M42club.com - Home of the BMW E30/E36 318i/iS
DISCUSSION => Engine management => Topic started by: Cobra Jet on February 18, 2009, 02:05:15 PM
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has anyone attempted or performed this AFM to MAF conversion? I know there is alot of discussion on this site about trying to do such a conversion (and there are some expensive "kits" available, based on other threads on this site), however, I've never seen the below discussed here (or know of anyone attempting it):
(http://img.photobucket.com/albums/v297/gizmo316i/misc/untitlededit.jpg)
AFM to 3" MAF conversion (E36 328i MAF)
taken from here:
http://forums.bimmerforums.com/forum/showthread.php?t=433471&page=13
Seems like it worked for that user and he even states it was close to an 8hp gain...
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Well, for one it depends on the MAF you are using...the wiring connections differ between many (some have 12V power & a 5V reference, some have a power ground & an analog ground, others have one of each or the other, some have a high-power burn-off connection, etc). Aside from that, ALL of them have different transfer functions (volts vs. flow rate).
I remember seeing that BFc post long ago, and it is amazing that it worked. It works by feeding the MAF signal (proportional to kg/sec) into the connection that was for the VAM's output. The ECU is programmed to take the VAM signal (m3/sec) AND the ambient temperature, and use them to produce a (kg/sec) value with an ideal gas law relation (there is also a pressure switch in the car as well since pressure DOES play a part in the air's density, but the ECU only has 2 settings for low/high altitude).
What the project above does is feed the mass flow rate to the ECU, and the 02 sensor's output is fed into the air temperature sensor input to the ECU. Based upon the way the O2 sensor's voltage swings when rich/lean & the way a NTC thermistor behaves, the ECU will see big enough swings in the perceived air temperature to effect the calculated density, and somehow the person who did this saw it work out ok. I cannot really figure out the purpose of the resistors though...they don't do anything to the O2 sensor's output as far as I can tell.
Anyway, from what I have learned over the years, messing with the air temperature in that fashion is not the right way to do it. The ECU will retard the ignition timing when it thinks air temperatures are over 38C or something to reduce the chances of detonation. So, when the O2 sensor is putting out a voltage that the ECU would interpret as hot air conditions, the ignition gets pulled. Conversely, on a really hot day, the O2 sensor will be doing its thing, and the ECU will think that the air is nice & cool at times and NOT pull the timing when perhaps it should.
Basically, the way this thing works is really clever and I think it is a cool concept. When the output from the MAF leads to the ECU making a lean condition, the O2 sensor will output a lean-voltage, and a lean voltage when seen on the air temperature input looks like really cold air, which is much more dense, leading to the ECU pumping in a bunch more fuel, and this just so happens to correct the problem. Well, it will probably way overcompensate and cause a rich condition, which via O2 sensor hooked up to something thinking it is a thermistor, reads a hot air condition, leans the micture, and so-on. This is called an under-damped system, and is highly subject to wild oscillation. Who knows, maybe the guy who did this got really lucky and it all works out as being critically damped & it does not oscillate too badly.
Either way, it would be fun to try this & get a wide-band O2 sensor in there at the same time to see how the AFR's behave. It is brilliantly simple, but I have my own personal doubts about how well this will work for performance, mileage or efficiency. Honestly, there just is not much to be gained by going to a MAF on a normally aspirated M42. Really, there is not, other than the fact that rebuilt AFM's are $$$ & hard to find. Throttle response is a fair bit snappier, but after repeated dyno runs, there is NO power to be had, at least on the E30 M42 (assuming a performance chip is used, who knows what happens with a stock chip & MAF).
For reference, M42 fuel injection wiring diagrams are here:
http://www.m42club.com/forums/showthread.php?t=5098
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Well, for one it depends on the MAF you are using...the wiring connections differ between many (some have 12V power & a 5V reference, some have a power ground & an analog ground, others have one of each or the other, some have a high-power burn-off connection, etc). Aside from that, ALL of them have different transfer functions (volts vs. flow rate).
I remember seeing that BFc post long ago, and it is amazing that it worked. It works by feeding the MAF signal (proportional to kg/sec) into the connection that was for the VAM's output. The ECU is programmed to take the VAM signal (m3/sec) AND the ambient temperature, and use them to produce a (kg/sec) value with an ideal gas law relation (there is also a pressure switch in the car as well since pressure DOES play a part in the air's density, but the ECU only has 2 settings for low/high altitude).
What the project above does is feed the mass flow rate to the ECU, and the 02 sensor's output is fed into the air temperature sensor input to the ECU. Based upon the way the O2 sensor's voltage swings when rich/lean & the way a NTC thermistor behaves, the ECU will see big enough swings in the perceived air temperature to effect the calculated density, and somehow the person who did this saw it work out ok. I cannot really figure out the purpose of the resistors though...they don't do anything to the O2 sensor's output as far as I can tell.
Anyway, from what I have learned over the years, messing with the air temperature in that fashion is not the right way to do it. The ECU will retard the ignition timing when it thinks air temperatures are over 38C or something to reduce the chances of detonation. So, when the O2 sensor is putting out a voltage that the ECU would interpret as hot air conditions, the ignition gets pulled. Conversely, on a really hot day, the O2 sensor will be doing its thing, and the ECU will think that the air is nice & cool at times and NOT pull the timing when perhaps it should.
Basically, the way this thing works is really clever and I think it is a cool concept. When the output from the MAF leads to the ECU making a lean condition, the O2 sensor will output a lean-voltage, and a lean voltage when seen on the air temperature input looks like really cold air, which is much more dense, leading to the ECU pumping in a bunch more fuel, and this just so happens to correct the problem. Well, it will probably way overcompensate and cause a rich condition, which via O2 sensor hooked up to something thinking it is a thermistor, reads a hot air condition, leans the micture, and so-on. This is called an under-damped system, and is highly subject to wild oscillation. Who knows, maybe the guy who did this got really lucky and it all works out as being critically damped & it does not oscillate too badly.
Either way, it would be fun to try this & get a wide-band O2 sensor in there at the same time to see how the AFR's behave. It is brilliantly simple, but I have my own personal doubts about how well this will work for performance, mileage or efficiency. Honestly, there just is not much to be gained by going to a MAF on a normally aspirated M42. Really, there is not, other than the fact that rebuilt AFM's are $$$ & hard to find. Throttle response is a fair bit snappier, but after repeated dyno runs, there is NO power to be had, at least on the E30 M42 (assuming a performance chip is used, who knows what happens with a stock chip & MAF).
For reference, M42 fuel injection wiring diagrams are here:
http://www.m42club.com/forums/showthread.php?t=5098
What happened to the ones you made?Are yoyu still selling/developing them?
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If you really wanted to get rid of the MAF, just go MegaSquirt. They make an adapter board, and diagrams for wiring the stock sensors are out there. You could use either the Alpha-N or hybrid modes with a wideband O2. Probably get it all wired together with custom motorcycle throttle bodies for less than $1k. There isn't really an easy way to convert these voltages unless you're handy with a Basic Stamp, and the improvements won't justify the cost.
http://en.wikibooks.org/wiki/BMW_E30_Megasquirt_&_Wasted_Spark_ECU_Convertion
Granted, it's for an M20. We're a few steps ahead of the game with our coil packs and cam position sensor.
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What happened to the ones you made?Are yoyu still selling/developing them?
Nope, I have no plans to make kits any longer. No power is to be gained from a MAF conversion, and that means like 95% of the market is not interested, despite its other benefits. I can't afford to produce & sell for 5% of the market :p
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Just to let you know that I have planned to replace my MAF for an AMS. I have purhased one through BMP Design. I can't do the switch yet because i am currently in Alaska and won't be home until July. With the replacement, I will upgrade fuel pressure regulator and a couple of other things. I will try to document and photo the whole procedure when i get back. I know it sounds like ..Talk. Just wait. Cheers.
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Nope, I have no plans to make kits any longer. No power is to be gained from a MAF conversion, and that means like 95% of the market is not interested, despite its other benefits. I can't afford to produce & sell for 5% of the market :p
That's to bad!:(
So i'm keeping my AFM:p but do you know how to check if your stock afm is working correctly?:confused: