Why is there MAP?

[abcdpeterson]

MAP = Manifold-Absolute-Pressure = Indicates the pressure in the intake manifold

Why would there be pressure on the intake side?
The air is being pulled in, shouldn't that result in negative pressure?
Aren’t vacuum lines taken off the intake side?

I can see Pressure with a turbocharged as it pushes air in.

[Shrek]

That's exactly why it is MAP. Absolute pressure means pressure as compared to the only stable pressure comparison: perfect vacuum.

The other alternative would be amount of negative pressure compared to ambient at startup,
and thus would change during your drive.

[lightfoot]

Originally Posted by Shrek

That's exactly why it is MAP. Absolute pressure means pressure as compared to the only stable pressure comparison: perfect vacuum.

The other alternative would be amount of negative pressure compared to ambient at startup,
and thus would change during your drive.

Yup, exactly. To expand on this a bit, normal atmospheric (barometric) pressure at sea level is around 14.4psi. The engine may pull a vac relative to this but the manifold pressure cannot be <0, since 0 is a perfect vacuum. At wide open throttle, the MAP sensor will read close to the atmospheric pressure.

Atmospheric pressure decreases with altitude and other factors. The MAP sensor is one way of calculating the air mass going into the engine so that the fuel delivery can be adjusted appropriately.

When people talk about manifold vacuum, they mean vacuum relative to atmospheric pressure.

[abcdpeterson]

Thanks!
I know it's going to be a good day, when the day starts with new knoledge.

I added MAP to scangauge, it never reported anything. is MAP not always reported?

[Right Lane Cruiser]

Not all vehicles have that sensor.

[lightfoot]

Originally Posted by abcdpeterson

I added MAP to scangauge, it never reported anything. is MAP not always reported?

Not sure if this is true of the Vibe (which is a rebadged Toyota Matrix??), but as I understand it most GM cars use a MAF (Mass Airflow Sensor) which indirectly measures air flow rather than pressure. Don't know how OBD handles this, but MAF might be reported on a different channel from MAP and you would therefore get no reading on MAP?

More info here:
http://en.wikipedia.org/wiki/Mass_airflow_sensor

I'm not an expert so I may be wrong here??

[Shrek]

My car, which is of the same platform as Matrix, Vibe etc reports only LOAD too.
No MAP, MAF nothing.

But I notice LOD is very similar to MAP 99 when stopped, and down to 7-8-9 when dfco at high rpm.