Baro Minus Map equals manifold pressure

So... I was de-carbing an engine the other day and (I always monitor a scan tool while do this) it occurred to me, hey I should test Paul's subtract Map from Baro tip! S0...

and do the math

look at the vacuum gauge next to the modis! 21 inches!!! (I always run a long vacuum line from the manifold into the cabin so I can nail the key to off if I have to, never have had to but lets be safe)
Holy crap!!! it works!!! all this time like 25 yrs plus using a scan tool and I never realized that the .84 -.90 volt map sensor reading I always look for is realy 21 inches of vacuum!!!
I just started on the Premium channel because I bought a Modis and wanted to "brush up on my Diag" and holy #$%& I'm Rusty as heck! Paul your site is so fricken awesome!!! i'm a 30 year factory tech ASE L1 certified back in '94 when the L1 test first came out...
and I just cant believe how much I love working on cars now! I feel like i'm reborn using lab scopes again and ditching the factory troubleshooting mazes.... (i almost never use them, just want to know for the results for some direction) Thanks for the Clarity!!

Actually the 0.9 volts DO NOT reflect 21 inches of vacuum.

As per name - Manifold Absolute Pressure -; this is what it really means. The (low) pressure created in the intake manifold at idle will translate into a voltage reading on the scanner. This pressure and the corresponding voltage would not change with different altitudes but the engine vacuum/gauge pressure would!.

Since the age of the Tech 2 the GM diagnostic software offers 3 pics: one being the baro, the second map and third the difference between those two, you may have a look at this concept.

In my opinion the map reading is of more value than the engine vacuum just because it eliminates variables not related to an engine itself. An engine in mechanical sound condition should produce the same amount of intake pressure independently from altitude.

In other words Volumetric efficiency Always remains constant?
Is that a correct statement? The volume of air the engine "consumes" is constant independent of atmospheric Pressure?... I'm digging this concept (oops just showed my age)....

wowcoop wrote:

In other words Volumetric efficiency Always remains constant?
Is that a correct statement? The volume of air the engine "consumes" is constant independent of atmospheric Pressure?... I'm digging this concept (oops just showed my age)....

Yes ......the volume stays constant (how would the cylinder size change?), what changes is the air MASS with different altitudes or engine loads.

Volumetric efficiency of the same engine in theory wil be constant, independently of the absolute atmospheric pressure.

Why that? Because it is a RATIO /comparison between the mass /matter in a given space (volume of the cylinder) that really enters into the cylinder and the mass/matter that in theory would enter into this same space/volume at atmospheric pressure, may that atmospheric pressure be high or low.

This ratio is not affected by changes in absolute pressure as these changes do affect the mass/density in the same way at any atmospheric pressure as well as they do at intake/cylinder pressure.

What DOES change with different altitudes and atmospheric pressures is the absolute mass that is inhaled by the cylinder. And this is what in the end defines the power output. Less airmass due to lower density equals to less oxygen and hence less fuel.

With regards to this topic I feel the term "volume" often times is used in a negligent manner, using it to describe what should be qualified always as mass, eg. mass of air instead of volume of air. There is a reason why stoiciometry is a MASS ratio and the MAF sensor measures air MASS.

HTH ;.)