Someone explain the MAP/KPA correlation

Ok so I am a little bit confused here and I am hoping that someone can explain this to me.
So engine/manifold vacuum has always been a never used thing for me in the past. I never really had any experience with it until I started tuning my fuel injection. So all the experience I have with it is from map sensor readings through a data log. All my datastream readings have always been labeled kilopascals. With 100 being no vac/wide open throttle, and 20 being lots of vac during decel. This is how I have learned it and how I am used to it.
So I thought I would pick up a vacuum gauge to check readings on a vehicle I can not data log. Most all the gauges out there I see are in inches of mercury. So we know its tough trying to learn a new unit of measurement (like being used to fahrenheit and having to switch to celsius). So I thought I would search for a vac gauge that was in kpa, but none seem to match. Or they have both in/hg and kpa, but are opposite of how it is done in the data stream of these ecm's.
So looking at gauges, it looks like 0 in/hg is no vac, and 30 is high vac. And if the guage also has kpa on it, it is 0 for no vac, and 100 is high vac. For example, here is a random picture pulled off google: http://www.mityvac.com/hq_images/MVA6178.jpg

So can someone shed some light on this for me?

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Originally Posted by JeepsAndGuns

Ok so I am a little bit confused here and I am hoping that someone can explain this to me.
So engine/manifold vacuum has always been a never used thing for me in the past. I never really had any experience with it until I started tuning my fuel injection. So all the experience I have with it is from map sensor readings through a data log. All my datastream readings have always been labeled kilopascals. With 100 being no vac/wide open throttle, and 20 being lots of vac during decel. This is how I have learned it and how I am used to it.
So I thought I would pick up a vacuum gauge to check readings on a vehicle I can not data log. Most all the gauges out there I see are in inches of mercury. So we know its tough trying to learn a new unit of measurement (like being used to fahrenheit and having to switch to celsius). So I thought I would search for a vac gauge that was in kpa, but none seem to match. Or they have both in/hg and kpa, but are opposite of how it is done in the data stream of these ecm's.
So looking at gauges, it looks like 0 in/hg is no vac, and 30 is high vac. And if the guage also has kpa on it, it is 0 for no vac, and 100 is high vac. For example, here is a random picture pulled off google: http://www.mityvac.com/hq_images/MVA6178.jpg

So can someone shed some light on this for me?

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You are correct in the way you think about vacuum using Kpa and Hg.:thumbsup:

I don't like the scale on the Mityvac gauge you posted a link to, it's confusing.

Maybe this gauge will work for you? http://www.ebay.com/itm/Wallace-and-...-/160958129419

dave w

That gauge is backwards from what the kpa readings are in a datalog also. Use this chart to convert vacuum gauge inches of mercury to kpa. The last two columns should be all you need. I went through this a couple of years ago and found this chart handy when using a vacuum gauge.

Hope this helps.
Brian

Crap, how did this end up in OBD2? I could have sworn I put this in the general GM EFI systems. All my tuning has been on OBD1 systems, not sure if obd2 if different. Maybe a mod can move it?

I am still confused. I was reading something online about in/hg and it was saying, basically normal ambient air is like 30 in/hg. But all the gauges I see are opposite. Then I see the chart you posted up and the last two rows seem to help. However there are two in/hg columns. One is "absolute" and the other is "gauge", they are complete opposites.
So do you use a different scale when measuring engine vacuum Vs. something like barometric pressure? Is that the difference I am seeing?

you're on the right path with gauge vs absolute, but what you're seeing in the datastream is pressure, while a gauge only tends to show vacuum. 100kPa of vacuum at sea level is near 0kPa pressure. 100kPa of pressure is near 0 vacuum at sea level(and would actually be a bit above barometric pressure if you were in Denver, for instance).

that's why they can appear backwards at times and still be in the same units: kPa-pressure vs kPa-vacuum.

following post is way better

Explaining difference between Absolute pressure and Gauge vacuum: Absolute pressure is based on the idea that zero pressure happens off planet, where there is no atmosphere. Since off planet = zero pressure then areas we measure on Earth will experience pressure. And the pressure will increase as you get closer to the planet's center. Gauge pressure assumes zero is at sea level on earth. As you get further from the planet pressure drops. Gauge vacuum is negative gauge pressure. It starts at zero at sea level and gets higher as pressure gets lower. In summary:

MAP: 100 kPa at sea level --- > 0 kPa in space
Gauge Pressure: 0" Hg at sea level ---> -29.99" Hg in space
Gauge Vacuum : 0" Hg at sea level ---> +29.99" Hg space

Now in the engine, measuring vacuum is a way of measuring how much restriction is keeping air from getting into the manifold. If you have zero restriction then vacuum and atmospheric pressure be equal and the difference between them would be zero vacuum. If you have a large restriction, say when coasting down a steep pass in low gear with closed throttle and a full load, the manifold pressure will be very low (less then zero) while the atmosphere will still be zero. The result will be high vacuum. One difference to note is that most engine vacuum gauges are referenced to local atmosphere rather than sea level.

You can match up vacuum gauge to scantool or ecm MAP by subtracting the MAP from the BARO then converting to inches of mercury. For example:

Engine idling = 35 kPa
BARO = 86kPa

35 - 86 = -51

Vacuum = 51 kPa (throw out the "-" since it's already negative pressure)

convert kPa --> inches mercury (Hg)

51kPa * .2953 = 15.06"Hg

There are conversions from kPa to inches of mercury. Here's one in chart form that you can print out for handy reference:
http://www.aqua-calc.com/convert/pre...nch-of-mercury

HTH

I can move this post if desired but it's just as relative to OBDII as OBDI.

Great explanation, but here's the short attention span theater version:

"Gauge pressure should really be called "Pressure delta" (or difference)... but noone would understand that. :)

IOTW, the Gauge measures the difference between ambient pressure and what's being seen inside the pressure vessel (passage into which the pressure sensing device is poked).

Am I completely wrong in this? Just making sure I have it straight.

Yes,gauge pressure measures pressure delta. But that phrase is generally reserved for measurements taken against two specific points in a system. Pressure delta could refer to readings taken before and after an elbow in an air intake to show flow loss, or to boost pressure readings taken at a wastegate and at the manifold to determine signal loss.