High Altitude performance of ECU Tunes

[CJ_S4]

Just wanted to post up about an issue we were chatting about in another thread. Particularly to get the attention of people who are more knowledgable and to get some vendor support.

A few of us were having a conversation in a recent thread that had to do with high altitude performance of supercharged vs turbo vs NA cars. It was generally agreed upon that FI cars lose less power at altitude than NA, which is long been recognized, but do supercharged applications lose more power with altitude than their turbocharged counterparts? A fixed rotor speed with a supercharger may only allow it to compress a certain ratio of air as compared to the ambient pressure/temp. Whereas a turbo may be able to spin it's compressor blades faster in thinner air to create a higher compression factor than a supercharger for a given ambient pressure (altitude). So feel free to debate that at your leisure.

My question in regards to the ECU tuning of the B8 S4 is this: if the ECU parameters are changed to keep the boost bypass valve shut at higher rpm based on a programmed boost pressure what happens if that Pre-programmed pressure is never met? In other words, what if the altitude is high enough that even under stock conditions the pressure at which the bypass is opens is never met, so the bypass never opens? This implies that any ECU tune that is based on closing the bypass valve later in the rpm range is essentially doing the same thing as stock - save for maybe a few timing or air-fuel parameters. Under this assertion, would it be true that an ECU tune may make little or no power over stock at high altitudes?

Just throwing that out there...othe than just a general idea of the logic, I really have no idea how the tunes work. Thanks!

[snoroller]

Hi there,

Yeah, those are the burning questions, I agree. A couple notes:

- I did find thru some research that turbos are definitely the way to go for high altitude. You've got it right I think that since they are not tied directly to engine speed, provided that they are sufficiently large, they will be able to spin faster as required and therefore bring more air to make up for the density loss at elevation. The more I read, turbos are the shizzle for up here.

- re the APR tune, great question. I'd love to hear from APR. I tried contacting Eaton for a few technical questions about roots chargers to help me understand how intakes work with them, but no dice yet. Would be great to talk to an Audi tech for s4 specifics but not sure how to contact them. I'm assuming that APR would not be quick to jump in this conversation if our assumptions are true. But maybe???

-Tim

[redrocker55]

Absolutely true that turbo's are better at altitude...in the mountains for example. I've driven a MR2 turbo at 14,000 feet and couldn't believe how much power I had that high up. Turbo's can spool faster at lower rpm's by controlling boost and continue to compress air at higher altitudes so combustion chamber really doesn't know the difference at what altitude it is operating. Look at how many Subaru turbo's there are in Denver...that is real word proof...you get AWD plus turbo...lot's of vag cars there too!

Can't comment on the adjustments APR tune might add...my guess is that altitude adjustments might be left up to the audi engineers and that part of the ECU is not affected by the APR tune. But that is simply a guess and it needs to be checked for accuracy by someone who knows more about the tunes than me...NSW4guy for example...or APR.

[camlacen]

Turbine>turbo>supercharger>n/a

Airplanes are a perfect example of high altitude f/I. Small cessna's such as the 210 have a turbo model and a non-turbo model. Obviously the turbo has more power but can also sustain the power at altitude.

[CJ_S4]

Absolutely true that turbo's are better at altitude...in the mountains for example. I've driven a MR2 turbo at 14,000 feet and couldn't believe how much power I had that high up. Turbo's can spool faster at lower rpm's by controlling boost and continue to compress air at higher altitudes so combustion chamber really doesn't know the difference at what altitude it is operating. Look at how many Subaru turbo's there are in Denver...that is real word proof...you get AWD plus turbo...lot's of vag cars there too!

Can't comment on the adjustments APR tune might add...my guess is that altitude adjustments might be left up to the audi engineers and that part of the ECU is not affected by the APR tune. But that is simply a guess and it needs to be checked for accuracy by someone who knows more about the tunes than me...NSW4guy for example...or APR.

Yep - tons of Subaru's up here. There was also testimony by someone in another thread who watched a ZR1 run low 13's at like 118 at Bandimere...Total bummer!!

Just waiting for APR to chime in... :)

[snoroller]

Hmm... I'm looking everywhere and can't find anything online that gives information whether the stock ECU automatically compensates for decrease in air density. Seems like it would be a pretty simple thing to do, just tap into an internal barometer (or gps quad-angulation)...

I'm pretty concerned the more I think about it. I have the Stasis tune and as you know, live at 7000'. I think we agreed that closing the valve could compensate for around 28% power loss = ~8500'. So did I pay $1800 for a fraction of the potential gain?

I wonder what percentage of s4's are in high elev markets? Gotta be pretty low I'd imagine...

-T

[wwhan]

Hmm... I'm looking everywhere and can't find anything online that gives information whether the stock ECU automatically compensates for decrease in air density. Seems like it would be a pretty simple thing to do, just tap into an internal barometer (or gps quad-angulation)...

I'm pretty concerned the more I think about it. I have the Stasis tune and as you know, live at 7000'. I think we agreed that closing the valve could compensate for around 28% power loss = ~8500'. So did I pay $1800 for a fraction of the potential gain?

I wonder what percentage of s4's are in high elev markets? Gotta be pretty low I'd imagine...

-T

All the newer fuel injection systems adjust for air density. That is why the MAF and/or Absolute barometric pressure sensors are generally used. The S4 does not have a MAF sensor, so it must be using pressure sensors.

The smaller blower pulley from APR (and APR tune) would help some at higher altitude, since the blower would spin faster to move more air volume, but a positive displacement blower capability is limited for large elevation swings (sea level to 14k feet).

The turbo setup should produce a better solution at high altitude, if it has enough dynamic operating range (as long as the turbine does not reach an RPM limit). One needs a lot of extra air compression capacity to handle high altitude.

A dyno comparison at low altitude versus high alttitude should show the difference.

[redrocker55]

All the newer fuel injection systems adjust for air density. That is why the MAF and/or Absolute barometric pressure sensors are generally used. The S4 does not have a MAF sensor, so it must be using pressure sensors.

The smaller blower pulley from APR (and APR tune) would help some at higher altitude, since the blower would spin faster to move more air volume, but a positive displacement blower capability is limited for large elevation swings (sea level to 14k feet).

The turbo setup should produce a better solution at high altitude, if it has enough dynamic operating range (as long as the turbine does not reach an RPM limit). One needs a lot of extra air compression capacity to handle high altitude.

A dyno comparison at low altitude versus high alttitude should show the difference.

Great points...the turbo's have the ability to generate much more boost than what they use...so at sea level they bleed a ton of boost...this extra boost is contained at altitude so they will generate close to what they do at sea level.

Any system a manufacturer used to adjust for air density not only compensates for altitude...but also barometric pressure changes. They are that sensitive. They have to be to squeeze every ounce of efficiency out of every gallon of gas.

[CJ_S4]

Hmm... I'm looking everywhere and can't find anything online that gives information whether the stock ECU automatically compensates for decrease in air density. Seems like it would be a pretty simple thing to do, just tap into an internal barometer (or gps quad-angulation)...

I'm pretty concerned the more I think about it. I have the Stasis tune and as you know, live at 7000'. I think we agreed that closing the valve could compensate for around 28% power loss = ~8500'. So did I pay $1800 for a fraction of the potential gain?

I wonder what percentage of s4's are in high elev markets? Gotta be pretty low I'd imagine...

-T

I'm sure its less than 25% but probably not much less than that. High elevation would probably be defined as >4000'? That is a pretty big area in the Rocky Mountains. A TON of Audis in Denver, I don't see alot of S4's though...

[CJ_S4]

Just wanted to post up about an issue we were chatting about in another thread. Particularly to get the attention of people who are more knowledgable and to get some vendor support.

...

My question in regards to the ECU tuning of the B8 S4 is this: if the ECU parameters are changed to keep the boost bypass valve shut at higher rpm based on a programmed boost pressure what happens if that Pre-programmed pressure is never met? In other words, what if the altitude is high enough that even under stock conditions the pressure at which the bypass is opens is never met, so the bypass never opens? This implies that any ECU tune that is based on closing the bypass valve later in the rpm range is essentially doing the same thing as stock - save for maybe a few timing or air-fuel parameters. Under this assertion, would it be true that an ECU tune may make little or no power over stock at high altitudes?

Just throwing that out there...othe than just a general idea of the logic, I really have no idea how the tunes work. Thanks!

Bump...Anyone else have any ideas here? Arin??

[snoroller]

Bump...Anyone else have any ideas here? Arin??

Definitely interested. I'm beginning to think the worst here...

[CJ_S4]

Definitely interested. I'm beginning to think the worst here...

Same here...At least for the stand-alone tune anyway. Looks like it may have to be a pulley & tune - which is what I am planning on doing anyway. I'm guessing there are a fair amount of people who are looking to just do an ECU tune that are interested in the answer to my questions as well.

I even PM'd Arin to comment in this thread - so either no one knows the answer, or the vendors are purposely choosing not to comment. I figure they would instill as much confidence as they could in order to sell as many of these as possible...

[jeffc31337]

The best way to do this is to have someone who is stock at altitude run logs of their boost bypass during a WOT run. We can then compare that to logs taking at sea level. That will tell us if the stock ECU tune is doing any altitude compensation.

[Brendon1098]

I'll do my best to answer this for you guys in a more technical manner.

First off lets remember superchargers were first designed to fill cylinders at altitude for war planes, then came turbochargers to do the same. It is widely known that turbochargers are more efficient than superchargers at doing this.

Spool of a turbocharger is directly controlled by the waste gate being closed and heat. Heat spools turbo's, increased load is increased heat which means spool. (hang onto this thought for a minute okay?)

Now onto superchargers. I don't have any technical data in front of me for the B8 S4 so bare with me on the exact numbers Say on the B8 S4 with factory tune at sea level it takes 5100 rpm to reach it's boost TARGET. This is the point where the ECU goes, okay, you've met your REQUESTED/TARGET boost limit... time to open the bypass valve! At this point the car has made 13psi and is venting everything above that.

Same scenario but at altitude, lets say 5000 feet. Now because of the lower air density the supercharger now needs 5700rpm with the factory tune to reach the same 13psi the factory tune does at sea level at only 5100rpm. At 5700 rpm the factory tune now begins bleeding boost because it has made it's TARGET boost.

The same scenario plays out with a tune, except now the boost target is say... 15psi. Now that the ECU is requesting 15psi, it will keep the bypass valve closed until it sees that 15psi which at sea level may occur at 7000rpm. However at 5000 feet altitude the supercharger doesn't have the ability to turn fast enough with the factory pulley to ever meet it's boost target of 15psi within the targeted RPM range the engine runs which is less than 7200rpm.

The way to get around this is to obviously spin either the engine faster (maybe you can't due to the rev limiter to protect engine components) so you must spin the charger faster (pulley) to get the 15psi of targeted boost the ECU is looking for in order to open the valve.

This relation in the ECU is known as Requested and Targeted boost. This has been in play since the 1.8T/2.7T days and even before that. Simply put, the ECU has a table it looks at for all this data including fueling and timing. If you make less boost you can run more timing in some cases to make up additional power and if the ability to do that within it's safety parameters then it will.

I doubt you'll see a Altitude tune or Altitude Pulley, both the factory and aftermarket tune already compensates as they are 3 dimensional maps and part of the way the systems work. A even smaller pulley for cars with an altitude disability would surely spin the charger faster to recreate sea level conditions but it would do so at the cost of reliability due to higher RPM in the supercharger.

Turbo's get away with this because there is no mechanical link, it's ECU is thinking this "okay I want 15psi, give it to me!" the waste gate closes and stays closed until the point 15psi is generated and then opens to relieve any extra exhaust gases. VERY similar to our supercharged setup except that there is no mechanical link. RPM makes boost on superchargers not Heat/Load like on a Turbocharger.

Long story short, what we are looking for is Requested/Actual/Target Boost levels and their relation. If someone log's any tune, whether it's the 3.0T, 2.7T or 1.8T and you're comparing altitudes. You need to look at Actual and requested boost levels and also compare actual ignition timing not knock values at bare minimum.

In the B8 S4 case, we see the majority of our power gains from Stock to Tune where the bleed off occurs. That bleed off point will be higher at altitude on the factory tune and therefore you won't be making large power gains over stock until you get to the point where the factory bypass valve would be open. What is the magic altitude where the engine at its RPM limit on factory tune never opens the bleed off valve? That is your question for APR and I very seriously doubt they'll tell you. The tune would still be of some value because of it's RPM limit as well as timing and fueling curve changes, but from a boost stand point, at that altitude (what ever it may be) would make no change.

[NWS4Guy]

Here you go Brendon: http://www.audizine.com/forum/showth...ighlight=eaton

[CJ_S4]

Brendon - Thanks for the reply.

I think a few of us figured that this requested vs. actual boost level was happening as part of the ECU operation. Are you confiming this is how the APR and other tunes work? On the surface, this seems like exactly how they would work with the boost chart and bypass valve discussions in other threads and on vendor websites. It also seems that the ECU tune part of it just increases the requested and target parameters - thus keeping the bypass closed longer.

I think what I and others want to know, more specifically, is this: Does the mechanical link between the engine and supercharger afford enough extra spin-up and therefore boost for the ECU to activate the bypass at 5000' of altitude? Or, due to the lack of atmosphere, this bypass valve opening target will never open because the supercharger can't physically spin fast enough due to rev limitation of the IC engine. If the latter is true, then having an ECU tune based on increasing boost by keeping the bypass valve closed seems pointless - because even in stock conditions the bypass valve may never open because the requested boost value is never met.

I'm going to try to put some numbers to it and maybe someone who knows the real values can correct me:

Sea Level Air Pressure = 14.7 psia
Sea Level Peak Boost (requested) = 13 psig
Sea Level Peak Boost (actual) = 13 psig @ 5000 rpm
Bypass valve opens at 5000 rpm (?)

5000' Air Pressure = 12.3 psia
5000' Peak Boost (requested) = 13 psig
5000' Peak Boost (actual) = Who knows? I guess this is why we have to data log. Unless there is a way to calculate this empirically??
Bypass valve opens: Who knows?

And the ECU doesn't just take into account manifold pressure, it is also measuring mass airflow which is dependent on temperature and relative humidity as well. So that is another question - does the ECU bypass boost based on just pressure alone? Or mass airflow?

Has anyone logged boost at >5000' in the B8 S4?

[silentbob]

What the ECU wants, is to meet a torque request that is requested by the driver through the pedal. To reach the torque a certain air mass flow is ajusted by the ECU which leads to a corresponding MAP. The B8 S4 doesn't have a MAF. It uses a Intake Model that is based on MAP. Humidity is usually not considered in the calculation.

[Brendon1098]

Here you go Brendon: http://www.audizine.com/forum/showth...ighlight=eaton

Thank you

What the ECU wants, is to meet a torque request that is requested by the driver through the pedal. To reach the torque a certain air mass flow is ajusted by the ECU which leads to a corresponding MAP. The B8 S4 doesn't have a MAF. It uses a Intake Model that is based on MAP. Humidity is usually not considered in the calculation.

you are correct, in part. The car is a MAFless car. However, even with a MAF, it's always been calculated torque. The car's in reality use one signal, say TPS to look on the map, Okay so at 20% TPS the car should be making X calculated TQ and X Boost and A/f should be X. Where can the timing be with these factors? okay it can be here at position Y. If there is no knock then the ECU will increase it until there is knock. Otherwise if there is knock the ECU will pull back timing or over fuel the engine to control it if it can. Whichever way is SAFER typically.

Brendon - Thanks for the reply.

I think a few of us figured that this requested vs. actual boost level was happening as part of the ECU operation. Are you confiming this is how the APR and other tunes work? On the surface, this seems like exactly how they would work with the boost chart and bypass valve discussions in other threads and on vendor websites. It also seems that the ECU tune part of it just increases the requested and target parameters - thus keeping the bypass closed longer.

I think what I and others want to know, more specifically, is this: Does the mechanical link between the engine and supercharger afford enough extra spin-up and therefore boost for the ECU to activate the bypass at 5000' of altitude? Or, due to the lack of atmosphere, this bypass valve opening target will never open because the supercharger can't physically spin fast enough due to rev limitation of the IC engine. If the latter is true, then having an ECU tune based on increasing boost by keeping the bypass valve closed seems pointless - because even in stock conditions the bypass valve may never open because the requested boost value is never met.

I'm going to try to put some numbers to it and maybe someone who knows the real values can correct me:

Sea Level Air Pressure = 14.7 psia
Sea Level Peak Boost (requested) = 13 psig
Sea Level Peak Boost (actual) = 13 psig @ 5000 rpm
Bypass valve opens at 5000 rpm (?)

5000' Air Pressure = 12.3 psia
5000' Peak Boost (requested) = 13 psig
5000' Peak Boost (actual) = Who knows? I guess this is why we have to data log. Unless there is a way to calculate this empirically??
Bypass valve opens: Who knows?

And the ECU doesn't just take into account manifold pressure, it is also measuring mass airflow which is dependent on temperature and relative humidity as well. So that is another question - does the ECU bypass boost based on just pressure alone? Or mass airflow?

Has anyone logged boost at >5000' in the B8 S4?

Yes, this is how the ECU works. Not how the tune works necessarily. You are correct, there is an altitude (air pressure) at which the bypass valve will never open on the stock tune (in theory) without seeing the actual S4 map, I wouldn't know for certain and at what point that is. That is something that people that have seen the actual ECU map could answer. People like Audi, APR, Stasis? Revo? They could tell you at what air pressure the stock file at 7200rpm would no longer open and vent. Could you calculate this? sure you could get close.

Now remember that you must remember boost is purely the number OVER atmospheric pressure. So at sea level, actual pressure in the charger is 27.7psi. At 5000 feet it may only be 25.3psi.

The system we are dealing with is one called "air density" which uses the MAP sensors to figure out how to fuel these things.

Log's would be the easiest way to determine at about what altitude does the charger's valve no longer vent boost.

I want to note also that we are using the words map alot. There is the MAP sensor or Manifold Absolute Pressure Sensor and there is the ECU Map which is a 3 dimensional map (think, like an atlas of sorts, directions even) as to where to look for data on how much timing, fuel etc to apply to the engine.

If someone had VAG-Com log's of Stock vs Tuned S4's I think I could plug some real numbers in all of this guessing and give you an idea of the altitude range of where the stock blower with the volumetric efficiency if this engine no longer allows for requested boost to be met. Or APR could just save me the trouble and tell us :)

[NWS4Guy]

APR has said that they have kept the ability of the bypass to open in order to dumop boost, as this is a safety net Audi put in and it works well. For this reason, running the pulley hardware on the stage 1 tune WILL cause the bypass to open at the top end, since the SC will push more boost than the safety limit allows. Granted, with stage 1 at 1ATM (atmosphere, aka sea level) has the bypass limite raised so it never typically opens.

Other instances when it opens:

- Under engine protection (overboost) conditions when perhaps the air is very cold and dense, it might be so efficient to hit the bypass limit in certain situations
- Under extreme heat conditions (see the APR graphs on their new coolant system) when IAT's get too high, the engine will not only extremely richen the F:A to use the evaporative cooling effect of the extra fuel to keep things under control, it will also start bleeding boost
- The issue with the limp mode on the 2010 DSG models was apparently the ECU dumping 100% boost at shift points.

[Keith@APR]

Its all based on calculated load. Its really quite simple once you take the safety features out. ECU is programmed to acheive X load %. When the engine is not meeting that targeted load or X, it uses other opportunities to generate more load. The biggest hammer it swings is boost. So if Actual Load = Y the ECU + PSI until Actual Load = X or Requested Load.

To make more power you have to ask the ECU to reach a higher load on the engine. The way you recalibrate the ECU to achieve this load is where the work comes in but in regards to altitude, X get's harder to meet at altitude when chipped because chipping requires using almost all of the boost that's available so the ECU loses its biggest hammer.

So, yes, there will be a point of altitude where adding an ecu upgrade doesn't not provide the power increases that comes from adding boost or keeping the bypass closed longer. What that is exactly is left to be determined. It can be calculated to within a degree of accuracy but real world will still tell the true story. We've tested at reasonably high altitude for North America and never found that number but we didn't go to the highest peaks. Just ask someone in Colorado to log their bypass duty cycle when stock at a range of altitudes and you can get a better idea.

Turbocharger's historically have more bypass so to speak so they can generally make up more load with boost than a supercharger can. So, if consideration of what real world actually is isn't important, a blanket statement can be made that turbo's are better at altitude than a super but again, we come back to what altitude? If the s/c can make it to the top of K2 and still deliver the load, the extra boost typically available in turbochargers isn't needed and therefore they are equal. However, if the s/c car runs out of breath but the turbo can still add duty cycle, the turbo wins.

Regarding tuned ecu's, if the tuned s/c can still make load on K2 than you get all of the gains. If target load isn't being met and the bypass is at 100%, you won't get the same gains as those further down the mountain.

[silentbob]

Its all based on calculated load.

This is not Bosch. There is no calculated load on B8 S4 ECUs

[Brendon1098]

Its all based on calculated load. Its really quite simple once you take the safety features out. ECU is programmed to acheive X load %. When the engine is not meeting that targeted load or X, it uses other opportunities to generate more load. The biggest hammer it swings is boost. So if Actual Load = Y the ECU + PSI until Actual Load = X or Requested Load.

To make more power you have to ask the ECU to reach a higher load on the engine. The way you recalibrate the ECU to achieve this load is where the work comes in but in regards to altitude, X get's harder to meet at altitude when chipped because chipping requires using almost all of the boost that's available so the ECU loses its biggest hammer.

So, yes, there will be a point of altitude where adding an ecu upgrade doesn't not provide the power increases that comes from adding boost or keeping the bypass closed longer. What that is exactly is left to be determined. It can be calculated to within a degree of accuracy but real world will still tell the true story. We've tested at reasonably high altitude for North America and never found that number but we didn't go to the highest peaks. Just ask someone in Colorado to log their bypass duty cycle when stock at a range of altitudes and you can get a better idea.

Turbocharger's historically have more bypass so to speak so they can generally make up more load with boost than a supercharger can. So, if consideration of what real world actually is isn't important, a blanket statement can be made that turbo's are better at altitude than a super but again, we come back to what altitude? If the s/c can make it to the top of K2 and still deliver the load, the extra boost typically available in turbochargers isn't needed and therefore they are equal. However, if the s/c car runs out of breath but the turbo can still add duty cycle, the turbo wins.

Regarding tuned ecu's, if the tuned s/c can still make load on K2 than you get all of the gains. If target load isn't being met and the bypass is at 100%, you won't get the same gains as those further down the mountain.

Keith,
In regards to the bigger factor, the hammer or boost in our case. At an absolute lowest mbar where does the factory tune no longer vent boost at it's rev limit? This would tell the guys at about what altitude the car will no longer be trying to vent boost and will then be looking for other, tinier hammers to swing to meet it's load target.

I think what they're trying to get out of you is, wouldn't it be true that the higher the altitude the later in the RPM band someone will see gains from boost and the bypass valve being closed on a stock vs tuned comparison? Which after all is where the majority of the gains are made on these cars correct?

[CJ_S4]

silentbob, Brendon, Neal, Keith: Thank you guys for the responses.

So as I suspected, there is an altitude at which the % gain diminishes. I guess one way to validate at what altitude this happens would be to log boost as mentioned earlier.

Keith - when you mention you've tested at reasonably high altitudes, do you have a number or a location? Just curious.

[CJ_S4]

...

I think what they're trying to get out of you is, wouldn't it be true that the higher the altitude the later in the RPM band someone will see gains from boost and the bypass valve being closed on a stock vs tuned comparison? Which after all is where the majority of the gains are made on these cars correct?

This is absolutely correct. Sounds like he eluded to this being the case in the beginning of the 3rd paragraph of his post. I would like to dial in around what altitude the gains start diminishing.

[silentbob]

Here is a stock MAP Graph



Denver is a common place were Audi/VW tests the vehicles that are sold in the US for altitude influences btw

[Brendon1098]

This is absolutely correct. Sounds like he eluded to this being the case in the beginning of the 3rd paragraph of his post. I would like to dial in around what altitude the gains start diminishing.

I have the answer for you, at any altitude the gains start to diminish. ;)

[Brendon1098]

Here is a stock MAP Graph



Denver is a common place were Audi/VW tests the vehicles that are sold in the US for altitude influences btw

So at 7000rpm the car is making a total 26.1psi or 11.4psi after atmospheric pressure is taken out.
You need a log of a tuned car at sea level with the data point for 7000rpm, 100% load, actual mbar and closed bypass. Get that and you could tell what the car is capable of making (boost wise) at about absolute best mechanically. You could then take that figure and figure out at about what altitude, at max RPM the car no longer would need to open the bypass valve.

here is a table of atmospheric pressures for you: http://www.engineeringtoolbox.com/ai...ure-d_462.html

[silentbob]

So at 7000rpm the car is making a total 26.1psi or 11.4psi after atmospheric pressure is taken out.
You need a log of a tuned car at sea level with the data point for 7000rpm, 100% load, actual mbar and closed bypass. Get that and you could tell what the car is capable of making (boost wise) at about absolute best mechanically. You could then take that figure and figure out at about what altitude, at max RPM the car no longer would need to open the bypass valve.

here is a table of atmospheric pressures for you: http://www.engineeringtoolbox.com/ai...ure-d_462.html

The dotted line is MAP. The other one BMEP

[Keith@APR]

Keith,
In regards to the bigger factor, the hammer or boost in our case. At an absolute lowest mbar where does the factory tune no longer vent boost at it's rev limit? This would tell the guys at about what altitude the car will no longer be trying to vent boost and will then be looking for other, tinier hammers to swing to meet it's load target.

I think what they're trying to get out of you is, wouldn't it be true that the higher the altitude the later in the RPM band someone will see gains from boost and the bypass valve being closed on a stock vs tuned comparison? Which after all is where the majority of the gains are made on these cars correct?

Yes. but what remains to be seen is at what altitude the gains begin to become negated.

Someone is going to have to just drive up a mountain and test it.

The OEM programming does allow for 100% duty cycle if necessary and Audi has a responsibility to ensure their power rating at all altitudes but I doubt Audi intended to ever run the s/c at 100% duty cycle. How much duty cycle they do run at the highest altitudes is something that again, someone is simply going to have to test.

[Brendon1098]

The dotted line is MAP. The other one BMEP

Nice, it helps when I pay attention. I found something else we can work with for conversation purposes :)

http://www.goapr.com/includes/img/pr...tock_boost.gif

So mechanically at 7000RPM on APR's graph the valve is shut with their tune. Which is excellent for our conversation.
This is all hypothetical and i'm sure there are holes in it all but we should be able to get close.

We know the car likes to see a peak of 11.6psi of boost. It's factory peak target for sea level.

We know that the car mechanically is capable of making 12.7psi of boost at sea level at 7200rpm. We don't have data for 7000rpm but per their graph shown above, it's making more than the 11.60psi at that data point. So we know the car is capable of making the target boost level at sea level at that RPM.
Now before we go ANY further, lets look at the bleed off.... Audi SEVERELY bleeds this car off to 7psi at 7000rpm.

I think that no matter how high you realistically go, the car is going to make that, but obviously the question at hand is where can the car no longer make the 11.6psi that it's going to be using as a hammer against elevation and effect the comparison of tune vs stock? Isn't that ultimately what we are going after here?

The car loses 4.6PSI at 7000RPM compared to it's peak of 11.6psi due to supercharger bleed off. So okay you have 4.6psi in excess. So where do you lose 4.6psi in altitude compared to sea level? around 10000 Feet.

Now if we factor in that extra 1psi that the car mechanically can make, and that Keith says the OEM will allow if needed via a closed bypass. Then we could bring the altitude up to around 12,000 feet. However due to efficiency's and whatnot that 1psi is probably going to be lost somewhere along the line in our CRUDE math etc.

For making this easy, effectively, around 10000 feet your S4 will not make any more power based on boost alone when comparing stock vs tuned. However you will make power with fueling, timing and anything about 7k is a plus as well.

So if you live in Alma Colorado at 10,500 feet you may not gain much but also, you're probably out of breath and you're the highest city in the US so get off that mountain and breathe!

[Arin@APR]

My question in regards to the ECU tuning of the B8 S4 is this: if the ECU parameters are changed to keep the boost bypass valve shut at higher rpm based on a programmed boost pressure what happens if that Pre-programmed pressure is never met? In other words, what if the altitude is high enough that even under stock conditions the pressure at which the bypass is opens is never met, so the bypass never opens? This implies that any ECU tune that is based on closing the bypass valve later in the rpm range is essentially doing the same thing as stock - save for maybe a few timing or air-fuel parameters. Under this assertion, would it be true that an ECU tune may make little or no power over stock at high altitudes?

Hmm, seems like a lot of thread for a simple answer:

Altitude pressure is measured by an internal pressure sensor, so your ECU knows when you are operating at higher altitudes. With this information, altitude correction factors are used so you can operate at sea level or high altitude without recalibrating the ECU.

Also we don't calibrate the ECU to "force" the bypass valve shut after a specified RPM. Essentially there isn't a "Boost Bypass valve Map" that's being told to be 100% shut at all RPM's.

Log your bypass valve at high altitude. It'll still open. :)

[B8_Jim]

The car loses 4.6PSI at 7000RPM compared to it's peak of 11.6psi due to supercharger bleed off. So okay you have 4.6psi in excess. So where do you lose 4.6psi in altitude compared to sea level? around 10000 Feet.

First off - interesting stuff in this thread - thanks all.

Now - Brendon - I Don't think that's right. You not only lose the atmospheric pressure at altitude, you lose capability of the charger to create boost. Let's take 5000 ft for example - sea level is 14.7 psi, 5k ft is 12.2 psi. You are down 2.5 psi or 17% in atmospheric pressure, and on top of that, the charger's ability to produce boost is down 17% since the air it's grabbing at the start of the compression is 17% lower in pressure and it's a fixed geom compressor. Right/wrong?

So for stock @ SL where APR says there is 11.6 lbs of boost the manifold pressure would be 26.3 (14.7+11.6). To get that at 5000 ft you need 14.1 lbs of boost (26.3-12.2) - which is beyond the charger's capability even at SL - thus the supposition that the bleed valve will not activate until later. At 5000 ft, I'd guess the charger is capable of about 10 (12 * 83%) lbs of boost. So I'd imagine your MAP would be 12.2+10 = 22.2 - so you are down ~17% on MAP and probably to a rough estimate, torque.

At redline where the most boost bleed occurs for stock - at stock SL is requesting 7 psi boost + 14.7 psi atmos = 21.7 psi MAP. At 5k altitude you need 9.5 psi of boost to make it back to that target - so it would have 0.5 psi of headroom and still be able to meet the stock HP level.

Anything logically wrong with this? If not it looks like at ~5700 ft is where the ability to compensate would run out. The tune gains would be something like (rough estimate) of the avail boost vs. the value acheived for stock. Charger capability at 5000 ft would be 22.2 MAP, where stock would be 21.7 (assuming above is correct) so you would be looking at something like 2.3% gain, vs. at SL you have 26.7 which would be a 23% gain. (.5 psi MAP gain vs 5 psi MAP gain).

[NWS4Guy]

Jim's correct, you also have to account for the air you are compressing being less dense as well, exacerbating the effect. So many moving parts, this is why it's much easier to just see what your car, or at least someone else in your area with the same model is logging for boost, bypass actuation and all the other data - then you know for sure, and don't have to worry about trying to spreadsheet jockying it.

[Brendon1098]

Jim's correct, you also have to account for the air you are compressing being less dense as well, exacerbating the effect. So many moving parts, this is why it's much easier to just see what your car, or at least someone else in your area with the same model is logging for boost, bypass actuation and all the other data - then you know for sure, and don't have to worry about trying to spreadsheet jockying it.

lol this is all assumption here with very little tech data. We just need someone to go to US34 or I70 and do some pulls up the grade to achieve 100% load in 4th gear and provide logs with an altimeter on board haha

You're both correct that air density is decreasing with altitude and it will be harder and harder for the compressor to grab that air to compress. Hence why I said it's crude and that we would lose that 1psi that the factory tune and bypass valve remaining closed could provide as per APR's graph. After-all the only facts we have here are sea level pressures and that's it. We don't know if the car actually loses 17% or less. I just used pressures to make it easier to look at instead of throwing percentages in as well. The vacuum of the blower pulling air in may not be affected as much as you think just due to altitude. At altitude it is also colder than at sea level, so we are assuming temperatures haven't changed then aren't we? If we are factoring in temperature, then air is going to get colder the higher up you go and that will bring density back up a bit and will help the inter coolers perform better as well.

Now keep in mind that stock at sea level the car is bleeding boost at red line and it is not doing it because the ECU is saying "I want 7psi damnit" It's doing it because the car has met it's load target. It's probably doing that since RPM has climbed so much and it is a larger factor than boost as pertaining to calculated load at this point. It's not like the car at altitude is going to say, well shit, i'm at 10,000 feet but the ECU wants only 7psi.... sooo here's 7psi. But come on ECU I have another couple of PSI to offer you!!! says the S/C... Yeah, well I don't give a fuck!... says the ECU, i only want 7, because at sea level that's what I do!

We have to remember as Keith said, the largest "hammer" or the best way to combat altitude that this car has is boost, it will exhaust that until it's no longer capable of developing boost mechanically which is about 12.7psi in perfect conditions.

Jim, you are correct in that there are many other loses to presume here.

What we need to know as well is how efficient is the blower at altitude at creating a pressure differential.

I found this which is interesting to read http://en.wikipedia.org/wiki/Superch...titude_effects
The simple fact that in WWII the engines had too large of a supercharger at sea level for the power they were needing to produce so they had to watch the throttles to avoid over boosting the engine. Our bypass valve is audi's throttle so to speak.

I also found it interesting that aircraft fuels back then were up to 150 octane and that they had carb heaters because of the venturi from the amount of air being sucked in would actually start to freeze due to the volume of air entering the engine. Obviously I doubt we would ever see that issue, but there is a cooling effect we should note.

[B8_Jim]

At altitude it is also colder than at sea level, so we are assuming temperatures haven't changed then aren't we? If we are factoring in temperature, then air is going to get colder the higher up you go and that will bring density back up a bit and will help the inter coolers perform better as well.

My 17% did include temps decreasing (76 std atmosphere #'s) but 20 deg is like 3ish percent - nothing we've been talking about is that accurate - we have no hard #'s. I think it's unlikely that the small decrease in temp due to altitude would help an air-air intercoolers enough to make up for the loss of density and thus cooling ability - but the 3.0T is air/water anyway...

Now keep in mind that stock at sea level the car is bleeding boost at red line and it is not doing it because the ECU is saying "I want 7psi damnit" It's doing it because the car has met it's load target. It's probably doing that since RPM has climbed so much and it is a larger factor than boost as pertaining to calculated load at this point. It's not like the car at altitude is going to say, well shit, i'm at 10,000 feet but the ECU wants only 7psi.... sooo here's 7psi. But come on ECU I have another couple of PSI to offer you!!! says the S/C... Yeah, well I don't give a fuck!... says the ECU, i only want 7, because at sea level that's what I do!

What is load here - not torque? Seems that would be what you control with your right foot. My #'s were not assuming that the ECU was asking for 7 PSI, rather that it would ask for a torque value - torque is going to be roughly related to air available, thus I figured to get the same target the engine would need about the same total manifold pressure. No?

We have to remember as Keith said, the largest "hammer" or the best way to combat altitude that this car has is boost, it will exhaust that until it's no longer capable of developing boost mechanically which is about 12.7psi in perfect conditions.

Right - which is the whole point of this thread and the wild speculation on my part :) without hard numbers - determine what the unit's capability to produce boost is in less than perfect conditions - i.e. at altitude .... ;)

Quick and dirty - this is what I assume would happen with HP at altitude - this seem reasonable?

[silentbob]

What is load here - not torque? Seems that would be what you control with your right foot. My #'s were not assuming that the ECU was asking for 7 PSI, rather that it would ask for a torque value - torque is going to be roughly related to air available, thus I figured to get the same target the engine would need about the same total manifold pressure.

[/IMG]

That's 100% correct.

Beside using the bleed of air the ECU has close to 0 capabilities to compensate altitude. Ignition angle is always on the limit with the fuel it was calibrated with for best efficiency ( except were a torque reserve is requested like for example idle speed) so nothing to gain here. In terms of fueling: on high loads/rpms component protection is the dominating factor. So nothing you can do here either.
All Audi/VW FI engines (no matter if SC or turbo) are designed to compensate altitude under defined conditions (...°[email protected]).

[NWS4Guy]

Nav shows altitude to a very reasonably accurate degree

[CJ_S4]

First off - interesting stuff in this thread - thanks all.

Now - Brendon - I Don't think that's right. You not only lose the atmospheric pressure at altitude, you lose capability of the charger to create boost. Let's take 5000 ft for example - sea level is 14.7 psi, 5k ft is 12.2 psi. You are down 2.5 psi or 17% in atmospheric pressure, and on top of that, the charger's ability to produce boost is down 17% since the air it's grabbing at the start of the compression is 17% lower in pressure and it's a fixed geom compressor. Right/wrong?

So for stock @ SL where APR says there is 11.6 lbs of boost the manifold pressure would be 26.3 (14.7+11.6). To get that at 5000 ft you need 14.1 lbs of boost (26.3-12.2) - which is beyond the charger's capability even at SL - thus the supposition that the bleed valve will not activate until later. At 5000 ft, I'd guess the charger is capable of about 10 (12 * 83%) lbs of boost. So I'd imagine your MAP would be 12.2+10 = 22.2 - so you are down ~17% on MAP and probably to a rough estimate, torque.

At redline where the most boost bleed occurs for stock - at stock SL is requesting 7 psi boost + 14.7 psi atmos = 21.7 psi MAP. At 5k altitude you need 9.5 psi of boost to make it back to that target - so it would have 0.5 psi of headroom and still be able to meet the stock HP level.

Anything logically wrong with this? If not it looks like at ~5700 ft is where the ability to compensate would run out. The tune gains would be something like (rough estimate) of the avail boost vs. the value acheived for stock. Charger capability at 5000 ft would be 22.2 MAP, where stock would be 21.7 (assuming above is correct) so you would be looking at something like 2.3% gain, vs. at SL you have 26.7 which would be a 23% gain. (.5 psi MAP gain vs 5 psi MAP gain).

These are the types of numbers I was looking for. I know this is all theoretical, but based on the logic you've presented - I would guess this would be the correct assertion.

My 17% did include temps decreasing (76 std atmosphere #'s) but 20 deg is like 3ish percent - nothing we've been talking about is that accurate - we have no hard #'s. I think it's unlikely that the small decrease in temp due to altitude would help an air-air intercoolers enough to make up for the loss of density and thus cooling ability - but the 3.0T is air/water anyway...



What is load here - not torque? Seems that would be what you control with your right foot. My #'s were not assuming that the ECU was asking for 7 PSI, rather that it would ask for a torque value - torque is going to be roughly related to air available, thus I figured to get the same target the engine would need about the same total manifold pressure. No?



Right - which is the whole point of this thread and the wild speculation on my part :) without hard numbers - determine what the unit's capability to produce boost is in less than perfect conditions - i.e. at altitude .... ;)

Quick and dirty - this is what I assume would happen with HP at altitude - this seem reasonable?

This chart seems reasonable. The question is, at what altitude does that small delta between "stock @ 5000'" and "chipped @ 5000'" shift so far to the right that they are essentially the same line all the way to redline? That is what boost logging would show. It would be subject to a couple hundred feet plus/minus due to ambient conditions and load conditions, but I think we can get a pretty firm number of where that is. Jim, if your logic is correct that would probably be around 6000' - which is an altitude that is enountered almost anywhere during mountain driving. Even some parts of south Denver over the Palmer divide hit 6000'. The vast majority of the Denver metro area is over 5000'.

Good stuff! Thanks for the replies everyone!

[L0U]

at least the air temperature is much cooler at elevation. That helps offset the thin air.

[CJ_S4]

at least the air temperature is much cooler at elevation. That helps offset the thin air.

Not always...It was in the 80's in Vail last weekend and still 75 at the top of Eisenhower (>11,000'). In the winter, yes. Not so much in the summer.