pHEnomIC

iTrader: (14)

I am having a hard time understanding the relationship between MAP and vacuum. I believe that vacuum is the difference between atmospheric pressure and MAP.

When people say you want the most vacuum at idle, does that mean you want the MAP to be as low as possible? It seems that when i raise idle spark it actually increases MAP.

Blown06

iTrader: (32)

MAP is just an acronym for the sensor itself. Manifold Absolute Pressure. The MAP measures manifold vacuum or pressure in some cases. Tuning software typically displays MAP measurements in KPA or kilopascals. I don't really know why except that it's an easy way to use one unit of measure for both vacuum and pressure. Vacuum is typically measured in inhg, or inches of mercury and pressure is typically measured in psi, or pounds per square inch. I assume it would be a pain for the software engineers to deal with this change in units, but I also suspect that it has something to do with "absolute pressure", meaning it doesn't take into account that our atmosphere is naturally at about 14.68 psi (about 105 kpa or 1 bar). I'm rambling on cause I'm bored and I might be full of ****.........so someone correct me if I have stated anything wrong.

To answer your question, Yes the MAP will display a lower kpa value for increased vacuum.

Blown06

iTrader: (32)

When tuning ignition timing or fueling at idle.....it is typically best to go with what increases vacuum. So if your timing change is increasing the kpa.....then your going backwards. JMO.

jimmyblue

iTrader: (11)

BARO is your barometric pressure, taken at low RPM high
TPS or pre-crank by the PCM.

Vacuum = BARO-MAP.