Altitude compensation (Re: 286 Vindication)]

[Johnny]

Bruce Bowling wrote:
>
> X-Hpvue$Revision: 1.8 $
> Mime-Version: 1.0
> Content-Type: Message/rfc822
> X-Vue-Mime-Level: 4
> X-Mailer: Elm [revision: 109.14]
> Sender: owner-diy_efi at coulomb.eng.ohio-state.edu
> Precedence: bulk
> Reply-To: diy_efi at coulomb.eng.ohio-state.edu
>
> ~
> ~ MIME-version: 1.0
> ~ Content-type: TEXT/PLAIN; CHARSET=US-ASCII
> ~
> ~ ~
> ~ ~ >Isn't it true that for a speed density efi system with an _absolute_ MAP
> ~ ~ >sensor, then altitude compensation is not needed?  For example, if you
> ~ ~ >changed altitude, the barometric pressure changes, which changes the
> ~ ~ >pressure in the manifold, which the MAP sensor directly measures.
> ~ ~ > Temperature (air density) compensation is still needed.
> ~ ~ >
> ~ Yes, this is true iff one cares only about pressure differential across
> ~ the throttle plates.  This is the method employed in air flow benches.
> ~
> ~ - Bruce
> ~
> ~
> Forget my statement directly above - the original statement (at the top) is in error.
> If you are using an absolute pressure sensor, and you wish to perform differential
> pressure measurements, you need a reading reflecting the barometric pressure
> (taken when the engine is off or a hardwired value) in order to determine
> the differential.
>
> My statement above applies if you are using a differential sensor, with one side
> looking above the throttle plates, and the other below.
>
> - Bruce

Some boxes, the Haltech comes to mind, have a sensor in the box itself
for barometric pressure. The Haltech uses a MAP vs. TP vs. RPM array
and includes a modifier table for barometric pressure along with all of
the other modifiers like intake temp, engine temp, etc.

It would seem that when comparing MAP to TP and RPM you would have to
have a provision for changes in barometric pressure. Your WOT MAP
changes with the changes in barometric pressure, but I don't think that
the engines needs are going to be the same as what is reflected in an
unmodified table. Example:

Sea-level, not WOT, 4000 rpm stablized, 22" MAP
              compared to
8000', WOT, 4000 rpm stablized, 22"MAP

If all other things were equal like intake air temp etc, would this two
conditions be identical in the "eyes" of the engine?

Text book physics would tell you that the difference in air density is
reflected in the fact that at the higher altitude you are at WOT at the
same stablized rpm yet you are only getting 22" MAP and this is how your
program would know the density is less, but I think that in the "real
world", applied physics is going to bring other factors to prevail. Like
the air flow difference in the plenum and runners between WOT and
partial throttle for example. This difference in volumetric
efficiency is difficult to calculate. You need to run the engine and
program the table accordingly. The problem is that you are going to have
a tough time programming the engine on the dyno at Sea-level and
simulating a WOT w/stabilized mid rpm and 22" MAP senario.

The only way to really do it is with a altitude test cell. Only then can
you really measure whats going on and program the box accordingly. At
that point you could tell what the engine sees as the operating
difference between MAP as a function of engine load and MAP as a
function of barometric pressure, especially at extreme altitudes or at
altitudes with the extended table programming brought on by turbo
charging for altitude compensation.

I will be flight testing my V8 test bed before the end of this year to
explore this very scenario. When I get the results I will let you all
know what they are.

hmm, flying laptop, I can hardly wait.

-j-