be used to limit manifold pressure. In most software version it isn't flat.

milehighassassin said:
Which is why when you VAG-com Data log, you can read atmospheric pressure, correct? I assumed the ECU knew that, but I will say at elevation it seems the OEM tune could use some work.

The ECU has it's own internal pressure sensor that it compares to the MAP sensor on start up. This is where the DTC comes from when installing larger map sensors.

Nevada_TDI said:
Mike,
Thanks for the offer, but I don't know if it's necessary and maybe you can clear this up for me... I realize that my current altitude causes the pressure in milibar reading to be "incorrect", but let's say I compensate and set the boostvalve to 18-19psi at my current (and usual) altitude. If I go to sea level will have I less boost, and if I go across the Sierra mountain pass at 8000' will I have too much, and will it even matter, really? I guess the real point is: since I like where the vacuum actuator is set right now, would having you reset the altitude compensation fix my current spike issue?

I don't think that your boost pressure will change much because a boost valve sets the pressure according to spring tension. The spikes are simply there because you tightened the actuator and now the vanes, more or less, aren't where the ECU thinks they are. It could be tuned out with some logs and such, but the boost valve will also do the job. Resetting the altitude compensation would just cause boost pressure to drop at altitude in order to further protect the turbo.

ArturCosta · Oct 24, 2009

I also had recently some problems with high altitude.
As 99% of the time I drive at more less sea level I have my remap tuned for this level.
The thing is that at high altitude (more less 1000m) I start having boost spikes at low RPM.
At sea level I get 1.8bars at 2200rpm , above 1000m I dont get much boost at 2000rpm and at 2500rpm it spikes to 2bars... above 3000rpm I only notice a bit more lag.

In my case its just the VNT tune at the remap had to be redone and tested at high altitude.

Nevada_TDI · Oct 24, 2009

vwmikel said:
Yes, there are other factors that play into it, but that 10x10 map is what would normally be used to limit manifold pressure. In most software version it isn't flat.

The ECU has it's own internal pressure sensor that it compares to the MAP sensor on start up. This is where the DTC comes from when installing larger map sensors.

I don't think that your boost pressure will change much because a boost valve sets the pressure according to spring tension. The spikes are simply there because you tightened the actuator and now the vanes, more or less, aren't where the ECU thinks they are. It could be tuned out with some logs and such, but the boost valve will also do the job. Resetting the altitude compensation would just cause boost pressure to drop at altitude in order to further protect the turbo.

Mike,
Rather than drive you nuts resetting my ECU, i'll go with the BV and just not worry about it. If I had the altitude compensation turned on, I would have less power than I have now i'm sure, and that isn't what I want anyway...

Farfromovin · Oct 24, 2009

vwmikel said:
The ECU has it's own internal pressure sensor that it compares to the MAP sensor on start up. This is where the DTC comes from when installing larger map sensors.

No kidding? I feel smarter (maybe dumber) every day

Now I wanna go open up my ECU to find this sensor LOL. I thought that was an older ECU thing. Do EDC-16 and 17 have it too? It must be some tiny IC type sensor I would imagine

Mike96ZJ · Oct 25, 2009

I have noticed a significant decrease in performance after driving from basically sea level in Maryland to 4700 ft. in Arizona. It also tends to smoke a bit more when I romp on it.

I am getting a delay in spool up as well, which is likely causing the smoke. I thought about getting it retuned, but I am only here another month before I head back to MD.

Diesel_Benz · Oct 25, 2009

Have any of you driven up to an altitude that is actually high?

Try living at 5200ft and driving over 10,000ft highway passes.

milehighassassin · Oct 25, 2009

Diesel_Benz said:
Have any of you driven up to an altitude that is actually high?

Try living at 5200ft and driving over 10,000ft highway passes.

Did you miss my post? I drive from Fort Collins to Vail every weekend and then back.

Nevada_TDI · Oct 25, 2009

milehighassassin said:
Did you miss my post? I drive from Fort Collins to Vail every weekend and then back.

Okay, got me there milehigh, and no I don't...

But the question still needs to be answered (please) although I (or you) may have a boost gauge connected to the output of my turbo, isn't the differential pressure an issue? More atmospheric pressure at sea level and less at high altitude.
If I were to place the connections of a manometer to my pressure side of the turbo, and the other side out to atmosphere at sea level and I was set to 21psi boost, I would read 21psi, right? But if I took that car to 6000ft, wouldn't the pressure read higher due to less atmospheric pressure at that altitude?
It's been years since I worked in an engineering office as an intern, but I seem to remember the differential pressure is the more important part of the equation, especially since we are talking turbos.

Here's why I say this: using the Vag Com MAP readings, I get 2400 milibar at full boost. But since I am at 4000 feet, and have less atmospheric pressure than sea level, I don't subtract 1000 from 2400 then divide by 68 to get psi, I subtract 823 from 2400 and then divide by 68 to get psi.

Someone very wise explained this one to me, so that's why i'm dragging it up again. But here is the real question I think: If I set my boost valve to an altitude compensated 19psi, and I go to sea level will I still get 19psi or more like 17psi? Anybody wanna answer this one? Thanks.

Diesel_Benz · Oct 25, 2009

Nevada_TDI said:
If I were to place the connections of a manometer to my pressure side of the turbo, and the other side out to atmosphere at sea level and I was set to 21psi boost, I would read 21psi, right? But if I took that car to 6000ft, wouldn't the pressure read higher due to less atmospheric pressure at that altitude?

Yes, it would read about 24psi. Turbos work on absolute pressure but the gauge is just a differential measurement.

Increased altitude moves the compressor's map plot point up and left, closer to the surge line, if manifold pressure remains the same.

Nevada_TDI · Oct 25, 2009

Diesel_Benz said:
Yes, it would read about 24psi. Turbos work on absolute pressure but the gauge is just a differential measurement.

Increased altitude moves the compressor's map plot point up and left, closer to the surge line, if manifold pressure remains the same.

Thanks, that is what I thought. I hope mile high reads this and takes into consideration that his boost gauge may be very "wrong".

I figured out the other question too: compensated pressure as far as the BV goes will make the actual boost less at sea level because it is just a spring and a ball set to a certain pressure.

Even though I have read many times about VNT-15 boost limits, what really is the safe maximum pressure to run them at? 17, 18.5, or 19psi ?

This question does refer to sea level pressure, not altitude adjusted.

milehighassassin · Oct 25, 2009

I disagree. Pressure is pressure. At elevation there is less atmosphere pressure.

Example:
My tires have 40 lbs of pressure in them. Really they have 40 lbs + atmosphere pressure (at sea level that is 14.7 lbs). But you cannot read that pressure on a tire gauge because that pressure also exists on the OUTSIDE of the tire as well as the inside.

Today scangauge told me at idle Manifold pressure was 10.8 in Silverthorn. In Fort Collins it read 12.5
So there is some change but the gauge is not off. It reads zero when there is no pressure. Like I mentioned earlier I have another gauge from an old gas turbo which has vac and it always read zero (no vac) when off (under idle on a gasser you are in vacuum). MY gauge is usually VERY close to what I read with vag-Com or scangauge.

1 BAR is always 1 BAR, while the number might change depending on both elevation and weather (that is how a barometer works).

The biggest problem I see is that at sea level the turbo already has 14.7 PSI "helping" it. Today I only had aprox. 11 PSI "helping" my turbo. Really that is just the ZERO point. The gauge is not off because everything else around the gauge is only experiencing that BAR as well. Pressurized systems will equalize. To make 22 PSI at my elevation compared to sea level the turbo has to compensate and make an additional 3.7 PSI. Now the turbo is NOT boosting to 25.7
It just has to work harder.
Pressure is pressure, it is just harder to make it higher up.

I hope this is making sense, I know I am all over the place and this is not well written.

The reason there is less pressure is because you have less atmosphere pushing down on you (in this case 9,000 feet less of atmosphere). This is the same reason it is harder for a human to breath at elevation. In order to inhale your lungs need to fill up with air. At elevation there is less pressure pushing that air into your lungs. It is exactly the same thing with a turbo. There is less atmosphere pushing air into the turbo at 9k feet than sea level. If you were to go to Death Valley and you were below sea level you would have more pressure (BAR).

Nevada_TDI · Oct 25, 2009

milehighassassin said:
I disagree. Pressure is pressure. At elevation there is less atmosphere pressure.

Example:
My tires have 40 lbs of pressure in them. Really they have 40 lbs + atmosphere pressure (at sea level that is 14.7 lbs). But you cannot read that pressure on a tire gauge because that pressure also exists on the OUTSIDE of the tire as well as the inside.

Today scangauge told me at idle Manifold pressure was 10.8 in Silverthorn. In Fort Collins it read 12.5
So there is some change but the gauge is not off. It reads zero when there is no pressure. Like I mentioned earlier I have another gauge from an old gas turbo which has vac and it always read zero (no vac) when off (under idle on a gasser you are in vacuum). MY gauge is usually VERY close to what I read with vag-Com or scangauge.

1 BAR is always 1 BAR, while the number might change depending on both elevation and weather (that is how a barometer works).

The biggest problem I see is that at sea level the turbo already has 14.7 PSI "helping" it. Today I only had aprox. 11 PSI "helping" my turbo. Really that is just the ZERO point. The gauge is not off because everything else around the gauge is only experiencing that BAR as well. Pressurized systems will equalize. To make 22 PSI at my elevation compared to sea level the turbo has to compensate and make an additional 3.7 PSI. Now the turbo is NOT boosting to 25.7
It just has to work harder.
Pressure is pressure, it is just harder to make it higher up.

I hope this is making sense, I know I am all over the place and this is not well written.

The reason there is less pressure is because you have less atmosphere pushing down on you (in this case 9,000 feet less of atmosphere). This is the same reason it is harder for a human to breath at elevation. In order to inhale your lungs need to fill up with air. At elevation there is less pressure pushing that air into your lungs. It is exactly the same thing with a turbo. There is less atmosphere pushing air into the turbo at 9k feet than sea level. If you were to go to Death Valley and you were below sea level you would have more pressure (BAR).

I guess I wasn't clear. You're right pressure is pressure; but that wasn't what I was talking about. I was talking about differential pressure. Let me rephrase my point and use the term absolute pressure.

In terms of a pressure gauge reading zero when there is no pressure applied to it's input, you are absolutely right. But, if you used a laboratory grade pressure gauge and went to sea level and filled your tires to 40psi, and then drove to 6000ft and let the tires cool overnight the pressure would be higher in the tires because the atmospheric pressure at 6000ft is less than it is at sea level.

I didn't mean to offend you and say your gauge was actually wrong, I meant relative to your elevation, your scale isn't accurate and here is why:
like you stated atmospheric pressure at sea level is 14.7psi, that's right. And, atmospheric pressure at 6000ft is 12.7psi approximately, right?

So if you use Vag Com on your car and it reads MAP is 2400 millibar at sea level, then you subtract 1000 milibar, then your absolute manifold pressure is 1400 milibar or 20.58psi, right?

But here's the kicker: if your MAP reading is 2400 millibar at 6000ft and you subtract 795 from 2400, that leaves an absolute pressure of 1605 millibar.
Therefore 1605 divided by 68 equals 23.6 psi, even though your pressure gauge is only reading 20.5psi.

I'm not trying to start a war here, and I may very well be wrong. But if i'm not, I hope you have a bullet proof turbo.

Diesel_Benz · Oct 25, 2009

milehighassassin said:
To make 22 PSI at my elevation compared to sea level the turbo has to compensate and make an additional 3.7 PSI.

No. At 6000ft, to move the same mass (pounds) of air as 22psi you will have to make 29.5psi.

At elevation there is less pressure pushing that air into your lungs.

Not just pressure, mass (volume/density) as well. 22psi at sea level moves about 290cfm at 21.1lbs/min. To get that same 21.1lbs/min at 6000ft you'll be moving 362cfm.

Nevada_TDI · Oct 26, 2009

D_B,
Thanks for the calculations. Like I said, my engineering days were long ago.

milehighassassin · Oct 26, 2009

Diesel_Benz said:
No. At 6000ft, to move the same mass (pounds) of air as 22psi you will have to make 29.5psi.

Not just pressure, mass (volume/density) as well. 22psi at sea level moves about 290cfm at 21.1lbs/min. To get that same 21.1lbs/min at 6000ft you'll be moving 362cfm.

That is correct and I was talking about pure pressure not the same mass of air, I do realize the air is less dense.

milehighassassin · Oct 26, 2009

Nevada_TDI said:
In terms of a pressure gauge reading zero when there is no pressure applied to it's input, you are absolutely right. But, if you used a laboratory grade pressure gauge and went to sea level and filled your tires to 40psi, and then drove to 6000ft and let the tires cool overnight the pressure would be higher in the tires because the atmospheric pressure at 6000ft is less than it is at sea level.

That is correct,

Nevada_TDI said:
I didn't mean to offend you and say your gauge was actually wrong, I meant relative to your elevation, your scale isn't accurate and here is why:
like you stated atmospheric pressure at sea level is 14.7psi, that's right. And, atmospheric pressure at 6000ft is 12.7psi approximately, right?

So if you use Vag Com on your car and it reads MAP is 2400 millibar at sea level, then you subtract 1000 milibar, then your absolute manifold pressure is 1400 milibar or 20.58psi, right?

But here's the kicker: if your MAP reading is 2400 millibar at 6000ft and you subtract 795 from 2400, that leaves an absolute pressure of 1605 millibar.
Therefore 1605 divided by 68 equals 23.6 psi, even though your pressure gauge is only reading 20.5psi.

I'm not trying to start a war here, and I may very well be wrong. But if i'm not, I hope you have a bullet proof turbo.

No offense taken here, I am trying to learn as well.

It has been stated that the ECU knows atmosphere pressure and that it compares that info with the MAP info. I assume it is making corrections to it's own scale from this information.

Maybe I just don't get it but I don't see how the gauge or the ECU will be seeing the wrong information. Even if atmospheric pressure changes the gauge is always going to be reading X-Amount above that correct? It doesn't matter if it is at sea level or at 30,000 feet.

Because the turbo is sitting in the same environment (more important it has the same atmosphere pressure working on it) it might have to work harder but I don't see it actually boosting a higher PSI than is actually being read by both the boost gauge and ECU. It works harder because it has to move more grams of air because the air is not as dense.

I think a lot of people get held up on PSI. PSI is not equal. Technically we are moving grams of air. Which is why a larger turbo that operates at say 10 PSI can produce more power than a smaller turbo that operates at 24 PSI. It is like moving water through a garden hose at 24 PSI or moving water through a fire hose at 10 PSI. It is about volume not pressure.

So at elevation there is less "volume" of air than at sea level. The motor doesn't run on pressure it runs on air and a specific amount of air for a specific amount of power. In order to move that air the turbo has to produce higher boost for an equal amount of air. Well that is not always possible because you are going to be operating the turbo out of efficiency.

With everything else but elevation being equal, you will never produce the same amount of power.

robnitro · Oct 26, 2009

Nevada_TDI said:
Someone very wise explained this one to me, so that's why i'm dragging it up again. But here is the real question I think: If I set my boost valve to an altitude compensated 19psi, and I go to sea level will I still get 19psi or more like 17psi? Anybody wanna answer this one? Thanks.

Let´s say you are at sea level and set the valve to 19 psi (33.5 absolute).
At an altitude where atmosphere is 12 psi, it will still open at a 19 psi differential, but the absolute pressure will be 30 psi. The gauge will still read 19 psi, as I believe most mechanical gauges are reading the pressure differential between the signal and ambient.

According to those numbers above, at sea level the turbo would be working at a 2.3 Pressure ratio. At altitude, 2.5 PR.

The PR is what a turbo is really rated for. Thats why compounding turbos allow them to run very high boost, because the second turbo is being fed already pressurized air.

Nevada_TDI · Oct 26, 2009

robnitro said:
Let´s say you are at sea level and set the valve to 19 psi (33.5 absolute).
At an altitude where atmosphere is 12 psi, it will still open at a 19 psi differential, but the absolute pressure will be 30 psi. The gauge will still read 19 psi, as I believe most mechanical gauges are reading the pressure differential between the signal and ambient.

According to those numbers above, at sea level the turbo would be working at a 2.3 Pressure ratio. At altitude, 2.5 PR.

The PR is what a turbo is really rated for. Thats why compounding turbos allow them to run very high boost, because the second turbo is being fed already pressurized air.

@robnitro thanks for the calculations. I did actually my own question prior; According to my calculations to get 18.39 psi at an altitude of 4000 ft, I have to set the BV to 16 psi, and at sea level it will still be 16 psi.

coronan · Oct 27, 2009

What Taxes a turbo?

Press. Differential?
RPM / CFM?
Or outlet press (ie shaft load)

To me outlet pressure would provide the most stress on the shaft. I've seen a thread or two arguing for equal MAP and Exhaust pressures.

But I know nothing about physics.

Diesel_Benz · Oct 27, 2009

Pressure ratio and RPM.

Tweekster · Nov 8, 2009

robnitro said:
Let´s say you are at sea level and set the valve to 19 psi (33.5 absolute).
At an altitude where atmosphere is 12 psi, it will still open at a 19 psi differential, but the absolute pressure will be 30 psi. The gauge will still read 19 psi, as I believe most mechanical gauges are reading the pressure differential between the signal and ambient.

According to those numbers above, at sea level the turbo would be working at a 2.3 Pressure ratio. At altitude, 2.5 PR.

PR and shaft rpm are the turbo killers at altitude, one of the reasons a turbo like a 17/22, with a larger compressor capable of higher PR's, may be a better option for those of us at higher altitudes, even with a mild tune.

Vacuum actuator settings at high altitude

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