ARGH! How do you change the Boost gauge?

Discussion in 'SRT Hellcat General Discussions' started by T Bone, Mar 11, 2016.

  1. T Bone

    T Bone SRT Hellcat Supporter Gold Supporting Member

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    I don't understand Inches of Mercury..... How can I change this to PSI or even BAR?

    I selected pressure to PSI in the settings screen but it doesn't affect boost....

    Is there a secret menu??
     
  2. Hemi Rocket

    Hemi Rocket Gold Member

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    Inches of mercury measures vacuum... PSI or Bar measure pressure.
     
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  3. Zebug

    Zebug Gold Member

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    When you get on it, it'll automatically change to PSI
     
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  4. T Bone

    T Bone SRT Hellcat Supporter Gold Supporting Member

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    Ah....so it goes from inches for vacuum and on boost it goes to psi??

    Why would they do that? They could have used a color scheme to denote vacuum and boost.
     
  5. Hemi Rocket

    Hemi Rocket Gold Member

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    You ask...Why do they do that... Inches of mercury is how you measure vacuum which is a negative pressure all based on atmospheric pressure of 14.7. A perfect vacuum is 0 which equals 29.92 inches of mercury. Your boost gauge, when in a vacuum is based on that scale of 0 to 29.92 Hg. That is the scale you see on your boost gauge while in the state of a vacuum. When you go into boost (Positive pressure) then the gauge reads in PSI.
     
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  6. T Bone

    T Bone SRT Hellcat Supporter Gold Supporting Member

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    I guess I am asking why can't they use psi or bar for vacuum too. Why inches of mercury? Convention? Tradition?
     
  7. Hemi Rocket

    Hemi Rocket Gold Member

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    Because its an actual measurement of "liquid mercury" in inches. Picture a clear tube on a vertical surface that forms a "U" and each leg is 35" long and it is filled 1/2 full with mercury. Both levels of mercury will be equal heights. Now place a vacuum on one of the tubes and the mercury will rise in that tube and lower on the other. Now measure the difference with a ruler if it's 10" that means there is 10" of vacuum. (Hence why it is referred to as Inches of vacuum) The greater the vacuum the higher it will rise to a Max of 29.92 inches. That's why the scale on a vacuum gauge read to is 30. If you used water instead of mercury in the tube it would rise to a different height so it would be a different scale. Hence the term "inches of mercury". Does that make sense now?
     
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  8. T Bone

    T Bone SRT Hellcat Supporter Gold Supporting Member

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    Thanks and I do understand how it is measured but what I don't get is that inches of mercury can be converted to PSI or Bar. Why bother with 2 scales.

    Didn't the old Indy cars uses inches of mercury to measure boost as well?

    In the end I guess it doesn't matter. Just curious.

    Thanks.
     
  9. Hemi Rocket

    Hemi Rocket Gold Member

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    I see where you're coming from... I think it has to do with the scale that would need to be used but I don't know that. Because its not linear meaning at zero boost/vacuum your starting at 14.7 PSI (atmospheric pressure) and a perfect vacuum is zero or I guess you could say - 14.7 PSI. Interesting question...I'm guessing at some point in time they needed a way measure a vacuum and the liquid mercury scale is what they came up with an that method stuck?
     
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  10. AvsBest

    AvsBest Silver Member

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    Very interesting read.

    To make sure I got this right: Once the hg gauge goes to -1 (I know it doesn't), it then goes into PSI? In other words, 1 hg in a very good vacuum and 29hg is a very bad one or the other way around?

    On the same trend, while in hg, it is using negative pressure in but when it's in psi, it's using the pressure positively to produce the power
     
  11. mfendley

    mfendley Silver Member Hellcat Car Club Gold Supporting Member

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    29.97 is the most vacuum, 1 is almost no vacuum. They use inches of mercury so they have a scale of 0-30, where if they used absolute pressure, it would be a scale of 0-15 (14.7 actually). If they stayed with absolute pressure, you would have to remember that 0-15 was at a vacuum, and 15-30 was under boost. Additionally, when the gauge was at 20, you would have to know you were at 5 pound of boost.

    It's mostly convention; vacuum is at inches of mercury (in the US automotive realm) and boost is in psig, not psia.
     
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  12. TIGwelder

    TIGwelder SSDD

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    The bypass valve allows the engine to run under normal vacuum conditions while idle, cruising under light load, easy acceleration, etc.

    Once the load/rpm is increased, the available boost comes up and then the bypass is adjusted closer to closed.

    For drivability and economy.

    And InHg is a standard like mentioned above. And wouldn't be as precise in -PSI , because we live under pressure all around us because we have an atmosphere. We just say 0 PSI is "atmospheric" pressure.

    Picture an old school carburetor car. Crusing down road would be a high Hg number/more vacuum because the throttle is near closed. If you were to punch it, at that moment it's nearer to 10-0 Hg because you lost vacuum by increasing the amount of air that is available to be used. You lift off the throttle and the vacuum number shoots back up.

    Big cam cars with a more overlap and large plenum areas need to be adjusted with lower number "power valves" in Holleys to account for the low vacuum during idle/light cruise.

    Engines are just air pumps with combustable fuel to control them. The supercharger just allows more air to be shoved in under "pressure" instead of the engine always "vacuuming" it in like a normally aspirated engine.
     
  13. Racist Cat

    Racist Cat Silver Member

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    It would just be weird to show negative psi.

    To have -14.0psi, and then 0psi, and 11psi. Wouldn't be "right"

    The proper way to measure vacuum is not negative psi, but in inches.

    That's industry standard.

    To measure vacuum in negative psi would be like asking to show the gear ratio, rather than gear number. IMHO
     
  14. Catless

    Catless Silver Member

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    The real answer is because that is way it is done by convention. Much like positive current flow is used in Electrical Engineering by convention.
     
  15. AvsBest

    AvsBest Silver Member

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    I know -PSI is not used, I was just using it to confirm the fact that "in hg" was kinda the -PSI equivalent. The opposite of boost if you will. To make easier to understand the science behind it.
     

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in the hellcat how do i change the boost pressure gage to show psi