Posts: 74
Registered: Oct 2001

posted 11-28-2001 01:06 PM            

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Hi all. I`ve just seen some pictures of M3`s with custom air boxes often made of carbon fiber and i noticed something wrong.
Some cars did not had the airflow meter anymore. The plug wasn`t even there. Does someone have any idea on how to eliminate the afm and why these guys do that?
Thanks. Eduardo.

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Posts: 78
Registered: Nov 2001

posted 11-28-2001 04:18 PM            

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I'm pretty new to BMWs so I don't know exactly what they're doing, but if you don't have a flow-based system you have a pressure-based system. IOW, the ECU hits the timing/fuel tables based on readings from absolute manifold pressure.

Positives to doing this are

1) Losing restriction since you can sample MAP through a simple vacuum line rather than a unit in the intake path.

2) It works better over a high pressure differential, pretty much the standard way of doing things on a forced induction engine with any moderate amount of boost.

The downside is sampling air pressure overall doesn't work as well as sampling airflow - take two gases at the same pressure but at different humidity, different temps, etc. the oxygen content is different between the charges. Using other sensors to offset the MAP readings is then necessary to optimize timing/fueling selections. - Beau

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Posts: 74
Registered: Oct 2001

posted 11-29-2001 05:32 AM            

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Thanks Beau. I`ll stick to my AFM.
Eduardo.

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posted 12-03-2001 03:34 PM              

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I think there are at least 2 modes that can run without HFM:

Speed/Density Mode:
Uses engine vacuum to sense engine load
Best for street-driven vehicles
Achieves optimum response and driveability

Alpha-N Mode (for race):
Uses throttle position as indication of engine load
Engines with wide fluctuations of manifold vacuum at idle due to radical cams

Your can run your car with HFM disconnected (don't take my word for it & do it at your own risk with all necessary precautions), fuel will be richer because computer thinks that air flow is infinitely high. But feedback can get too funny from lamda so ECU goes into protection mode (the 3rd mode!).

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