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Username Post: MAP question        (Topic#349102)
bowtie44s 
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bowtie44s
Age: 35
Loc: wv
Reg: 08-29-12
01-09-18 09:13 PM - Post#2720643    

I do not know much about the metric system and therefore I avoid it. The English system works great for me but my MAP sensor reads in metric. What is it reading in? I thought KPA but everything I look up says KPA is pressure. I have an Autometer vacuum gauge that reads in good old inches of mercury. When I floor it, the vacuum drops to 0 inhg but the MAP goes to 100.

Is it a different way of measuring the same thing? How do you convert one to the other?

Jeff

'88 Chevy K3500, aluminum head roller cam 511in³ stroker 10.5:1 compression, 96 NV 4500, 94-98 grille, 305/70-16 (33x12) BF Goodrich KM2s, 91 cluster swap


 




454cid 
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454cid
Age: 44
Loc: West Michigan
Reg: 02-18-12
01-10-18 12:17 AM - Post#2720651    
    In response to bowtie44s

I had to look up MAP sensor, for some reason I thought you were talking about the MAF, at first.

You're vacuum gage is telling you "gage" pressure, which is zero at your local air pressure. The MAP sensor measures "absolute" air pressure, which is measured against a true 0 pressure. 1 atm (atmosphere) is about 30 inHG, and 30inHG is about 100 Kpa.....not exact, but ball park. So your two readings are the same, but in two different units and from two different reference points.

99 K3500 RCLB


Edited by 454cid on 01-10-18 12:22 AM. Reason for edit: No reason given.

 
Shepherd 
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Posts: 1033

Loc: Lake George, NY
Reg: 11-11-15
01-10-18 05:45 AM - Post#2720664    
    In response to bowtie44s

If you are using a scanner, go to settings and switch to english.



 
65_Impala 
Very Senior Member
Posts: 3919

Reg: 12-29-02
01-10-18 07:29 AM - Post#2720675    
    In response to bowtie44s

Same reading, just different scales.

MAP is pressure referenced to absolutely no pressure. So, 100kPA is ~standard~ atmospheric pressure. It's made with a pressure sensing element that basically has the engine vacuum on one side and a complete vacuum on the other side. So, no engine vacuum at WOT reads higher than having engine vacuum at idle because the atmosphere is passing into the intake and to the sensor and then pressing on the element when there is no engine vacuum.

Historically, vacuum was first measured in reference to the atmosphere using a manometer. A U shaped tube with mercury in it, one end open to atmosphere and the other end being sucked on by the vacuum. The height the mercury lifted was equated to the amount of vacuum, hence the "Hg (inches of mercury) as the units. The tools to measure vacuum improved, but the units stuck.

It's not an simple one step multiply or divide conversion.

To do it take the MAP reading, divide by 100, multiple by 29.61 and then subtract 29.61.





 
bowtie44s 
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bowtie44s
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Loc: wv
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01-10-18 08:30 AM - Post#2720680    
    In response to 65_Impala

Thank you. Using your formula, my 47 at idle converts to -15.6 which is about where I'm reading in inhg.

Jeff

'88 Chevy K3500, aluminum head roller cam 511in³ stroker 10.5:1 compression, 96 NV 4500, 94-98 grille, 305/70-16 (33x12) BF Goodrich KM2s, 91 cluster swap


 
454cid 
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454cid
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Loc: West Michigan
Reg: 02-18-12
01-10-18 12:20 PM - Post#2720699    
    In response to 65_Impala

  • 65_Impala Said:

To do it take the MAP reading, divide by 100, multiple by 29.61 and then subtract 29.61.





Could you explain where your formula is coming from? It's not making any sense to me, and I'm pretty good with units.


99 K3500 RCLB


Edited by 454cid on 01-10-18 12:43 PM. Reason for edit: No reason given.

 
bowtie44s 
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bowtie44s
Age: 35
Loc: wv
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01-10-18 07:06 PM - Post#2720726    
    In response to 454cid

  • 454cid Said:
  • 65_Impala Said:

To do it take the MAP reading, divide by 100, multiple by 29.61 and then subtract 29.61.





Could you explain where your formula is coming from? It's not making any sense to me, and I'm pretty good with units.




It works, I compared my map to my vacuum gauge at a few different places and his formula works.

Jeff

'88 Chevy K3500, aluminum head roller cam 511in³ stroker 10.5:1 compression, 96 NV 4500, 94-98 grille, 305/70-16 (33x12) BF Goodrich KM2s, 91 cluster swap


 
454cid 
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454cid
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Loc: West Michigan
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01-10-18 08:48 PM - Post#2720747    
    In response to bowtie44s

  • bowtie44s Said:

It works, I compared my map to my vacuum gauge at a few different places and his formula works.



I guess he's just using the 100 to shift the decimal place. The number 29.61 doesn't match what I can find, However. It's a bit off, and isn't due to rounding. Probably close enough for our purposes.

What exactly are you doing?


99 K3500 RCLB


 
bowtie44s 
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Posts: 3806
bowtie44s
Age: 35
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01-11-18 08:48 AM - Post#2720776    
    In response to 454cid

I am guessing the 29.67 is normal barometric pressure. Ours is currently 30.02 so I'm just guessing that's where some discrepancy could be.

I have a FiTech TBI and the handheld has map readings. I have had a vacuum gauge for a long time. I was just wondering how the two correlate.

Jeff

'88 Chevy K3500, aluminum head roller cam 511in³ stroker 10.5:1 compression, 96 NV 4500, 94-98 grille, 305/70-16 (33x12) BF Goodrich KM2s, 91 cluster swap


 
65_Impala 
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Reg: 12-29-02
01-11-18 06:03 PM - Post#2720820    
    In response to 454cid

I just threw out a number that I though was the reading for a vacuum. To be more precise consider the following.

0kPA is 0 atmospheres and 100kPA is 0.986923 atmospheres.

Engine vacuum is considered -29.92 "Hg at 0 atmospheres and 0"Hg at 1 atmosphere.

This means the vacuum changes by 29.53"Hg as you go from 0 to 100 kPA.

So, a more accurate formula should be something like this.

Figure out the percentage of kPA by dividing the reading by 100kPa.
Multiply the percentage by 29.53 "Hg to get the amount it's away from a total vacuum in "Hg.
Add the amount it's away from a vacuum to -29.92 "Hg.

You do have to decide if you want the vacuum reading to be positive or negative. I'm giving a value that is more negative for more vacuum.

You have to be careful of what standard pressure the reading are referencing or comparing to. The MAP reading is the pressure above a vacuum. The "Hg used in a car is how much the mercury falls on the side of the manometer open to a 1 atmosphere standard atmosphere when the manifold vacuum sucks on one side of the manometer.





 
454cid 
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454cid
Age: 44
Loc: West Michigan
Reg: 02-18-12
01-11-18 08:42 PM - Post#2720832    
    In response to 65_Impala

Wow, to me, that's making it way more complicated than it needs to be. I don't understand why you want to use a percentage.

MAP kpa x .2953inHg/1KPA - 29.9213 inHg (or local pressure) = gage pressure inHg





99 K3500 RCLB


Edited by 454cid on 01-11-18 08:55 PM. Reason for edit: No reason given.

 
65_Impala 
Very Senior Member
Posts: 3919

Reg: 12-29-02
01-12-18 03:02 PM - Post#2720880    
    In response to 454cid

No, the long form is just explaining where the calculation comes from.

It's not like the formula MAP divided by 100 x 29.53 - 29.92 is so much more difficult to calculate.

29.52 divided by 100 is 0.2953, so you just hid the divide by 100 into the formula.



 




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