I always wondered this....on a MAF engine, like my '95 LT-1, at WOT does the PCM get it's airflow mass measure from the fuel table calculations (RPM vs. MAP + Baro + IAT, etc.) or does it ignore the fuel tables and strictly use the MAF sensor output? I'm thinking it uses the MAF and the fuel tables are used for backup pusposes if the MAF fails. Anyone know for sure?
What about at cruising RPM? I guess the main question is why have fuel tables on a MAF car? All SD systems I've worked with use the fuel tables to untimately calculate airflow. Seems like it's not needed (unless for backup) if you have a MAF sensor.
I always wondered this....on a MAF engine, like my '95 LT-1, at WOT does the PCM get it's airflow mass measure from the fuel table calculations (RPM vs. MAP + Baro + IAT, etc.) or does it ignore the fuel tables and strictly use the MAF sensor output? I'm thinking it uses the MAF and the fuel tables are used for backup pusposes if the MAF fails. Anyone know for sure?
What about at cruising RPM? I guess the main question is why have fuel tables on a MAF car? All SD systems I've worked with use the fuel tables to untimately calculate airflow. Seems like it's not needed (unless for backup) if you have a MAF sensor.
The VE maps are used for drivability and for WOT as well. The VE tables are what the PCM looks for fueling should the MAF sensor fail. Also the VE maps are used for fueling even with the MAF. Contrary to belief the VE Maps are used extensively as a predictor for the PCM fueling strategy. The VE table is used for throttle transient airflow estimation. It works like this, lets say that you are running along at part throttle cruise conditions, with high vacuum at the intake manifold, the MAF sensor is reading the amount of air that enters the engine.
Then lets say that you suddenly go to WOT. When this happens the engine would get a big rush of air into the manifold and vacuum goes low. The air that enters the manifold is measured by the MAF but doesn't actually go into the cylinders. At this point the MAF is reading far more air than the cylinders actually has entering the cylinders and the result would be a rich condition, as more measured air = more fuel. If you look at your MAF data logs you can see this "spiking" during positive throttle position changes such as going from cruise to WOT. The opposite is true negative position changes where you are snapping the throttle shut ( when the MAF stops reading airflow while the manifold builds vacuum, would result in a lean condition).
Now this is where the VE strategy comes into play, the PCM uses the MAF as an estimator. The MAF input and the VE cells cells that are accessed at that moment are used in an advanced calculation, also taking into account TPS values into quite a long predictive airflow/fueling routine, with varying coefficients depending what "zone" or "cell" of operation the engine is in. The result is an air mass value that the PCM can actually use for the fueling needs of the engine.
While in WOT conditions all of the above is used along with the PE tables to caculate WOT fueling requirements.
So yes the VE tables are used for back up in case of MAF failure. But they are also used along with the MAF for more accurate fueling.
I always wondered this....on a MAF engine, like my '95 LT-1, at WOT does the PCM get it's airflow mass measure from the fuel table calculations (RPM vs. MAP + Baro + IAT, etc.) or does it ignore the fuel tables and strictly use the MAF sensor output? I'm thinking it uses the MAF and the fuel tables are used for backup pusposes if the MAF fails. Anyone know for sure?
What about at cruising RPM? I guess the main question is why have fuel tables on a MAF car? All SD systems I've worked with use the fuel tables to untimately calculate airflow. Seems like it's not needed (unless for backup) if you have a MAF sensor.
The VE maps are used for drivability and for WOT as well. The VE tables are what the PCM looks for fueling should the MAF sensor fail. Also the VE maps are used for fueling even with the MAF. Contrary to belief the VE Maps are used extensively as a predictor for the PCM fueling strategy. The VE table is used for throttle transient airflow estimation. It works like this, lets say that you are running along at part throttle cruise conditions, with high vacuum at the intake manifold, the MAF sensor is reading the amount of air that enters the engine.
Then lets say that you suddenly go to WOT. When this happens the engine would get a big rush of air into the manifold and vacuum goes low. The air that enters the manifold is measured by the MAF but doesn't actually go into the cylinders. At this point the MAF is reading far more air than the cylinders actually has entering the cylinders and the result would be a rich condition, as more measured air = more fuel. If you look at your MAF data logs you can see this "spiking" during positive throttle position changes such as going from cruise to WOT. The opposite is true negative position changes where you are snapping the throttle shut ( when the MAF stops reading airflow while the manifold builds vacuum, would result in a lean condition).
Now this is where the VE strategy comes into play, the PCM uses the MAF as an estimator. The MAF input and the VE cells cells that are accessed at that moment are used in an advanced calculation, also taking into account TPS values into quite a long predictive airflow/fueling routine, with varying coefficients depending what "zone" or "cell" of operation the engine is in. The result is an air mass value that the PCM can actually use for the fueling needs of the engine.
While in WOT conditions all of the above is used along with the PE tables to caculate WOT fueling requirements.
So yes the VE tables are used for back up in case of MAF failure. But they are also used along with the MAF for more accurate fueling. This is especiallyt true in the later LS1 based OBD2 cars where even more complex alogorithms are used for fueling strategies
I don't think so because they didn't use a speed density or VE based strategy. I am sure that in the disassembly there are some hard coded equations for VE based strategy. But I am not personally aware of them.
Thanks Tom. So it seems for general driveability the VE tables would not have as much of an impact on an engine with a MAF. For example, if you do a cam swap the MAF system would be able to adapt better. Correct?
Thanks Tom. So it seems for general driveability the VE tables would not have as much of an impact on an engine with a MAF. For example, if you do a cam swap the MAF system would be able to adapt better. Correct?
Yes, a typical MAF strategy is much more forgiving than a speed density strategy. The main reason is that the SD scenario relies on two inputs for its strategy, and they are engine RPM and the MAP sensor for its load calculations. Change the cam and you change the VE of the engine which means that instead of idling at say 40 kPA you are now idling at 55 kPA. This means that where at 40 the ECM was fueling at lets say 2.35 gm/sec of injector PW. At the same idle speed at 55 kPA the PW may now be 3.45 gm/sec, as you can now see there is significant more fuel at the same idle speed. Guess what, now you are pig rich, and unless you can burn a new chip the engine isn't going to like you very much
Now if you have an older 96 to 99 GMC of Chevy Vortec truck. These things for some reason, even though they are equipped with a MAF sensor. Are very picky about cams. I once installed a 502/502 crate engine into my old 99 Suburban thinking it was going to be an easy way of getting power over the slug of a 454 that was in it. I even swapped out the 502/502 cam for something more reasonable for a truck which was a custom ground 218/226 roller, much like a HOT cam. Well let me tell you when I got that engine fired up, my nightmares began. A few years ago, before we had the tools to reprogram the PCMs I couldn't get that PCM to work no matter what we did. Even Ed Wright at the time was stumped, I ended up with a FAST EFI system on that truck. On the GM cars with MAF strategies, I still say no problem to a cam within reason, big cams can be made to work fine with some other mods, on the older GM Vortec trucks, one had better have the tools or know a good tuner with the tools.
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WOT - Fuel Tables or MAF?
