Disassembly: MAF tables and Air Flow Calculation

[Greg E]

Try to let go of the notion that the stock MAF is some intelligent air counter and try to think of it more in the terms of any 5V sensor. There's a voltage and a scaling (the ECU associates the input pulses to some amount of air). Granted it's not a linear varying voltage signal like a wideband O2 sensor or a MAP sensor, but it's still a 5V pulse signal sent to the ECU. The faster the pulses, the more air.

The MAF sensor can only read air volume. Dennis has a great explanation for how the mitsu MAFs work.

The Mitsu Karmann air flow sensor is a volume sensor - so you are right in saying "it'll show the exact same pulses for 70deg air as it does for 110 deg air" because (assuming all other conditions are equal) the volume of flow is the same. What changes with the temperature change is the density, and with it, the mass.

So the Mitsu "MAF" must be temperature and baro pressure compensated in order to produce a mass value, which is what we want. And by no small coincidence, the sensor has a baro pressure and air temp sensor in the body of the "MAF".

Hot wire sensors - like the GM sensor - use an entirely different measuring system (the amount of cooling on a heated wire) and these sensors measure air mass directly with no need to temperature or pressure compensate (at least on paper - I would not be surprised to find out that GM ECUs use some "fudge factors" to cover for where theory diverges from practice)

Knowing the volume doesn't get you anywhere, since the number of air molecules in a given volume changes with temperature and pressure. If you know air volume, temperature and pressure though, you can get air mass. Air Mass can be used to determine the correct amount of fuel

This is one of the reasons for the closed loop feature of the ECU. Even with all the tables used to calculate air mass, your final number is only as good as the base number. Even a completely stock, perfect condition car will have varying fuel trims depending on the weather outside. You'll never ever have a perfect 0% trim under all weather conditions.

The trims are less about weather as they are about engine wear and general chaos (in the mathematical sense). Remember that the reasoning behind closed loop fuel control is emissions. The manufacturers were forced to go closed-loop-control by increasingly tighter emissions standards over increasingly longer periods of time. As engines wear, build up carbon, spark plugs get weaker, injectors clog etc the calibration changes (slightly) and your emissions change accordingly. Closed loop allows the ECU to compensate for these drifts away from the mechanical state of the engine as it was delivered from the factory.

Closed loop will also accomodate errors in your weather-related tables as well (and for tuners, your fuel tables) which is why I cheat like a bastard and run closed loop with a low lean-out authority and a high enrichen authority all the time (with a pair of wideband sensors), tweaking fuel tables to try and minimize the amount of O2 correction. The OEM, with a narrow band O2, can't cheat like this except on cells where it is supposed to be 14.7:1 so the weather based corrections are probably very good (thus the reason why I want to duplicate them)

So how does the ECU calculate airflow? It starts with the MAF Size and the MAF Pulses. The larger the MAF size number, the more air the MAF can count (in the code, you must set this value according to the MAF you're actually running)

AirCount = MAF Pulses Per RPM
MAF Hz = MAF Pulses Per Second = AirCount/RPM

Load is the term used for years as a number value to show how much air volume there is. More Load = More HP. So the object of the game is to get as much airflow in the motor as possible.

Load quite simply is just Air_Count * MAF_Size / 65536. This is uncompensated Load (meaning Air Temp and Baro haven't been applied yet).

Being able to read this number accurately is advantageous over the traditional boost gauge/butt dyno as you're able to see if your airflow numbers go up with each mod performed. When you get to the point where you keep raising the boost, but airflow isn't increasing, then you hit the limit of the efficiency of your turbos on your particular setup.


Caution! There's a MAF Pulse Limitation in the code. This isn't a physical limitation of the MAF itself, just an issue where the ECU will stop adding fuel past a certain PPS (Pulse per second) due to a 1 byte limitation in memory. Beyond this limitation, the ECU will stop adding fuel so be forewarned.

This limitation is 13.06 PPS or 319.7 grams of air/second (table is shown in the V2 xml). With a stock MAF, this is only about 450hp depending on which HP calculator you're using. This is coincidentally is about where the stock 3/S MAF also seems to run out of resolution. Thought that was interesting. Obviously the car is capable of making more power as people have been doing so for years by fooling the ECU into seeing less air with a piggyback.

I'm not sure what this limitation is with an EVO MAF yet. The EVO MAF won't pulse as often for the same airflow as the stock 3/S MAF and I need more data to make an educated guess. Wouldn't be surprised if the limitation is double the stock 3/S MAF. If the demand requires I come up with a way around this limitation, I can, but for now, I'm not going to worry about it till we start seeing more big builds with the Flash ECU.

AirFlowMAX = MAFLimit * AirTempComp * WaterTempComp

*Note: These 2 comp tables are only used for calculating MAX airflow NOT engine load. Also note they are just set to 1 so they are disabled anyway.





So that's airflow. Next the ECU calculates a MAF Correction value for the injectors during the IPW calculation. It does this using the MAF Scaling, Adder and VE tables. The MAF Smoothing table is just a way to measure the engines Volumetric Efficiency (VE).





Changes to MAF Smoothing (VE) Table will produce large results relative to the value in the Scaling Table. So if you need to adjust the amount of fuel based on airflow, adjust the values in the VE table. It's kinda like an SAFC in a way only you're going off the actual MAF values instead of RPM. The higher the number in the VE table, the more fuel the ECU will use. Lower values = Less fuel.

It's important to note that changes to your intake, innercoolers exhaust, head work, turbos... All this changes your particular engines VE and the MAF may not detect this change in airflow. This table is tuned for a 100% stock car. Something as simple as hard pre-turbo intake pipes can effect the readings from the MAF as more can sneak thru it without being detected.

When tuning cars, I don't mess with the fuel injector size and latency values beyond setting up the values for whatever injectors is being used. Most people also tend to want to play with the fuel table too much during initial tuning, but if you get your VE table setup just right, the AFR value you see in the fuel table is what you'll get on the wideband.

The fuel trims are handy for tuning this table. If your trims are increasing, add more to the VE table in that range to bring them back to 0. If your trims are negative, lower the numbers.




Something interesting in the code is an idle correction factor during it's airflow calc process. Since at idle vacuum is so great, it causes an anomaly with the idle AFR in that the ECU will think there's a lot more air there than what's actually flowing thru the MAF. This correction is based off the idle switch and it works to differentiate the low fuel trim between idle and just low speed driving.

When you install cams and vac is a lot less than stock, consider bringing up this idle correction factor a bit as it may cause too lean of an idle mixture (not that a lean idle is really is a bad thing).

[Greg E]

When you install an EVO MAF, these are the settings you need to make your tables. Leave everything else alone.

[Zybrainn]

Just to clarify if we are using an EVO MAF, the maf scaling table (V1 table name) should change to the values you listed from the airflow/hz raw scale table (guessing V2 table name) to the right?

[Greg E]

MAF scaling is the chrome V1 and V2 name.

The raw scaling name is what the EVO guys called it in their ROMs. Those 3 tables on the right are straight out of my old EVO 8 stock ROM.

...and if you have an EVO ROM (even a tephra one) you can just copy/paste all the values instead of manually typing in each one.

[Chris@Rvengeperformance]

I guess that is why my car has to add fuel at idle, I'm still using the 3/s scaling. Hopefully I can try this out over the weekend. It is good I had a chance to dial in the injector scaling and the airflow table while I'm still on the stock fuel pump. The FPR overrun I'll get later would lead me to the wrong numbers.

I take it there is hope for a voltage to grams scaling someday? That beast 3.75" draw through maf I had would sure be nice.

[Greg E]

Grams/ second scaling (instead of load) is already in the ROM. I was curious if anyone used it. It's a sinple matter of editing the table and selecting the g/sec scaling.

If you'd like to experiment with the GM MAF I'll send you the special ROM I've been working on which utilizes a linear 5V signal for the MAF input instead of the HZ signal like the Mitsubishi MAFs.

Believe you and Adam are my only beta testers still in the game during the winter.

[Chris@Rvengeperformance]

Grams/ second scaling (instead of load) is already in the ROM. I was curious if anyone used it. It's a sinple matter of editing the table and selecting the g/sec scaling.

If you'd like to experiment with the GM MAF I'll send you the special ROM I've been working on which utilizes a linear 5V signal for the MAF input instead of the HZ signal like the Mitsubishi MAFs.

Believe you and Adam are my only beta testers still in the game during the winter.

Sure I can test the GM maf. Let me get the few kinks worked out we talked about and we can go from there. I don't need a solid tune until the NG next year, so I have time to iron it out.

I'll keep my eye out for a z06 maf. I guess you have some preliminary scalings setup for some mafs? GM publishes some decent info don't they?

I guess there are other methods of obtaining a 5v signal, aren't there ? I assume it is not quite as simple though, so we can start simple.

FYI, the part # I ran before was 25168491, if there is no objections I'll get one of those again. Not interested in blow through.

[DG]

As is my usual practice, the summer ends and I start driving my car again. And once again, I start noticing the more ragged edges of my current AEM EMS tune, and start wishing I had the OEM ECU calibration translated into an AEM EMS calibration so I could leverage all the tuning work Mitsu did when they first developed the friggin' thing.

So I'm happy to see the OEM ECU disassembly stuff moving forward. Who knows, maybe this time I'll actually get it done.

Ok then, sooooo.....

The MAF sensor can only read absolute airflow, not relative (or also called Atmospheric) meaning it'll show the exact same pulses for 70deg air as it does for 110 deg air. Same with barometric pressure. This is one of the major advantages to using a ford or GM hotwire MAF. They measure air volume so your base signal is already compensated for atmosphere.

Not exactly.

The Mitsu Karmann air flow sensor is a volume sensor - so you are right in saying "it'll show the exact same pulses for 70deg air as it does for 110 deg air" because (assuming all other conditions are equal) the volume of flow is the same. What changes with the temperature change is the density, and with it, the mass.

So the Mitsu "MAF" must be temperature and baro pressure compensated in order to produce a mass value, which is what we want. And by no small coincidence, the sensor has a baro pressure and air temp sensor in the body of the "MAF".

Hot wire sensors - like the GM sensor - use an entirely different measuring system (the amount of cooling on a heated wire) and these sensors measure air mass directly with no need to temperature or pressure compensate (at least on paper - I would not be surprised to find out that GM ECUs use some "fudge factors" to cover for where theory diverges from practice)

Even a completely stock, perfect condition car will have varying fuel trims depending on the weather outside.

The trims are less about weather as they are about engine wear and general chaos (in the mathematical sense). Remember that the reasoning behind closed loop fuel control is emissions. The manufacturers were forced to go closed-loop-control by increasingly tighter emissions standards over increasingly longer periods of time. As engines wear, build up carbon, spark plugs get weaker, injectors clog etc the calibration changes (slightly) and your emissions change accordingly. Closed loop allows the ECU to compensate for these drifts away from the mechanical state of the engine as it was delivered from the factory.

Closed loop will also accomodate errors in your weather-related tables as well (and for tuners, your fuel tables) which is why I cheat like a bastard and run closed loop with a low lean-out authority and a high enrichen authority all the time (with a pair of wideband sensors), tweaking fuel tables to try and minimize the amount of O2 correction. The OEM, with a narrow band O2, can't cheat like this except on cells where it is supposed to be 14.7:1 so the weather based corrections are probably very good (thus the reason why I want to duplicate them)

Something as simple as hard pre-turbo intake pipes can effect the readings from the MAF as more can sneak thru it without being detected.

I've never seen anything that suggests that the "MAF" is anything other than perfectly accurate for its calibrated range. It should not be possible, assuming you aren't exceeding the flow limit, to "sneak air through the MAF" without it going uncounted.

What is more likely is that this really is a VE correction. The "MAF" is a volume sensor, and it sits at the head of a very long and convoluted airflow path. Due to flow efficiancies along that path, somewhat less air will arrive at the engine than was counted at the sensor - and even that isn't quite right. As long as air is not being lost (via a vacuum leak) the same mass of air is making it in, but the volume and density are being tweaked a little by flow losses.

Change the flow efficiency of this flow path into the engine and you change the amount of divergence between what is measured at the entry to the pipe and what comes out at the end of the pipe - that's the change in VE. And VE is indeed sensitive to things like hard pipes.

What I have noticed sort of emperically though is that the Mitsu "MAF" is very much less sensitive to VE changes than is a pure speed/density system (which implies volume flow rather than measuring it). The fuel maps still have a curve to them, but not much of one - and the very narrow range of that "MAF Smoothing" table bears that out. To the point where I wonder if it isn't so much about VE as it is about nonlinearity in the response curve of the sensor.

I'd give a testicle to see a 3S MAF and an EVO MAF hooked up to a flow bench (where flow volume is carefully controlled) and see the results of a stepped flow test, where sensor output Hz is plotted against known flow.

Greg, I also wish you and I could spend some time together in a room with a whiteboard... it would be SO much easier to do this in person.

DG

[Greg E]

I've never seen anything that suggests that the "MAF" is anything other than perfectly accurate for its calibrated range. It should not be possible, assuming you aren't exceeding the flow limit, to "sneak air through the MAF" without it going uncounted.

What is more likely is that this really is a VE correction. The "MAF" is a volume sensor, and it sits at the head of a very long and convoluted airflow path. Due to flow efficiancies along that path, somewhat less air will arrive at the engine than was counted at the sensor - and even that isn't quite right. As long as air is not being lost (via a vacuum leak) the same mass of air is making it in, but the volume and density are being tweaked a little by flow losses.

Change the flow efficiency of this flow path into the engine and you change the amount of divergence between what is measured at the entry to the pipe and what comes out at the end of the pipe - that's the change in VE. And VE is indeed sensitive to things like hard pipes.

What I have noticed sort of emperically though is that the Mitsu "MAF" is very much less sensitive to VE changes than is a pure speed/density system (which implies volume flow rather than measuring it). The fuel maps still have a curve to them, but not much of one - and the very narrow range of that "MAF Smoothing" table bears that out. To the point where I wonder if it isn't so much about VE as it is about nonlinearity in the response curve of the sensor.

I'd give a testicle to see a 3S MAF and an EVO MAF hooked up to a flow bench (where flow volume is carefully controlled) and see the results of a stepped flow test, where sensor output Hz is plotted against known flow.

Greg, I also wish you and I could spend some time together in a room with a whiteboard... it would be SO much easier to do this in person.

DG

Hey Dennis! Was hoping you'd come out of hiding and find my posts! You're one of the guys whom I am gearing this whole project towards.

You're right. I should edit my posts to be more clear about the difference between mass and volume. If you don't mind, I'd like to steal a few of your words from this post and move it up top.

If I didn't witness the direct changes in the fuel trims from swapping on different intake pipes, I would agree with your statement but time and time again the MAF needs to be rescaled for different intakes, exhausts, turbos... That's where the "air sneaking thru the MAF" statement comes from.

Remember the stock MAF has chambers divided up and only the vortex generator sits in the middle chamber. This means there is a lot of uncounted air going thru the other chambers at any given time.

While I've never had the equipment to measure the stock and EVO MAF side by side, its known that the stock MAF will pulse more times a second than the EVO for the same amount of air. This is the purpose of the single cell MAF size value. This number tells the code how much air flows thru the MAF for each individual pulse. This is why you need to set the size value to be so much bigger than stock for the EVO MAF.

Further evidence to support this comes from comparing my stock 98 readings to a few modded cars. Jesters 99 is a good example. He's running a montero MAF (very similar to the EVO MAF, the settings are close enough between the two). Adam is running 19Ts at 19ish PSI.

Comparing his logs to mine, the stock car shows about 220 load at 16psi vs 205 LOAD on Adams car. There is absolutely no way his car is flowing less air than mine. Looking at his raw HZ readings, they are also lower than my car. Why? Because the montero MAF doesn't pulse as much for the same amount of air.

I see this time and time again so I swapped the EVO MAF onto my stock car. Just as i suspected, lower HZ and load numbers. 160 load vs 220 with the same exact tune on the same exact day on the same exact stretch of road.

Something that will really blow your mind is I don't believe the MAF itself is a restriction like we are all accustomed to believing. In the code, there is a 1 byte limitation to the MAF signal at about 319 pulses per RPM. Without going into too much detail with the math (though I can if you'd like), this comes about 450hp. Now, that number about where people have been running out of power with the stock MAF. What happens is past 319 pulses per RPM the ECU just stops adding fuel as it doesn't see any more pulses past that limit. Air keeps coming in but the numbers are beyond max so the AFR just leans out.

I'd be willing to bet you if I increased this limit, we could get more power out of a stock MAF. Obviously this depends on sensor latency/accuracy past that point. Then of course there is the conundrum of breathing thru a straw but that's left to be seen.

Dennis, I would love to chat more ideas like this with you. I do this all the time with a few guys while I'm bored at work. Do you have my email?

[DG]

If you don't mind, I'd like to steal a few of your words from this post and move it up top.

Go ahead.

but time and time again the MAF needs to be rescaled for different intakes, exhausts, turbos

That makes sense though if what the "scale factor" is, is a VE factor. Changing the efficiency of the flow path behind the sensor is going to alter the relationship between air measured at the front of the pipe and air "measured" (by the calibration) at the end of the pipe. I'd bet, for example, that if you moved the MAF to right before the throttle body, you could change intake piping all you wanted without needing to rescale the MAF - because changes upstream of the sensor get measured by the sensor.

Similarly, I expect that changing air filter designs doesn't require MAF rescaling either.

My experience though with the AEM in MAF mode is that the MAF is very insensitive to VE changes, whereas the Talon (which was an AEM in speed/density mode) was very sensitive to VE - one of the reasons I retained the MAF for my Stealth.

Remember the stock MAF has chambers divided up and only the vortex generator sits in the middle chamber. This means there is a lot of uncounted air going thru the other chambers at any given time.

True, but the ratio of counted to uncounted is proportional and fixed. It would be quite the trick convince air to flow through the unmetered portion without a proportional amount flowing through the metered section.

The old "hacked MAF" from back in the early days of 1G DSM tuning was about changing the ratio of metered vs unmetered air, not so much about increasing the raw flow value.

its known that the stock MAF will pulse more times a second than the EVO for the same amount of air.

This is entirely possible, but is a separate issue - that's a difference in the calibration of two separate sensors, not a question of the sensor not counting air passing through it.

If you are using a frequency or count-based air sensor, there is going to be a maximum frequency over which your count window will be too short to catch all the pulses. It makes sense, then, to design the MAF so that the maximum amount of air you intend to be able to count generates this frequency, or a little lower to give you some wiggle room. Given that the EVO sensor is intended to count more air than the 3S one, I'd expect the max frequency to occur at a similar percentage of max flow and the rest of the airflow to be scaled accordingly, rather than keep the 3S airflow ratio and just add headroom. This would result in a lower frequency air count at the same air flow.

It might be interesting to connect a 3S MAF to an EVO MAF in series, use one to run the car, and log the other. This would get you airflow curves for both sensors at the same airflow value. Then switch them and see if the same relationship holds.

Something that will really blow your mind is I don't believe the MAF itself is a restriction like we are all accustomed to believing. In the code, there is a 1 byte limitation to the MAF signal at about 319 pulses per RPM. Without going into too much detail with the math (though I can if you'd like), this comes about 450hp. Now, that number about where people have been running out of power with the stock MAF. What happens is past 319 pulses per RPM the ECU just stops adding fuel as it doesn't see any more pulses past that limit. Air keeps coming in but the numbers are beyond max so the AFR just leans out.

I absolutely believe it - there is a similar feature in the AEM. It is the difference between the "software calibration" of the MAF (what the ECU thinks the maximum value of the MAF is) and the "physical calibration" of the MAF (the relationship between the volume of flow through the sensor and the output frequency)

This brings us back to the desire for a flow bench test to see exactly how much air volume can be passed through the thing and what the output signal looks like.

I would absolutely like to see the math, in detail, for how the OEM ECU calculates its load values, starting with the individual pulses measured at the MAF and ending with the load axis on the tables. I know exactly how this works on the AEM and I have it documented on farnorthracing.com If I knew how the OEM ECU did it, I could make a conversion function that would convert OEM calibrations to AEM calibrations and back again. One could have an AEM whose initial calibration was exactly the same as the OEM ECU (within the limits of the AEM control scheme to duplicate the OEM and vice versa)

I don't think I have your email address.

DG