When to change your MAF table
12-26-2005, 10:13 PM
#1
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When to change your MAF table I am having trouble understanding why you would have to change your MAF table, regardless of what other changes you make to the engine, if you keep the same MAF.
Can someone explain this to me?
Thanks
Can someone explain this to me?
Thanks
12-26-2005, 10:21 PM
#2
MAFs can be funny...not really "ha ha" funny...more like "ha ha, I just ran over your dog" funny.
If you have really changed your intake tract, or your engine very much, the stock calibration will be off. Even something as simple as descreening and porting the MAF ends will throw off its calibration. If you have a large cam, you will be dealing with reversion effects as the air is coming back the other way in greater quantities, and the MAF can't distinguish the flow direction.
The easiest way to tell if your MAF calibration is off is by watching your trims in closed loop operation (they'll be a decent amount off, dependant on your setup of course), or looking at a wideband O2 sensor at WOT in relation to what you are commanding. If the calibration is off, your engine won't give you what the PCM is commanding.
Bah. I'm tired and rambling. Time for bed.
If you have really changed your intake tract, or your engine very much, the stock calibration will be off. Even something as simple as descreening and porting the MAF ends will throw off its calibration. If you have a large cam, you will be dealing with reversion effects as the air is coming back the other way in greater quantities, and the MAF can't distinguish the flow direction.
The easiest way to tell if your MAF calibration is off is by watching your trims in closed loop operation (they'll be a decent amount off, dependant on your setup of course), or looking at a wideband O2 sensor at WOT in relation to what you are commanding. If the calibration is off, your engine won't give you what the PCM is commanding.
Bah. I'm tired and rambling. Time for bed.
12-27-2005, 12:45 AM
#3
graph Dynamic Airflow vs MAF Airflow (a question for general public tho is which one? SAE?) and if they're close, then you're good, if they're off, then MAF recalibration is in order.
But yea, it's not that hard to throw it off, and it's really easy if you're modding. The lo-tech check is to just disconnect the MAF physically, and if the car drives better without it, then you're in a most definite need for a MAF retune.
But yea, it's not that hard to throw it off, and it's really easy if you're modding. The lo-tech check is to just disconnect the MAF physically, and if the car drives better without it, then you're in a most definite need for a MAF retune.
12-27-2005, 07:42 PM
#4
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Inlet tract changes -can- bias the MAF off from its
calibration but this is not a given. It depends a lot on
how straight things are (as the stock screened unit
enforces a pretty even airflow areal distribution). On
my Camaro, Blackwing lid car a descreened truck MAF
is happy as can be with a truck table (though among
trucks there can be several percent worth of table
"disagreement" and I picked the fattest one, for luck).
My setup is straight, short and this probably helps.
Trucks and 'vettes which have arched or elbow inlet
tubing, seem to report more offset after descreening.
The Z06 MAF has a table that's somewhat different
than (not a whole bunch, but some) the screened
trucks despite being, near as I can tell, identical in
the guts aside from the screen. GM publishes a variety
of MAF cals, differing by platform, for platforms that all
use the same Delphi piece. And this is with the screen
helping out, uniformity-wise.
Some post-MAF effects, I've been told, can also muss
up its mind - bad reversion, stuff that makes the airflow
"messy".
But my opinion is, you should only want to change it if
you have changed the MAF, its geometry or the spatial
distribution of the airflow presented to it.
calibration but this is not a given. It depends a lot on
how straight things are (as the stock screened unit
enforces a pretty even airflow areal distribution). On
my Camaro, Blackwing lid car a descreened truck MAF
is happy as can be with a truck table (though among
trucks there can be several percent worth of table
"disagreement" and I picked the fattest one, for luck).
My setup is straight, short and this probably helps.
Trucks and 'vettes which have arched or elbow inlet
tubing, seem to report more offset after descreening.
The Z06 MAF has a table that's somewhat different
than (not a whole bunch, but some) the screened
trucks despite being, near as I can tell, identical in
the guts aside from the screen. GM publishes a variety
of MAF cals, differing by platform, for platforms that all
use the same Delphi piece. And this is with the screen
helping out, uniformity-wise.
Some post-MAF effects, I've been told, can also muss
up its mind - bad reversion, stuff that makes the airflow
"messy".
But my opinion is, you should only want to change it if
you have changed the MAF, its geometry or the spatial
distribution of the airflow presented to it.
12-27-2005, 09:49 PM
#6
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There are two main tables that control the fueling of the engine - VE and MAF. They are the main tables that you use to get to the commanded AFR that is defined by the stoich value (during closed loop) and PE value (during open loop WOT). Think of stoich and PE as a target, and the VE and MAF tables are the bullets to get you there - you set the target where you want it (i.e. 14.6 for part-throttle and 12.8 to 13.0 for WOT), and then tune the VE and MAF to get you there. The VE and MAF work together and weigh in differently during steady throttle, changing throttle rates, and above 4,000 rpm. That is why its essential to tune first the VE (in open loop speed density) and then the MAF (in open loop) with a wideband. Does that help?
