Sensor Questions?

[SRavet at bangate.compaq.com]

John T Stein <JSTEIN at dpc2.hdos.hac.com> Wrote:
| 
| On Friday Jan 26, "SRavet" wrote the following in response to a 
| request to contrast MAF vs. MAP sensor-based systems:
| 
| 
| 
| > The difference is that MAF is more complicated, but is more tolerant of 

| > engine modifications.  MAP (or SD, speed density) use a pre-calculated 
| > volumetric efficiency value.  Basically, rpm determines the volume of 
air 
| > moving through the engine, and the pressure determines the amount of 
oxygen 
| > in that volume.  The computer then injects the proper amount of fuel.  
If 
| > you increase the breathing ability of this engine (headers or intake 
modes, 
| > etc), the computer doesn't know about it and will inject too little 
fuel.
| > 
| > MAF systems are really SD based, but the computer is able to modify the 

| > injector on time based on the reading from the MAF sensor.  It is much 
more 
| > tolerant of future engine modifications without requiring the prom to 
be 
| > updated.
| > 
| > Steve Ravet
| > sravet at bangate.compaq.com
| > Baby you're a genius when it comes to cooking up some chili sauce...
| 
| I have heard this discussion of the "shortcomings" of MAP-based 
| systems before and have always assumed that as the engine management 
| computer observed a long-term lean condition (from the O2 sensor 
readings), it 
| would adjust the injector "on" time to correct, thereby accommodating 
| any changes to engine volumetric efficiency brought about by engine 
| hardware modifications.  In the case of a GM system, I would expect 
| the block learn values to be above nominal (128). 
| 
| Is this assumption correct, or have I credited the engine computer
| with too much "intelligence"?? 
| 
| John
| 

John, you are correct, my description above was a little simplistic.  As 
long as the ECM is in closed loop, it will adapt (within the block learn 
limits).  If you aren't closed loop, though, pre-programmed values are used 
which will result in a lean condition.

Steve Ravet
sravet at bangate.compaq.com
Baby you're a genius when it comes to cooking up some chili sauce...