Diminishing Boost at High Altitude
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This is a discussion on Diminishing Boost at High Altitude within the Engine Modifications forums, part of the Tech & Modifying & General Repairs category; I noticed heading up Loveland Pass in CO this weekend that at 10k ft I was only seeing 10psi on ...

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    Diminishing Boost at High Altitude

    I noticed heading up Loveland Pass in CO this weekend that at 10k ft I was only seeing 10psi on the boost gauge full throttle at 4.5k (I normally see about 13psi at 5400 ft). Is this limitation due to boost control or the limits of the turbocharger.

    I have a turbo miata which when using wastegate control only will deliver a constant psi regardless of altitude on my boost gauge. When I go to closed loop map control (via a link ECU) the gauge psi goes up with altitude (about 3psi per 5k ft).

    I would expect the WRX behavior to be similar to the miata.

    Gary

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  3. #2
    Registered User pace's Avatar
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    Given that your gauge reads boost above the ambient, and that your MAP sensor reads absolute; I see three plausible mechanisms of metering 'boost'.

    #1. Open-loop where you effectively dictate a maximum wastegate 'position'.
    - This will cause increased manifold pressure and increased boost with increasing ambient barometric pressure.

    #2. Closed-loop, where the wastegate is mapped from relative 'gauge' boost.
    - You should see constant gauge boost regardless of altitude or weather conditions. Conversely, your manifold absolute pressure should increase slightly with increasing ambient barometric pressure.

    #3. Closed-loop, where the wastegate is mapped from absolute 'manifold' pressure.
    - Your manifold would see constant pressure regardless of altitude or weather conditions. Conversely your gauge should show increasing boost with decreasing ambient barometric pressure.

    I don't know for sure, but I would assume that the WRX ECU uses open-loop control and hence you see the symptoms described in #1. Regardless, you are likely correct in asserting that the behavior is somewhat attributable to the TD04 running out of breath at very high altitude.

    The behavior you describe for your Link closed-loop control is consistent with a system that is being driven off the factory MAP sensor, as in #3. The 3psi/5,000ft sounds about right. At my house which is at 8,500ft my MAP sensor reads -4.7psi at rest (0psi on my boost gauge).

    -Pace

    -Pace

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    Originally posted by pace
    Given that your gauge reads boost above the ambient, and that your MAP sensor reads absolute; I see three plausible mechanisms of metering 'boost'.

    #1. Open-loop where you effectively dictate a maximum wastegate 'position'.
    - This will cause increased manifold pressure and increased boost with increasing ambient barometric pressure.

    #2. Closed-loop, where the wastegate is mapped from relative 'gauge' boost.
    - You should see constant gauge boost regardless of altitude or weather conditions. Conversely, your manifold absolute pressure should increase slightly with increasing ambient barometric pressure.

    #3. Closed-loop, where the wastegate is mapped from absolute 'manifold' pressure.
    - Your manifold would see constant pressure regardless of altitude or weather conditions. Conversely your gauge should show increasing boost with decreasing ambient barometric pressure.

    I don't know for sure, but I would assume that the WRX ECU uses open-loop control and hence you see the symptoms described in #1. Regardless, you are likely correct in asserting that the behavior is somewhat attributable to the TD04 running out of breath at very high altitude.

    -Pace
    I never heard of boost control being done as #1 but it could have its advantages in a turbo which was undersized and hence would be either really inefficient or be spinning like crazy at higher altitudes as it had to deal with higher and higher pressure ratios if using#2 or #3.

    It seems that using a MBC(for the sake of this discussion only) would turn the behavior into more like #2. Did you ever notice such things (assuming you used a stock turbo and MBC)?

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    Registered User pace's Avatar
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    I think my terminology 'wastegate position' may have been ill advised, but I was essentially trying to convey the fundamental difference in the function of an open-loop system that has no sensory feedback and does not adjust the wastegate on-the-fly. MBCs fall into this category and behave accordingly with changing barometric pressures. Most if not all aftermarket EBCs that I am aware of offer an open-loop or 'manual' mode.

    Colorado has a pretty big WRX community and most of us have had to deal with the issue you describe at one point or another.

    -Pace

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    limits of turbo

    I tried some expeiments when I first got my WRX and found that even if I crank the boost controller to a ridiculous number the turbo will run out of breath at very high altitude.

    With an EBC set for very high boost I could not reach 1 kg/cm^2 boost at 13,600 ft altitude. The pressure ratio is just too high for the turbo, you push it way out of its effeciency range and it becomes a circular feed back.

    Hot air out of turbo, reduces mass flow even though your holding boost. Lower mass flow means less exhaust gas mass flow, less power to drive turbo.

    At the same time, as you climb the inlet pressure on the compressor keeps falling so to hold the same boost the turbo has to actually operate at a higher pressure ratio. Which requires more power from the hot side tubine.

    At some point you either bump into the max rpm of the compressor, or the max flow limit. Your moving a lot of volume but it is hot and not much actual mass, so after intercooling, less boost.


    Anyway thats my experience.

    Larry

  7. #6
    Registered User pace's Avatar
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    Good points all, Larry.

    I used to run an MBC with the stock turbo and had no problems building 16psi at my house which resides at 8,500ft. It seems odd therefore that an additional 1,500ft in elevation would prevent him from building even 10psi. I have to conclude that the stock boost control methodology is at least partially the culprit here although I have no doubt that the TD04 sizing is somewhat to blame.

    -Pace

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    Registered User hotrod's Avatar
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    turbo boost and waste gate

    That is one of the reasons I look at dyno charts reporting spool up characteristics of turbos with some caution.

    The low rpm behavior of a turbo as far as boost onset is greatly affected by how the boost controller is set. If I knew I was not going to get into a prolonged high boost situation in a higher gear. I could make the stock turbo hit so hard if almost felt like N2O.

    In situations like rallyx you can get away with that because your never on the loud peddle for more than a couple seconds at a time. ON the street that same boost controller setting would result in severe overboost, so the stock boost controller could very well be a limiting factor for him.

    My tests were with a Blitz DSBC, and an ECU that had boost cut eliminated.

    Larry

  9. #8
    Registered User pace's Avatar
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    Slightly OT: Larry did you have any further thoughts on the fuel cut situation, whereby traditional MAP-clamp defender disabling does not appear to be functioning as intended? Specifically, David and myself are still experiencing issues with the UTec allowing the ECU to throw a cut at ~1.4 bar once over 6,000ft elevation. I have heard nothing more from TXS on this issue.

    -Pace

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    Actually Pace I have talked with Phil about this after hitting fuel cut quite a bit last week on a ski trip. It seemed that I could only boost about 16psi through Monarch pass (~12,000ft. above sea level) before hitting fuel cut.

    Phil explained that the UTEC FCD is hardwired so that the stock ECU only sees about 13-15psi AT SEA LEVEL. Which means that up here, say at 6000ft, since we have a 3.5psi deficit thanks to altitude/atmospheric loss the ECU has visibility up to about 18.5psi (relative gauge measured) = fuel cut area.

    I ordered a FCD from TXS and it should be here this week sometime and hopefully I will install it before I go back up skiing next weekend and let you know how it works. I can tell you this though, even at only 16-17psi the car gets to 140mph pretty quickly .

    Oh and as far as why a stock WRX only runs 10psi up at high altitude is becuase a) the stock turbo running out of breathe and 2) the ecu reducing the boost levels AND fuel cut threshold becuase it was designed with the EVER so crappy TD04L turbo in mind!

    Ah the extra tuning BS we high altitude people have to deal with ... [sigh]
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  11. #10
    Registered User pace's Avatar
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    Larry had problems with his aftermarket FCD IIRC. It seems like this would function the same way as the UTec FCD (MAP voltage clamp), so I don't see how this would work any better. I'll be curious to see how yours works out.

    Given the size of the WRX market up here, I am a little amazed that nobody has ever been approached to be an 'official' beta tester for any of these tuning shops that are offering EMS solutions. I don't know the proficiency level of TurboXS' field testers, but I personally discovered three 'bugs' within a couple of days of ownership (one of which was immediately addressed in a patch). Don't get me wrong; I love my UTec and think it is an absolutely outstanding product (the best of its kind, in fact). As a professional software engineer I fully understand the implications of bringing a new product to market. I also know that these minor niggles and glitches can be caught in Q/A if the correct procedures are put into place.

    Essentially, the unique challenges of high altitude tuning seem to be disregarded in the design and testing of numerous performance products (turbochargers are another good example) and it invariably comes back to haunt the manufacturer and/or the consumer in some capacity. Boulder County reportedly has the highest concentration (units per capita) of Subarus of anywhere in the United States. I'm not sure that it makes good business sense to classify our issues as those of a small isolated group.

    It would be nice to see TurboXS include a disclaimer/warning in their product literature that the FCD disable feature is ineffective at high altitude. This will allow customers to plan ahead and budget for the purchase of a third party FCD if that is what is needed to fix the problem.

    Just my 2c.

    -Pace

  12. #11
    Registered User hotrod's Avatar
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    I agree

    Pace:

    Yes Your right I did have similar issues, but since my car is on jack stands at the moment I haven't done much about it other than let a few of you know it seems to be a generic issue.

    I did post some comments on the Cobb's forum about the issue and requested they look at the issue in their ECU reflash as they are physically closer to the problem than any other major vendor.

    I also mentioned we could find a half dozen test mules if they want them, that can test from 5000 ft alt to 14000 ft altitude in a matter of a couple hours.

    I also find the lack of attention to it somewhat of a puzzle. As many subaru owners that live up here and given the large numbers of visitors both for skying and summer travel, it should be more of a concern. Not to mention the California folks that will see the same issue topping Donner Pass to Reno.

    Larry

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    Registered User djrez4's Avatar
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    Also, perhaps we're just a vocal minority, but it seems that there are more people per-capita in the high country that are actually interested in tuning their vehicles. We also happen to be intelligent people, fully capable of doing said tuning.

    Why won't you companies believe us???

    I think it's time to design a line of high altitude turbo-chargers....
    Deliberating enclaves of like-minded people are often a breeding ground for extreme movements - Cass Sunstein

  14. #13
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    Originally posted by djrez4
    I think it's time to design a line of high altitude turbo-chargers.... [/B]
    Way ahead of ya
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  15. #14
    Registered User pace's Avatar
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    Originally posted by davidm_sh
    Way ahead of ya
    Did you make it up here? I dropped by TEC twice (5.15 and 5.45) and tried calling on your cell but there was no answer. I was guessing you got held up in traffic and/or figured it was futile and turned around.

    If you did get some face time with them, AIM me tommorrow so we can further discuss how to proceed.

    -Pace

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    So given the fact that we have pretty much determined that the stock TD04 is out of breath at these altitudes, is there really any advantage to doing something like a ecuTek reflash..i.e. vishnu stage subzero(which I believe raises the boost).

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