M42 AFM -> M44 MAF conversion

[mrjezza]

Has anyone done this conversion?

As I understand it the easiest way to make this work with an M42 would be to create some sort of circuit that converts the outpot of the M44's MAF sensor to match the output of the M42 AFM's variable resistor that is actuated by the flap.

How exactly would one calibrate the circuit?  It would be easy to match up the reistances so that the resistance of the MAF with no air passing across it matches the resistance of the AFM in the closed position, but how would you calibrate the MAF to match the fully open position?  Seeing as theres no restriction on the MAF as to a max position, just the limit of how much air you can make flow across the sensor wire, how do you know how much air actually equals the airflow in the fully open position? (Also, even if you can ascertain this figure by playing the MAF inline with your intake and recording the resistance on WOT, this may not even match up as the airflow should theoretically be better with the MAF than fully open on the AFM making the figure you recieve useless).

Could anyone speculate as to the likely performance gains (or aural gains) this conversion would see?  Also, what's likely to go wrong if you don't end up matching up the outputs accurately - the car will run rich/lean?

[doigal]

try to find the specs on the MAF unit - ie what the resistance range is at zero and at WOT - it "should" be out there somewhere. Other than that... try to simulate WOT and use that as comparison?

Edit - after a little googling,

Bit of infomation here:
http://www.318ti.org/forum/printthread.php?t=5732&pp=40

and a guy thats done it here:
http://forums.bimmerforums.com/forum/archive/index.php/t-189571.html

Seems that theres a kit for $mega, but i'm sure it can be done for WAY cheaper.

[mrjezza]

If all that is involved is converting voltage, it would be a piece of cake to make a circuit to do that.

[doigal]

if you get it done, please let us know how!

[zaidddd]

PLEASE UPDATE US. if  anyone conversion maf on m42 318is e36. i would to install on my engine

[bmwman91]

It is not that simple. There is no way to directly plug & play a MAF on the M42. Simply converting the voltage will not work well. Yes the car would run, but poorly. There are multiple reasons for this.
1) The response speed of the MAF is much much faster than the AFM, around 40x faster. The MAF output is so fast that it can show every individual air intake pulse as valves open when you are at 100% throttle. The ECU is not programmed to read the voltage fast enough to deal well with this (the stock AFM signal does not show any pulsing at all and is more like a DC output at all operating points). So you would need some fairly significant low-pass filtering, and due to the non-linear nature of the MAF transfer function, it cannot be done with analog means; you need to do it in software on the actual mass air flow value not the voltage.
2) The stock AFM has acoustical-mechanical resonances that are excited at large throttle openings. Basically it lies and over-reports air flow in certain RPM ranges, and the fuel maps in the ECU are tuned to correct this. A MAF will not have any of these resonance effects and will remain accurate in its output under all normal conditions. As such, you would need to remap the fueling to work properly with the MAF so that you do not run lean at 100% throttle in certain RPM ranges. This is more of an issue with the E30 M42 than the E36 M42 since the DISA manifold helps a little, but it is still an issue.
3) There is zero power to be gained by going to a MAF. The stock AFM is not restrictive. I don't know why people are so convinced that it is. It simply is not. I took many datalogs with the stock AFM and my own MAF conversion and the engine flows exactly zero more air with the MAF. Throttle response might be slightly better (I have no way to actually measure it so it is subjective).

So basically, converting to a MAF requires an embedded processing system to low-pass filter the signal after converting voltage to mass-flow values, and then convert that into an equivalent volume-proportional signal matching the stock AFM transfer function. Also you need to measure intake air temperature so that density is corrected for in the conversion from mass to volume flow rate. On top of that, you would either need to remap the fuel tables, or have the converter monitor RPM and throttle position to mimic the stock AFM's resonance effects (remapping fuel would be easier).

Hopefully that illustrates the point that it is not easy, nor is there any real point or benefit. I know because I did it, expecting gains in performance, but got nothing (other than learning a lot).