Map Readings

[wa g6et]

Can someone please explain the MAP readings I get on my Scan Gauge readings from the OBD port, is the MAP available at all?. How does my boost compare to this? Being in Perth we are pretty much at sea level so atmo is pretty much 14.7psi, so if I see 22psi on my MAP gauge does this equal 7.3psi?

Thanks in advance

[Barra]

I think MAP on OBD is in kPa including vac. Do a search for map kPa to psi on google and find one with vac. I think 100 is 0 Psi boost. Used to go by the kPa, upgraded to digital Psi gauge. Look up Auber instruments digital gauge from the states on ebay. Hope this helps.

Edited 10/01/12 10:38 AM by Barra

[discostig]

Manifold Absolute Pressure. Use along with BAP (barometric absolute pressure aka air pressure outside) to figure out boost in PSI using this:

Boost PSI = (MAP - BAP) / 2.04

So yeah it's measuring in kPa I think? Anyway IH8TOADS helped me with that formula.

One of my pretty graphs that I love so much:

Edited 10/01/12 10:52 AM by -SteveR-

[ratter]

The ECU records in inches of mercury

1 psi = 2.036021inches of mercury

[wa g6et]

So from this,using psi because my MAP gauge is set to read psi at the moment:

22.4(MAP reading)-14.7(BAP Perth sea level)/2.04=3.8psi boost

Is this how it works?

Edited 10/01/12 11:14 AM by wa g6et

[wa g6et]

Found this converter/calculator, now I'm confused. Using this I get 7.6psi which feels closer to the mark as I was giving it a good squirt at 22.4psi MAP.

http://www.megamanual.com/v22manual/mturbo.htm

and this

http://www.chapelsteel.com/psi-mpa.html

[Wenier]

Also the atomosphere isnt always 14.7 at you have to take into account temp of the day etc.

[discostig]

Inches Hg eh, thanks Ratter! I only know the formula that I need to apply to the numbers I log.

Yeah atmos changes hence the BAP being read by the car all the time and used to process boost levels I'm guessing.

[wa g6et]

Also the atomosphere isnt always 14.7 at you have to take into account temp of the day etc.

Checked the air pressure here in Perth with BOM 1013.5hpa which equals 14.7psi

[discostig]

The plot thickens!

Where is the BAP sensor in the car, is it inside the intake system before turbo? Don't think it is as I've only noticed the BOV and PCV tube I think on the pre turbo piping?

Could the BAP as read by the car differ from the air pressure in still air outside?

Used my convert app on the BAP above (29.09151) and it comes out as 14.28841 PSI. Another log from months later was BAP = 29.09151 In Hg = 14.74838 PSI. Oooh the numbers!

Edited 10/01/12 12:13 PM by -SteveR-