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Reading your Boost Gauge


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quick question...i have a EBC...and when i drive normally it shoes like -11 etc...when i gun it it shows positive numbers....now i was told w/ a negative its vacuum and positive its the opposite.....but how is it 11psi's? that are bein vacuumed? please help thanks
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Its a different scale on the vacuum side (usuall inches of mercury). It's negative because your turbo isn't providing boost and your engine is "sucking" in air from an intake manifold at atmospheric pressure. Suck = vacuum.
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In the analog gauge above (and yours), you have two scales, positive pressure (PSI) and negative pressure/vacuum (inches of Hg or mercury.


inches of mercury refers to the ability of the vacuum in question to physically pull a column of mercury up a tube in inches. But given the mercury is poisonous, we settle for a nice 52mm gauge in/on your dash.


the trick is to remember both readings are all relative to atmospheric pressure which is already at 15psi (give or take depending on the weather and altitude)


So zero is neither pressure nor vacuum (0psi and 0"Hg),

27" of mercury is about the highest vacuum you can pull on this planet (about -15psi) and boost (or positive pressure) is limited only by your wallet and ability to deal with cracked engine blocks, but is typically about 12.5psi for a stock LGT, and up to about 18psi for a stageII setup.



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Since the gauge measures the difference between the absolute pressure in your intake manifold vs. the absolute pressure of the atmosphere, the absolute maximum vacuum reading is a negative number equal to whatever the current barometric pressure is at the moment (typically 29.5 in. of mercury (Hg.) although as Windex points out this number will rarely exceed 27 in Hg on a car engine.


The factory spec for a stock LGT is +13.5 psi and most will achieve this in second and third gear. This is almost a boost of +1 Atmosphere (14.7 psi at sea level).


Although the vast majority of common boost gauges are calibrated in inches of Hg for the vacuum side and pounds per square inch on the pressure side (ANSI Standards), the Subaru factory optional STI gauge uses the JIS/ISO/IEC system which measures absolute pressure in Maga Pascals MPa, so a real Subaru gauge is calibrated to -0.1 MPa (vac.) and + 0.15 MPa on the boost side.


A idling LGT should measure about -0.064 MPa on a factory gauge.

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  • 3 weeks later...

^ That sounds just fine, actually.



<-- I love Winky, my "periwinkle" (ABP) LGT! - Allen / Usual Suspect "DumboRAT" / One of the Three Stooges

'16 Outback, '16 WRX, 7th Subaru Family

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