Altitude compensation (Re: 286 Vindication)]

[Bruce Bowling]

~ 
~ 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?

But airflow is different in these two cases.  This is why when one performs
an air-flow on a cylinder head, carb, etc, the flow is given at a certain test
pressure.  When one states to me that their head flows 100cfm, I always
ask them at what test pressure did they measure it at?  The response is 
almost always a puzzled look.

To convert a given flow rate from one test pressure to another, they
are related by the root of the test pressures:

CFM(new) = CFM(old) * sqrt(TestPressure(new)/TestPressure(old))

(Hey, this is the same equation form used to determine new injector flow
from changing the fuel rail pressure).

Example: If the flow is 65 CFM at a test pressure of 5 inches of water,
what would flow be at 15 inches of water?

	CFM(new) = 65 * sqrt(15 / 5) = 112.5 cfm

Here is a schematic of a flow bench (kinda looks like a throttle body/intake
runner on a car):



                 Air In
                 |    |
                 |    |
                 |    |
                 |    |
            Cyl. Head, carb, etc.
                 |    |
                 |    |
               ---    |
Test Pressure Meter   |
               ---    |
                 |    |
                 |    |
                 |    |
                 |    |
          --------    |
          |      |    |
        Flow     |    |
     Manometer   |-  -|  Calibrated orfice
          |      |    |
          --------    |
                 |    |
             Big Vacuum Device

Hence my original statement that to get actual CFM airflow, one needs two
parameters (barometer and pressure drop across throttle plates).  How
one determines these quantities (using one MAP, or two MAPS, or one differential,
or 5,000 MAPs) is up to the designer.

- Bruce

--
-----------------------------------------------------
<<<<<<<<<<<<<<<<<<<<<<<>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
-----------------------------------------------------
               Bruce A. Bowling
  Staff Scientist - Instrumentation and Controls
 The Continuous Electron Beam Accelerator Facility
    12000 Jefferson Ave - Newport News, VA 23602
                 (804) 249-7240
                bowling at cebaf.gov  
        http://devserve.cebaf.gov/~bowling
-----------------------------------------------------
<<<<<<<<<<<<<<<<<<<<<<<>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
-----------------------------------------------------