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Hello guys,

Preparing myself to upgrade to the 3.25" ID BMW 540i V8 MAF housing and been reading almost all related posts since...

In the wikia we have a factor of 1.94 for scaling (due to max flow 3.25" divided by max flow stock).
If we rely on the surface proportion formula, it would be around 1.69 (5352.1 mm^2 / 3166.921mm^2)
Anyone found a "magic number" for this scaling ?
For the stock vs S90 the numbers are closer (1.34 in the wikia - flow division vs 1.21 - surface division).

I've cut the "neck" on the BMW MAF, removed around 1cm (around 0.4in) and re-glued the top over, and now the sensor is almost dead center. Anyone did the same modification and still goes by 1.94 factor ?

Thank you !

BMW MAF.jpeg

image.png.8d7cbf9d56651cb844caa39d01faa334.png

 

Also, if we use the calculations on the s4wiki : https://s4wiki.com/wiki/Mass_air_flow (category "Effect on airflow") then we would get a "real" reading of just +69% over the stock MAF. That meaning 826kg/h (@5V) becomes at most 1396kg/h, pretty far from the 1602kg/h in the wiki.

Or I'm making bulls**t calculations here ? :D


Later edit
From the VOLVO bluebook it seems that MAF measurement should be up to 408kg/h :

Clipboard01.jpg.c303b22f1057f4800968e4747016309f.jpg

which seems consistent with the Bosch HFM2 datasheet:
image.png.eb3495f58509bea170accfc447d446f7.png

Also, if we multiply 255 with the 1.6 scaling factor in the M4.4 RAM scaling we also get 408

Still couldn't understand where the 826kg/h was taken from...

Edited by Midnight Caller
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Investigated some more and if indeed MAF flow table is split in two high-bit / low-bit tables, then, for stock MAF, locations are 19h and D6h (19D6 is 6614 dec). Multiplied by 0.125 is indeed 826.75kg/h

But that doesn't really mean (I think) that this is what the MAF can really measure, but just a calculated conversion table.

Still can't make the connection to the VOLVO specs (408kg/h) and the BOSCH datasheet which mentions 480kg/h

Also, VOL-FCR reads max 408kg/h, and same issue is mentioned by another guy on a V90 with 3L N/A engine (also M4.4)

https://volvoforums.com/forum/volvo-260-760-960-28/maf-reading-full-throttle-v90-3-0-a-106366/
 

Could it be diag tool not reading data correctly ?

Edited by Midnight Caller
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Just to give you an answer, I don't think anyone here wants their catalytic converter to melt or damage to exhaust valves, per se....

As a side note, I think I've managed to tame the BMW 540i MAF. I used the x1.7 factor (not the 1.94 as stated in the Wiki, as that factor would take me to new heights in dumping fuel on the exhaust pipe :D)
So, in particular for my setup, where I tried to center the sensor in the MAF housing as good as I could, x1.7 factor (new MAF area / old MAF area) required increasing the values in the MAF lookup table with a range from 20% to about 3% in the 0.5-1.2V cells. That leads to an STFT of +/- 3% at idle and up to +/- 10-12% under light loads.
I am happy with the values, as the measuring range error for a certain voltage is around 10%
For example, for 1.4844V you are measuring between 94.25KG/h and 103.38KG/h, and for 1.9531V = from 175.38KG/h to 186.38KG/h...

Gotta get myself a partner in crime to see what happens on higher loads.

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The 1.94 'factor' was intended for a default off-center application of the sensor, no modifications to the housing. It would work 'relatively' well, but it is not a straight factor. Especially in low flow situations (<100kg/hr) the factor was quite variable, possibly since the actual air speed is just a bunch lower and you're working in the area of the sensor where small absolute variations (but very large relative / %-wise variations) are not recorded accurately due to the sensor's resolution.

I have characterized it like 5 years back to excruciating detail with bench-setups and 10kHz datalogging, and found that in the lower regions a factor of 2.4 was more accurate, which then slowly settled to the 1.94 reported here. The way 1.94 was derived (i was sent the source data) truncated any low-end accuracy.

That said, it doesn't really matter. It 'works', as long as you completely re-map your VE tables to account for these changes. I myself characterized an accurate translation curve for the MAF, but remapping VE would've given the same results. Not sure which one is more work. I wasn't able to find my factor graph just now or I would've posted a picture of mine - it was very oscillatory in the lower end with values between 2.5 and 1.8.

tl;dr - 1.94 isn't the whole story, that was a very simple linear regression that assumed ideal single-factor translation and ignored a bunch of low-end data as a result. Depending on how meticulate you are, you have to re-map or accurately characterize your own factor with bench-testing.

Edited by Boxman
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I'm thinking the off-centre position of the sensor would lead to measuring less air than reality, especially at low air speeds, thus the need for such a large factor. But in my case, if I use 1.94 indeed the idle would be ok (around 16kg/h) but after 2000rpm the STFT goes waaay under -25% to compensate for the error in over-measured air. That's something @Tightmopedman9 also confirmed and I want to really thank him for the heads up he gave me. So I rather used the 1.7 ratio and only adjust for the smaller number of values, problematic, low-flow area.

I thought it would be good to present this information here, maybe another OCD'st like me ( :D ) would like his sensor dead center and measure things more precise. I've been lucky I guess and sort things up from just the second iteration by checking MAF flow vs STFT values and modifying as such.

Yes, the VE Map was the first thing that came up to my mind, but that would be a RPM/Load map and more calculations were necessary. Taking into account that I will also have to upgrade to 650cc injectors, that way would've only screwed up more things :D

 

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You'd be correct on that one. Even though the air is straightened somewhat by the roster, flow in the middle of a tube is generally faster moving than at the edges. Hence, a sensor placed more in the middle might need a lower factor. (https://en.wikipedia.org/wiki/File:Development_of_fluid_flow_in_the_entrance_region_of_a_pipe.jpg)

Idle performance with the default factor is really poor. Like I said, mine went upwards to 2.3 at low idle. It seems you have similar experiences - needing a higher factor at idle than at load.

Indeed it's a very fair point to make - the factor isn't a straight-forward plug-it-in-and-it-works. I went with the characterize-my-MAF method since I felt it less work than remapping the entire VE without a dyno. Unfortunately, this kind of discussion was often slammed down by the ones who made the first approximation of the conversion, as an over-confident "it doesn't matter and this is diminishing gains". I think pride played a role - who knows.

Fact is, there is a lot to be fine-tuned, even in areas where certain people said there were no gains to be had.

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Here's my previous post about MAF factor in 540i housing, for anyone wondering, showing a raw data plot of an early measurement of the factor across the entire flow range. This was for a 540i housing specifically with default intake setup. I went on to measure and accurately characterize a proper curve for my specific setup.

The take-away is that for the meticulous ones, there is a lot of fine-tuning to be found especially on the lower end. High-flow variations are easier to catch with minor VE-table adjustments.
 

 

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FWIW I run a HFM-5 MAF as opposed to the original HFM-2. It did require hw mods as I had to swap the 12v feed to the original maf to a 5v one (aswell as repin to the new style plug), and also had to swap the ADC channel (I used the accelerometer chan) as the original one had a strange voltage offset.

The maf I used was 0280218089 iirc (came from my T6 xc90) which again iirc is 3 inch/78mm (and that's ID too not OD), and the conversion table was ripped straight from the original ME7 bin so no messing with conversion etc (just had to half the table resolution iirc as ME7 is 2x the size). 

Although not a biiig upgrade from original it gives me plenty of flow sensing for what I need atm and could always use a bigger HFM5 if needed and again rip the oe table from the source vehicle bin.

Edited by jenksta
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2 hours ago, Midnight Caller said:

That's cool ! How did you repurpose the input channel in the BIN (use acc sensor instead of original ADC channel ? )

Just need to change the one or two bytes that reference the maf adc to the accelerometer one.

Been meaning to check if the offset on the maf chan is in hw or sw as I believe the 8051 supports software voltage offsets for the ADC chans, so if it is sw then could remove the offset and use the original chan 🤔 

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As far as I see there should be no offset in the MAF channel. as ADC voltage starts from 0V in the MAF lookup table, and as far as VIDA says (at least not on the grounding side, as it is with the HO2S):

#A3
Power ground
mass air flow (MAF) sensor
Ulow
 
 
#A4
Signal
mass air flow (MAF) sensor (measured to terminal #A5)
U = 0.1 - 0.2 V
U ≈ 0.9 V
U increases with increasing air mass
#A5
Signal ground
mass air flow (MAF) sensor
Ulow
 
 
#A6
Signal ground
engine speed (RPM) sensor
Ulow
 
 
Edited by Midnight Caller
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13 minutes ago, Midnight Caller said:

As far as I see there should be no offset in the MAF channel. as ADC voltage starts from 0V in the MAF lookup table, and as far as VIDA says (at least not on the grounding side, as it is with the HO2S):

#A3
Power ground
mass air flow (MAF) sensor
Ulow
 
 
#A4
Signal
mass air flow (MAF) sensor (measured to terminal #A5)
U = 0.1 - 0.2 V
U ≈ 0.9 V
U increases with increasing air mass
#A5
Signal ground
mass air flow (MAF) sensor
Ulow
 
 
#A6
Signal ground
engine speed (RPM) sensor
Ulow
 
 

It's been a while but from what I remember 0v on the HFM-2 was giving 0.6 or 0.7v on the normal MAF channel. I think if you backpin the original MAF with a voltmeter you'll probably find this the same too. Took me a good while to figure out as I believed it to be a problem with the MAF table, but eventually found it to be caused by this offset. Also worth noting it's not on the ground side either as all I did was swap the 5v output from the MAF to the accelerometer channel and all was fine.

Edited by jenksta
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