[Gmecm] MAP and Vac

Tue Jul 18 11:51:56 UTC 2006

The relationship Andrew described is correct.

What is the lowest value allowed by the code for vacuum?  It appears that vacuum at WOT is limited by software values.  The vacuum reading should be zero with key on and engine off.

MAP and vacuum are equally effective as indicators of load.  My guess as to the reason for the vacuum based table is that it was imported from an older version of code written when engineers were more likely to use vacuum based units.

Zaphod

"Robin Handley" <Robin at FuryWorld.fsnet.co.uk> wrote:

>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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