Imperial MAP values seem backwards

[kp]

I could be wrong here but when inHG is used for MAP shouldnt 0 be wide open throttle and ~15inHG be at idle like a vacuum gauge? When using EIlive it displays idle close to 0inHG and WOT ~20inHG - seems backwards. I know kPA is the other way around (higher numbers for less vacuum). Just curious if my logic is off :)

[John Skiba]

oppps

[Blacky]

Measuring air pressure can be done in different ways:
1. as absolute pressure.
2. as gauge pressure wrt to atmosphere.
3. as vacuum wrt atmosphere.
The units (Metric or Imperial) do not influence how the measurement should be made. i.e. a measurement in kPa is *exactly* the same as a measurement in inHg.

The formula for converting between kPa and inHg is:
1kPa = 0.295299875 inHg
so
0 kPa = 0 inHg
50 kPa = 14.8 inHg
100 kPa = 29.5 inHg
101.3 kPa = 29.9 inHg = 1 atmosphere = 1 bar

Regardless of what units are used to measure it, the OBDII manifold pressure is always measured as an absolute pressure. That is defined by the SAE and is part of the OBDII speification.

The acronym MAP means literally, Manifold Absolute Pressure.
Which is measured with respect to 0 being the pefect vacuum.

Hope I did not confuse the issue even more.
Regards
Paul

[Bruce Melton]

I could be wrong here but when inHG is used for MAP shouldnt 0 be wide open throttle and ~15inHG be at idle like a vacuum gauge? When using EIlive it displays idle close to 0inHG and WOT ~20inHG - seems backwards. I know kPA is the other way around (higher numbers for less vacuum). Just curious if my logic is off :)

I was just reading mine via Scantool and ran from 13inHG/44Kpa at idle, ~11inHG/37kpa at light throttle and 29.5/99 at WOT which I hope is "normal"??

[kp]

You are right Paul, after 25 years of looking at vacuum gauges it is correct the way its converted. Absolute pressure is not manifold vacuum.

Technically, inHG of vacuum is what a typical vacuum gauge reads and thats where I got confused. Also the Gen7 stuff I have been working with lately lists the VE table using kPA or inHG VAC so it looked correct to me there compared to a vacuum gauge.

In other words, never mind :)