Sensor Questions?

[ARTHUR OKUN]

You wrote: 
>
>The Q of MAF vs speed density has come up in 2 mailing lists at once,
>so this goes out to both lists...
>
>
>> Interesting, Scot.  It doesn't seem that there is really a need for 
a MAF, 
>> then.  Is MAF just an aid to enable the CPU to learn better?  Or is 
there a 
>> MAF term in the fuel equation?
>> 
>> --steve
>
>I like to think about it this way (it is a bit simplified, but it's
>basically true):
>
>A speed density (SD) system will only know how much air is entering 
the
>engine as a percentage of the maximum flow. For instance, driving at 
half 
>throttle at a certain condition may tell the ECM the engine is using 
50%
>of maximum flow. Dyno test run by the manufacturer indicate that max
>flow is 600 cfm (cubic feet per minute) of air. The ECM then knows how
>much fuel to inject for 300 cfm of air. 
>
>This sounds like a perfect setup, until you start to think about it. 
>Production tolerances will affect max flow in an engine (casting 
flash,
>mismatched cylinderheads-to-manifold, etc). Also, max flow will change
>as the engine ages. And we haven't even started considering 
modifications
>such as ported heads, hotter cam etc.
>
>So as soon mods are made to a SD engine, a new chip has to be burnt. 
>
>The solution to this is the MAF (mass air flow) sensor. Instead of 
measuring
>the percentage of maximum flow, it measures the actual amount of air 
that
>enters the engine. After some corrections for air temperature (simple) 

>the ECM can then calculate the required amount of fuel. A MAF engine 
>will respond very well to modifications, aging, production tolerances
>etc. Only drawback is slightly higher cost, and the restriction of the
>sensor in the intake path. 
>
>Modern MAF systems also have a MAP (manifold absolute pressure) sensor 
that
>makes it a complete SD system. The ECM continuesly compares MAF to SD 
while
>in closed loop to try and catch a MAF failure. If it determines that 
the
>MAF has failed (ie MAF value is *really* out of whack compared to SD), 
the
>ECM will run in SD mode and disable the MAF.   
>
>
>> Steve Ravet
>
>Markus Strobl                96 Z28 6M Black/grph. Borla, !CAGS, PEG2, 
QLC 
>Dallas, Texas                1LE driveshaft,  Honker, T-tops,  CD, its 

>F-member 9/93                14.09 @99.40   - When stock -  27.5mpg 
@85mph
>
its hard to build a non  microprocessor based fi system without
a maf ;   think about it.