Diminishing Boost at High Altitude - Page 2
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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; Originally posted by linklemming So given the fact that we have pretty much determined that the stock TD04 is out ...

  1. #16
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    Originally posted by linklemming
    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).
    Yah there is. I mean espicially if you run seperate boost control to say 16-17psi. The stock ECU will only give you about 12-14psi up here. Larry was saying the stock turbo could only give 'everything it had', about 14psi, up at the top of Pikes Peak (14,580ft). But at 6000-9000ft you can get more power from pushing the stock turbo a bit.
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  3. #17
    Registered User hotrod's Avatar
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    boost is good -- but!!!

    There are two things I think that you need to realize about altitude effects and turbos.

    First the turbo doesn't know what altitude it is at or what engine it is connected to. All it knows is the density of the gas at its inlet, and the pressure ratio you are asking it to work at, and indirectly it knows the engine size because the engine can only accept so much flow at a given rpm with a given engine displacement.

    Second is that the engines actual power output is a function of the absolute manifold pressure not the boost. You have to remember to consider the effect of the drop in local air pressure too.

    What that means for us is that the stock WRX 2 liter at about 4500 - 5000 rpm wants to accept the right amount of flow to put the stock turbo in the sweet spot of its compressor map. At these mid range rpms you can successfully use all the pressure capability of the turbo. If you ask it to, it can deliver almost 26 psi boost at sea level at this rpm point. Thats a pressure ratio of just short of 2.8:1. Here at altitude you run the same pressure ratio and because the inlet pressure is lower you only get a fraction of that boost.

    For example the standard pressure here at 6000 ft altitude is 11.78 psi, so that same 2.78 :1 pressure ratio gives a boost of 20.9 psi, but because the local actual air pressure is only 11.78 psi the total absolute pressure in the manifold is 32.7 psi, compared to the absolute manifold pressure of 40.8 psi at sea level. To get an absolute manifold pressure of 32.7 at sea level you only need to run a 2.22:1 pressure ratio (which means cooler out let temps) or a boost of 17.9 psi.

    This is why you can never (practically speaking) completely negate altitude effects. If you try by running higher boost your running the turbo at a higher pressure ratio and it will almost always produce less actual mass flow.

    Due to the higher pressure ratios you need to run at altitude, turbos have to spin faster to make the boost, so lag is worse (takes longer to spin up the turbo to the higher rpm).

    I have run pretty high boost on the stock turbo up here (18.5 psi peak) to get the max out of the stock configuration. But this is only useful in the lower 2/3rds of the rpm range.

    I know that the high rpm end of the power band is pretty useless as the turbo just can't produce enough flow at that pressure ratio. I counter that by using an agressive waste gate duty cycle on my boost controller to get boost just as fast as I can and seldom take the engine past about 6000 rpm.

    Larry
    Last edited by hotrod; 01-15-2003 at 04:04 PM.

  4. #18
    Registered User pace's Avatar
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    Very well put Larry. As you said, the altitude works against us twofold:

    1. The turbo has to spin a little faster to make the same boost, as demonstrated by the PR calculation.

    2. Since boost is actually a relative measurement above the ambient, you end up with a lower absolute manifold pressure for any given indicated gauge boost. Start with a lower ambient, add the same amount of boost, net a lower end result.

    Essentially our turbos work harder and still net less absolute manifold pressure. So not only do we make less peak power for a given psig, but our turbos also take longer to spool to that point.



    -Pace

  5. #19
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    But just think about it guys. Things could be a LOT worse for us. We could be driving ... [GASP] ... naturally asperated vehicles [heh].
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  6. #20
    Registered User hotrod's Avatar
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    NA sucks --- think about it it really does ;)

    Yeah once you've enjoyed cruising up the west approach of Eisnhower tunnel at 90+ at part throttle in 5th gear and watched the winnebegos disappear in your rear view mirror its hard to go back.


    David, as I was composing that last post I got to thinking about rules to help select an appropriate turbo for up here.


    At first blush the following seem logical

    1) It should like to run at a slightly higher pressure ratio than a turbo of similar flow at sea level.

    2) Its compressor map should not be too close to max rotor rpm at typical pressure ratios, to avoid over speed at high altitudes.

    3) turbine side should be a little bit large for the nominal flow of the compressor to facilitate quick spool up and low exhaust back pressure.

    4) edit -- The A/R should be a little smaller to speedup the compressor to maintain pressure ratio.

    Do these seem to correspond with actual experience with turbo's that are very good at sea level but seem to be a little off up here ?

    Larry
    Last edited by hotrod; 01-15-2003 at 04:49 PM.

  7. #21
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    Re: NA sucks --- think about it it really does ;)

    Originally posted by hotrod

    At first blush the following seem logical

    1) It should like to run at a slightly higher pressure ratio than a turbo of similar flow at sea level.

    2) Its compressor map should not be too close to max rotor rpm at typical pressure ratios, to avoid over speed at high altitudes.
    3) turbine side should be a little bit large for the nominal flow of the compressor to facilitate quick spool up and low exhaust back pressure.

    Do these seem to correspond with actual experience with turbo's that are very good at sea level but seem to be a little off up here ?

    Larry
    Well those criteria do seem about right for us. But it's ironic that the 1820 seems to fit this bill farily well with only ONE problem. Or I should say one problem I have heard of and never tried/experienced. And that is the ability to build any amount of boost to get a good launch up here. I heard WRXThis car trapped at like 109 up here with a 1820 but he ran a 13.7 or somthing around there [heh].

    I heard you had a 16g. Are you going to just try that? What are your thoughts on an 18g for our car? Curious.
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  8. #22
    Registered User hotrod's Avatar
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    Yes have the 16G

    Right I have a small 16G, and will try to see what I can do with that.


    The 1820 was the turbo I was thinking about. I was trying to see if we can figure out why it does not build boost as quick as you would think it should.

    It is tough when you have a turbo that should be a killer, and you just can't get it spooled before the race is over.

    The other possibility is that the real problem is the 1820 gets you into the very long 4th gear too soon and that is the problem. You make so much power you spin through all of low gear (which kills the ET), and then zip through 2, and 3, trapping in 4th.

    Larry

  9. #23
    Wrinklechops
    Quote Originally Posted by davidm_sh View Post
    Yah there is. I mean espicially if you run seperate boost control to say 16-17psi. The stock ECU will only give you about 12-14psi up here. Larry was saying the stock turbo could only give 'everything it had', about 14psi, up at the top of Pikes Peak (14,580ft). But at 6000-9000ft you can get more power from pushing the stock turbo a bit.
    not sure how much more dead this thread could be, but it nevertheless caught my attention. I live up here in Avon (CO) at about 7000ft maybe, and with my Cobb Stage 1 map I'm having trouble getting past 12psi even... not sure what's up...

  10. #24
    Wrinklechops
    Quote Originally Posted by hotrod View Post
    Yeah once you've enjoyed cruising up the west approach of Eisnhower tunnel at 90+ at part throttle in 5th gear and watched the winnebegos disappear in your rear view mirror its hard to go back.
    Oh I love that stretch

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  12. #26
    Wrinklechops
    Quote Originally Posted by Sinister View Post
    Nothing beats the canyon runs...

    glenwood, coal creek, boulder...
    Oh hell yea...I guess there's also no cops running speed traps through the canyons

  13. #27
    Registered User Sarra's Avatar
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    Shut the car off at 10,000 feet, and your boost/vacuum gauge should read a slight vacuum. So, at high altitude, you're probably seeing the same absolute pressure in the manifold, but there's less pressure on the gauge itself, so it is off at altitude.

    If you shut the car off and the gauge reads 0, then... Well, it's a different kind of gauge.

    I noticed that my car freaks out when I go from 6,000 feet rapidly down to 2,000 feet. The ECU reduces boost down to the minimum 7 PSI or so, car feels sluggish. I shut it off and turn it back on and all is good again though. It has to do with the fact that when you go uphill, most cars at least, can actually tell, and will be able to adjust, but can't do the same when going downhill.

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