Sensor Questions?

[Markus Strobl]

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, leather
F-member 9/93                14.09 @99.40   - When stock -  27.5mpg @85mph