Use of MAP in FI calcs

[Garfield Willis]

On Fri, 08 Sep 2000 10:27:24 EDT, Heelandtoe at aol.com wrote:

>Ok, so baro is either sampled at power up or in some cases continuously
sampled.  I agree with Ludis that what is needed particularly in the
mass airflow calcs is the pressure inside the manifold, not outside.
So.. is the only purpose for baro then the correction for the effect
on exhaust scavenging that someone mentioned? I would think that a few
kPa variation in baro would have minimal effect on scavenging.

This is interesting; didya know that there are two separate and
parallel threads on this very subject going on in efi332 and gmecm?
Hee.

Anyhoo, I doubt if it's strickly related to scavenging, which is a
dynamic pressure effect that would be superimposed on the overall
pumping pressures the engine is seeing. I'd bet it's just simply the
issue of VE varying simply due to the inlet (i.e., intake) vrs outlet
(exh) pressures, IOW the basic air pumping issue; the engine ingests
air at a certain pressure, and expels it to ambient. MAP measures
absolute ingestion pressure, but without BARO, you have to estimate
the ambient pressure.

Perhaps the best way to consider this whole thing is to think of
variations in baro as equivalent to "a variation in total exhaust
back-pressure with altitude", sotaspeak, since the pressure the
exhaust dumps to (ambient) is changing with altitude. That way it's
easy to see the relative effect of baro changes, and to also see that
the VE tables can't take them into account like they do for whatever
exhaust system you have on the vehicle when the engine is tuned (since
the exhaust system if fixed, and hopefully doesn't change with
altitude :).

So merely from a pumping standpoint, you can see the effect of BARO.

Gar


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