# Does car ECU subtract atmospheric pressure from MAP reading

In calculation of MAF (g/sec), Does the ECU subtract local atmospheric pressure (baro sensor reading) from Manifold Absolute Pressure (MAP sensor reading) to determine the actual air mass entering the manifold at high throttle angle?

Here you are some data gathered from OBDII scanner :

RPM 2300

MAP 120 kPa

Baro 99 kPa

TPS 19%

So, does the ECU calculate the air mass by (120 kpa) or by (120 - 99 "21 kPa" )?

So MAF =

``````(RPM /120) x (MAP - Baro) / IAT + 273 x VE x ED x 28.966 / 8.314772
``````

Or

``````(RPM /120) x (MAP) / IAT + 273 x VE x ED x 28.966 / 8.314772
``````
• Perhaps density, area and velocity are used. Jun 11, 2021 at 10:57
• @Solar Mike, Yes, I know the MAF equation.. However what I don't get in the calculation I have to subtract atmospheric pressure from MAP sensor reading (as the manifold is losing vacuum and equalize with atmospheric pressure)? or I use the 120 kPa as this the actual pressure in the manifold Jun 11, 2021 at 11:00
• MAP stands for manifold absolute pressure, which is what counts. The air is in the manifold about to be burned, and with no more regard to what is happening (Baro) outside. Trust the MAP. Jun 15, 2021 at 0:20
• @John Canon, Yes, MAP reading should be below standard or local atmospheric pressure (100.1 kPa), and these 120 kPa was because of not applying the PID equation while reading the ECU, I got it right now & you confirmed what I have discovered, thank you so much.. Jun 19, 2021 at 9:26