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  1. #1
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    Boost at altitude?

    I have no idea about how things work. If your like me, you take the easy way and Google it.

    here is a short list of info ive found, links provided:
    "Now, the turbo. I'm going to try to explain this without many numbers. A turbo is a very simple device. It doesn't know gauge pressure. It doesn't know absolute pressure. It only does ratios. Imagine if you ask a guy to make $10 for you. If you give him $10 seed money, that's kinda hard. If you give him $1000, that's easy, right? A turbo works hard based on the pressure that is coming out of it divided by the pressure going into it. So a turbo at sea level (14.7psia) would work the same amount to produce 14.7psig as a turbo up here (12.2psia) would to produce 12.2psig. It would be a pressure ratio of 2.0 in both cases. Keep in mind the absolute pressure coming out of the turbo is atmospheric pressure plus gauge pressure. So that means with a 2.0 pressure ratio you'd have 14.7+14.7=29.4psi at sea level. At high altitude that would be 24.4psi... sucks, don't it? That's what sucks for us. But even so, THE TURBO DID THE SAME AMOUNT OF WORK TO DO 12.2PSIG UP HERE AS IT WOULD TO DO 14.7PSIG AT SEA LEVEL!!!

    Now, that's not being entirely honest with you. We measure pressure after the intercooler, and there is a drop there. We also are neglecting the intake, and there is a pressure drop there. You need to account for those things, and they usually make it even worse for us altitude folks. I'm not going to get into the details of that.

    So here's what I'm saying - you can't run the same boost pressure as people at sea level for any given turbo. There is a little uncertainty because compressor maps are generally shown in volume flow rate, and we're dealing with 20% less dense air... but I don't want to pretend to guess at that math. Well, maybe I could, but I couldn't explain it without words like "enthalpy", and I don't think that would accomplish anything for anyone who doesn't already understand it.

    There is the same % of oxygen in the atmosphere at 6000msl as there is at sea level.

    The turbo will overcome the lower barometric pressure of being at 6000ft, but only to the point of the what the boost controller commands. Stock LGT boost is @13.4psi above atmospheric. 13.4+14.7=28.1psia. Since you are at less than standard atmospheric pressure, I believe the boost controller will try to get you to @13.4 above your atmospheric pressure.

    You might only see +9lbs of boost, but your "zero" point is about -4psi => 13lbs total boost. IIRC, you lose @2"Hg(1psi) per 1000 feet of altitude gained. Depending on relative barometric pressure for where you are, your boost gauge should read -8" to -12"Hg with the motor off.

    So, back to your question. You are a little down on power compared to being at sea level, but you could tune to run leaner at altitude and be about the same power as the rest of us. But if you did that, going down to a lower altitude on a trip would be very dangerous, as you would be WAY too lean for a lower altitude.

    I actually am going to put this into practice tomorrow. I live at 2700msl. I am getting protuned at a shop that is at sea level. I need to tell him this, otherwise I will run a little rich at home. I will probably have him tune for middle of the road, since I do spend alot of time driving up and down the mountain I live on.

    Just for fun I will put some numbers to the Pikes Peak example:

    Lets take a stock STi with the stock VF39 turbo.

    1ST - calculate the absolute pressure/air that you are pushing through the engine:
    -At sea level:
    run 14.5 stock psi + 14.7psi (atmospheric pressure) = 29.2psi total.
    -At 6000ft ~ the beginning of Pikes Peak hill climb:
    run 14.5 stock psi + 11.4psi (atmospheric pressure) = 25.9psi total.
    -At 14,380ft ~ the end of Pikes Peak hill climb:
    going off the above example I have noticed about 1.1psi atmospheric drop for every 2000ft you go up (6000ft ~ 3.3psi pressure drop). So 14.5psi + 8psi (atmospheric pressure) = 22.5psi total

    2ND - calculate the % of air loss you experience compared to sea level (aka roughly the amount of hp loss you will experience with a stock STi):
    -at 6000ft 25.9 / 29.2 = 88% so you have 12% less pressure going into the engine
    -at 14,380ft 22.5 / 29.2 = 77% so you have 23% less pressure going into the engine

    NOW same comparison for a NA car in terms of how much % less air:
    -at 6000ft 11.4 / 14.7 = 77% so you have 23% less pressure going into the engine.
    -at 14,380ft 8 / 14.7 = 54% so you have 46% less pressure going into the engine.

    3RD - NOW for the good part - calculate the PR you would have to run on the STi to get the same 14.5psi of RELATIVE boost at each elevation:
    -at sea level - (14.5 + 14.7) / 14.7 = 1.98 PR
    -at 6000ft - (14.5 + 11.4) / 11.4 = 2.27 PR
    -at 14,380ft - (14.5 + 8) / 8 = 2.8 PR

    Some interesting notes on the above example:
    -Just to run the same relative boost pressures WHILE still experiencing a 23% loss in air/power you have to spin the turbo at a PR of 2.8 instead of 2.0 at sea level.
    -The above example doesn't take into account less efficient affects of intercooling with less dense air.
    -The above example doesn't take into account lower VEs from higher turbine speeds from higher PRs.
    -The fact that you have vacuum in the intake tube between your air filter and the compressor wheel/inlet. The short of this is you have to add an extra 0.2-0.3 to your PR since your turbo doesn't even get to work with the above atmospheric pressures I just told you. (This is starting to get into the real world side of things [heh]).

    Now you might say well just turn up the boost to compensate right? Well lets just see what that would do to the PR at 6000ft.

    So we want to run the same pressure through the engine at 6000ft as we did at sea level right?
    -Sea level total pressure = 29.2psi
    -So at 6000ft 11.4psi (atmospheric) + x = 29.2 where x is the amount of boost your turbo has to run which would be 17.8 psi.

    SO the PR for running 17.8 psi would be (17.8 + 11.4) / 11.4 = 2.56 PR compared to 1.98 PR at sea level.

    To help put the PR into perspective lets say for fun you decided to put a TD05H-18G on your STi - here is the comp map.

    So you can see at a PR of ~2 at lets say 500cfm (which might be close to what th engine could injest at redline) the turbine rpm would be 105,000rpm.

    Now at a PR of 2.56 ~ 2.6 at 500cfm the turbine rpm would be 120,000rpm. That is an increase in turbine rpm of 14%. Now I honestly don't know what direct/forumla related link that has to the EGBP but I can tell you it's not favorable. Not to mention the fact you lost about 2-3% effeciency from being on a different part of the map.

    Well sorry to ramble on this long but the short answer is yes you can get a MBC to set the boost up a bit higher at elevation. But you can see that it is pretty much a lose-lose game. Fortunately for you the stock TD04L-13G is so small no matter what you do (short of wiring the wastegate shut or preloading the HELL out of the wastegate with helper springs) you will not beable to hold more than about 10-11psi to redline (if memory serves me correctly... it's been about 3 years since Iran that turbo on a 2.0L [heh]).

    Here is a link to the stock TD04L-13G map if you want to do the same math I did above and see where you fall in the map.

    curently searching for more in depth and verified sources, but its common knowledge that higher alt = less power. how much is the question though

  2. #2
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    So pretty much this all sums it up to that AR design was right in having the pressure ratio at 2.5?

  3. #3
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    Too drunk to make any sense of this.

    Stage 2 or 2.5 E9X M3 S65 V8 supercharger kit for sale

  4. #4
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    ^ lmfao hahhaahha

  5. #5
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    Thanks Lost Marine!

    Read it again Sticky while not drunk, basic math and it makes perfect sense. I like this article, there isn't a bunch of jargon.

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