Boost and High Elevation question.
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This is a discussion on Boost and High Elevation question. within the High Altitude Tuning forums, part of the Engine Modifications category; When at higher elevations ie. 5000+ Ft, does the turbo achieve a full boost of 13.5-14.5 PSI? ? I know ...

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    Registered User bmxdave's Avatar
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    Boost and High Elevation question.

    When at higher elevations ie. 5000+ Ft, does the turbo achieve a full boost of 13.5-14.5 PSI? ? I know this is a touchy topic but would a MBC set slightly higher than the stock boost help out? Im not looking to make my car really fast, just a little quicker.

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    Registered User qoncept's Avatar
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    Doh! I lost the magazine (and don't remember for sure what it was..). I think it was Modified Exotic or something. They had an article about the Pike's Peak hillclimb and started discussing turbocharged cars and elevation. It said that an NA car will suffer from the lack of oxygen, but because the turbo is always pushing it's 13.5psi or whatever it's set at, it doesn't see the problem so much.

    Make's sense, but one could easily make up an argument that the opposite is true. Can anyone add anything?

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    Registered User ebr0t's Avatar
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    moving to the right forum.
    Quote Originally Posted by DTR rex

    Peter North > *

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    Registered User crazycam31's Avatar
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    Read this. Read Me

    So, esentially, NA cars do suffer more but you don't want to be running the same amount of boost becasue there is less air to push so you are over spinning the turbo.

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    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.
    www.stealth316.com/images/td05h-18g-jlspec.gif

    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.
    www.stealth316.com/images/td04-13g-jlspec.gif
    Stock 2004 WRX STi
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    Registered User bigd's Avatar
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    i'm going to read this when i have the attention span it requires. i've been curious about this as well.

    but for now, I =
    Dave
    01 RS

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    Registered User bmxdave's Avatar
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    So i'm seeing alot more drawbacks to using a MBC than im seeing advantages, I've been offered one for $20 but now i'm thinking its a bad idea.
    Basically its more wear on the turbo, on the engine, and could get partial throttle full boost(Can someone explain why this is bad, running lean perhaps?) just to go a little faster...I know its not the way to start when modding but I wasnt planning on cranking the boost way up.
    Mods - Perrin Uppipe, Crucial Racing Downpipe, SPT/Bosal Catback, Samco IC Hoses, Cobb Stage 2, K&N Panel, GPMoto Intake Elbow.

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    Well with all the above said I used a EBC on my 02WRX and also on my 04STi. The WRX especially the newer ones do suffer from PTFB (which will cause you to run a lot of boost at or near stoich afrs). The STi does not seem to suffer from this as verified by a wideband.

    But there are some gains to be had with a MBC/EBC ... I used to run about 16.5psi peak and it would taper down to 11psi or so by redline with my EBC. It is more wear and tear though on the turbo however.
    Stock 2004 WRX STi
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    Registered User qoncept's Avatar
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    Seems to me you can boost just as much and you're not going to lose horsepower because you can't get enough air, but I bet altitude affects efficiency and changes the compressor map.

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    Registered User FormerlyEuro's Avatar
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    wow. thats some facinating stuff davidm_sh. thanks. I live at the base of Pikes Peak in Colorado Springs and Ive always been curious about the exact math involved concerning air flows at this altitude. Thanks!

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    very informative post..that figures would provide much information concerning flowmaster air flows..
    Last edited by kurtdaniel; 06-08-2009 at 06:16 PM.

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