[Gmecm] MAP and Vac

[Robin Handley]

Plot here: http://213.162.107.39/misc/MAP_Vac_Baro.gif

Robin

----- Original Message -----
From: "Robin Handley" <Robin at FuryWorld.fsnet.co.uk>
To: <gmecm at diy-efi.org>
Sent: 16 July 2006 10:20
Subject: Re: [Gmecm] MAP and Vac


> 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!