Pressure-Vacuum Chart

Bob Owner/Technician East Longmeadow, Massachusetts Posted   Latest   Edited  

I added a nice pressure vacuum chart to my resource page that might interest everyone. It has Idle and WOT references which is nice. I wish I could tell you where it came from but for the life of me I can't remember where I got it and I can't find it online anywhere.

If you search for "pressure vacuum" it should come up at the top of the page.

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Victor Owner/Technician
Doral, Florida
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It was my belief that one inch of vacuum was equivalent to three psi of pressure.

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Bob Owner/Technician
East Longmeadow, Massachusetts
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Victor, I don't see that. There is a great website where you can play with all kinds of conversions to see for yourself. convertunits​.​com/type/pressure Where it says "convert between two pressure units" click on the arrow and you will get a drop down menu to select any type of unit you want to compare. Enjoy.

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Scott Manager
Claremont, California
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Hi Bob, Here‘s the link to the table I shared with you at the TST Calibration Insight class last May: engineeringtoolbox​.​com/vacuum-convert…

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Rick Technical Support Specialist
Saint Charles, Missouri
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Bob, That is my go to chart. I carry it with me always. If only all techs and engineers could agree what pid value would identify MAP consistently. I cant remember where we got it but we handed it out in our drivability classes at the Federal mogul training center in the early 2000s. I'm partial to KPA. Thanks

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Randy Analyst
Denver, Colorado
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That chart is very misleading. There isn't a direct correlation between RPM, BARO and vacuum. Here are 4 screenshots, idle and accel, from 2 different vehicles. Using your chart, what is the vacuum? After uploading I see the images aren't very readable, I will try to upload them again later. If you can read them, go ahead and look but apologies for the low quality.

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Bob Owner/Technician
East Longmeadow, Massachusetts
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I believe the chart is meant as a quick reference guide and not for specific vehicle specs. Manufacturers use many different values in scan data and sometimes it's nice to have a quick reference to see how the different measurements compare to each other. Especially at atmospheric (WOT) vs typical idle value. I'm certainly not using this for an exact spec. Some values are high at WOT and some…

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Randy Analyst
Denver, Colorado
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diag​.​net/file/f4z6jh1md…

The readings from the graph are posted above. As everyone knows, vacuum = BARO - MAP so once we convert BARO to the unit we desire, in hg, we simply subtract to get vacuum. The 2 accel's are near WOT. There is nothing wrong with either vehicle that would affect these readings (they have a slight rich condition). Using the…

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Bob Owner/Technician
East Longmeadow, Massachusetts
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Randy, I appreciate your post and I see what your saying. I took some numbers from my own vehicle for comparison. Here is what I see. Doing the math, Baro kpa - MAP kpa at WOT = 98.98kpa Baro kpa - MAP kpa at idle = 33.79 kpa. These numbers closely mirror the chart. If we convert to inhg I get 29.23 inhg at WOT and 9.98 inhg of pressure at idle (approx 20 in vacuum). This also closely mirrors…

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Randy Analyst
Denver, Colorado
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Was your idle taken in Drive? Defrost on? FYI my post was in drive for the CX7 and in park for the Ram.

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Bob Owner/Technician
East Longmeadow, Massachusetts
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In drive but I can't remember for sure about the defrost. I recorded this data about a month ago.

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Randy Analyst
Denver, Colorado
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Do you see why I asked? Using vacuum, you and I can’t compare data but I’m 5000 feet higher than you and the pressure at idle is virtually the same.

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Bob Owner/Technician
East Longmeadow, Massachusetts
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If you hooked up a mechanical vacuum gauge to your Ram, what would it read at idle?

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Randy Analyst
Denver, Colorado
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Same as baro - map. What do you use this chart for?

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Marlin Technician
Estacada, Oregon
Marlin Default
 

What is supposed to be the use of this chart? Manifold pressure depends on more things, such as elevation and engine design.

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Bob Owner/Technician
East Longmeadow, Massachusetts
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See my reply to Randy.

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Marlin Technician
Estacada, Oregon
Marlin Default
 

I read it, thanks. Maybe I am just dense (nice pun?) today, but the only value I find in the chart is easy correlation among various ways of measuring and reporting pressure. If all references to engine speed and/or throttle position were removed, I think the chart would be improved; not only are the references ambiguous, they are likely to be very misleading for anyone who would rely on the…

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Rick Technical Support Specialist
Saint Charles, Missouri
Rick Default
 

Most all charts will reference @sea level. Key words here Reference. Cant remember the calculation for each 1000 ft per 1"hg??, = 7KPA approximate. This looks like a great topic for a four class at Vision with "Bernie" LOL

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Randy Analyst
Denver, Colorado
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No, the problem with the chart is is makes a linear relation from pressure to vacuum. That's absolutely false. If you remove the second column, I would have no problem with it.

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