[Gmecm] MAP and Vac

[Robin Handley]

Thanks Andrew.

I was vaguely aware that the initial MAP measurement was used as a pressure
reference for calculating true engine load and therefore correct fueling.
AIUI this is one of the benefits of speed-density over Alpha-N (unless the
Alpha-N has a MAP sensor and incorporates some kind of baro correction, of
course).

I'm still digesting your words, but my recorded data doesn't seem to agree
with the simple relationship that you describe. Maybe I've misunderstood. In
my recorded data, I see a fixed value of Baro (~97kPa). This makes sense,
but my recorded MAP and Vac values do not add up to this.

Fitting a line to a (roughly elliptical) MAP vs Vac plot (that has a number
of outlying points) gives: MAP = 120 - Vac, when Baro = 97 throughout.

Looking at the absolute plots; while driving: Map + Vac ~= 120, but during
the following idling phase, Map + Vac ~= 100 (i.e. a lot nearer Baro),
except when the Fan kicks in. Then, because Vac goes up by 15kPa while the
fan is on, so does (Map + Vac).

What have I missed?

Robin



----- Original Message -----
From: "Andrew Gibson" <andrewsharyn at yahoo.com>
To: <gmecm at diy-efi.org>
Sent: 16 July 2006 04:32
Subject: [Gmecm] MAP and Vac


> Here's the scoop. Manifold vacuum is the best indicator of engine load.
Which is why it is used to control fuel and spark. (I'll explain the
vacuum/map difference in a moment.) The reason the vacuum changes when the
fan comes on is beacuse the fan uses electricity from the alternator which
is driven by the engine. Fan comes on, engine load goes up, vacuum goes
down.
>
>   Here's where the map/vacuum gets interesting. When you turn the key on
the computer takes a Barometric pressure reading for alititude spark/fuel
adjustment. It is taken from the map sensor. When the car is started the
manifold vacuum adjusts the reading at the map sensor according to how much
vacuum is present. Your MAP and Vacuum values in the datastream will always
add up to the BARO reading. For instance say you have a BARO of 25 inHg,
Vacuum of 17 inHg, then your MAP will be 8 inHg. The MAP reading is opposing
the Vacuum because it is Manifold Absolute Presssure - The difference
between barometric pressure and Vacuum. It is used because of the changes
that occur in pressure due to altitude. A vacuum only measurement is not
enough for accurate fuel and spark control at varying altitudes.
>
>   The reasoning is this: At higher altitudes less air is available. The
engine is also less able to pull a high vacuum. With no altitude
compensation the ecm will see a lower vacuum, think there is a higher load
and more air available, and enrich the fuel mixture. When in reality there
is less air and so what the ecm precieves as normal is creating a double
power enrichment without needing one. Lower altitudes will create a lean mix
and even advance the timing for a precieved cruise condition. Less fuel,
advanced timing = engine meltdown. So as you can see all three readings are
very important to proper running, fuel economy, power and engine longtevity.
Enjoy!
>
>
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