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Premium Member
2001 Subaru Impreza RS
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Discussion Starter #1
Okay, this is a newbie question (shrugges shoulders) here goes. I'm talking to an Audi guy about turbos, he's talking about the meter in his car that measures boost, and it's based on atmosperic pressure. I talk about psi and he's wondering if we are on the same page. Before here, I've always associated psi with superchargers. What's the deal? Why don't we use atmosperic pressure to measure boost, or why does he?

Nick C.
 

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Meany Head
2000 RSC
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10,235 Posts
I'll take a stab at this, If I understand the confusion.

Your friend is talking about atmospheric pressure, otherwise knows as "Bar". His Boost gauge will likely read between 0 and 1.5 Bar. This is basically the Metric version of Psi.

1 Bar = 14.7Psi

Air Pressure at ocean level is equal to 1 bar. Thus, one bar is one atmosphear.

Hope that is what you were looking for.
 

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Yes, the Audi guy is definitely talking about Bar per Skidd's post.

There is a third way to measure atmospheric pressure... and incidentally it's the way the optional WRX gauge pod's boost gauge is measured... in Inches of Mercury (in/Hg). As far as I know, no-one with even half a brain has used in/Hg for an automotive application in God knows how long. Somebody at Subaru must be smoking crack.
 

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2001 Silverthorn Coupe
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If I remember correctly, there are advantages to the different scales. You can also measure pressure in Pascals (I don't think that I spelled it right :rolleyes: ) Anyway, I will ask my wife about it, as she deals with the different pressure scales on a daily basis.

Just a FYI: Pressure at sea level is a good reference for 1 bar or 14.7 PSI, but remember it is not "equal" to it, because atmospheric pressure is not stable, and therefore not a constant 14.7

Weather or not there is any difference in the scales when simply measuring a relative pressure change in the intake manifold of a turbo application is another story though, I doubt that it makes much difference, but is just easier for the person who looks at them to comprehend what information is trying to be relayed. Think of it as just degrees Farenheit or Celcius. They both accomplish the same thing, people are just more comfortable with one or the other. I personally do not like the Bar scale as it is to big. Its like measuring the length of an item with a rule that is 1 meter long instead of a rule with centimeters marked on it.

Greg
 

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2003 Legacy L
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Porter, if you look at the gauge closely, there are two different scales in the boost gauge in the WRX. on the left side, it measures vacuum in cm/hg, and then it measures boost in Bar.
 

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2001 STM Sedan
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Impreza Rider said:
Porter, if you look at the gauge closely, there are two different scales in the boost gauge in the WRX. on the left side, it measures vacuum in cm/hg, and then it measures boost in Bar.
right vacuum is measured in inches of mercury and then boost or pressure is measured in bar....i thought they had a KPa guage as well(which NO ONE uses).
 

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2003 Legacy L
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Hndatch627 said:


right vacuum is measured in inches of mercury and then boost or pressure is measured in bar....i thought they had a KPa guage as well(which NO ONE uses).
I know that...but the way his post reads he makes it sound like the WRX gauge measures only in in/hg:)
 

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2001 Silverthorn Coupe
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Well, I didn't remember correctly, the important differences to my wife are absolute pressure and gauge pressure, the scales don't matter one way or the other.

Greg
 

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2001 Subaru Impreza RS
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Discussion Starter #10
Thanks for clearing that up folks.:rolleyes:

Nick C.
 
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