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How much Boost lost due to Altitude?

Discussion in 'West' started by SheepDog, Aug 14, 2018.

  1. SheepDog

    SheepDog Member Established Member

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    Hello All,

    I have recently installed a Procharger on my 17' GT with a P-1x headunit, and a 3.5" Pulley. I live in Colorado and was wondering how much boost loss others at this altitude are seeing? I was under the impression that the 3.5" pulley would be good for somewhere around 12 psi at sea level, but I am only seeing between 6-7 psi at redline. I also have Kooks headers so that should also create about a 1lb loss in measurable boost, but is losing another 4ish PSI reasonable at 5500 feet?
     
  2. Norton

    Norton Long-time SVT Enthusiast Established Member

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    Formulas involving ABSOLUTE air pressure (e.g., 14.7 PSIa @ sea level, ~12 PSIa in Denver, etc) are required to compute exact numbers. In my experience, however, boost loss due to altitude is about 1lb for every 2K feet of elevation. (e.g., My 2013 GT500 was rated to produce max boost of ~15lbs at sea level. This translated to ~12lbs at ~6K ft of elevation.)

    Based on that, I'd expect a loss of just under 3lbs at 5.5K ft elevation.
     
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  3. SheepDog

    SheepDog Member Established Member

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    Thank you Norton, That is the general consensus across a few other forums I have come across as well. My logs don't indicate a boost leak or belt slip, so I guess I just need to pulley down a bit.
     
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  4. luker669

    luker669 Active Member Established Member

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    Don’t just look at you altitude check the density altitude. I’m at 3200 feet but in the summer my density altitude is usually over 6000


    Sent from my iPhone using svtperformance.com
     
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  5. SheepDog

    SheepDog Member Established Member

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    Thank you, I didn't factor that in. I've only ever had Turbo cars before, and the Wastegate operation is based on actual pressure so the turbo will simply spin harder to achieve desired boost. (Provided it isn't mechanically limited) I guess since the Centri's are only ever going to be spun as fast as they are driven, a given impeller speed will indeed produce more or less boost based on air density.

    Looks like on average 90 degree summer day in my town, the Density Altitude is 8627 feet, so basically the Moon. This would certainly cause the 5 or so PSI loss I'm seeing. Looks like on a nice cold winter Day, I will get all of that back, with a DA of 4762 Feet. This also comes with frozen roads and cold tires, so more power and less traction. Maybe I need 2 Prochargers. ;)
    Thanks for the help Sirs.

    Here is a link to the calculator I was using. Density Altitude Calculator - English/Metric
     
    Last edited: Aug 15, 2018
  6. SheepDog

    SheepDog Member Established Member

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    I
     
    Last edited: Aug 15, 2018
  7. SheepDog

    SheepDog Member Established Member

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    Just thought I would follow up on this. I went down 1 size pulley to a 3.4". at 7200 RPM, I am seeing 8.3 PSI. At sea level this would be somewhere around 13 PSI. With the P-1x, OEM Harmonic balancer, and this 3.4" pulley the blower is spinning at 61,319 RPM at 7500 RPM which is basically the max recommended speed for the blower. I think that if anyone else is considering a Centri and lives at high altitude, I would have to recommend going with a larger head unit like the D-1x or Paxton2200. You won't have to spin it so hard to achieve the kind of boost you want.
     

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