Mass air flow

General information
A mass air flow (or MAF) sensor placed just before the intake manifold measures the instantaneous rate of air entering the combustion chamber. This is done using a thin wire or film suspended in the air stream.

Hot wire MAF
A hot wire MAF is composed of a thermistor wire and an ammeter. Since the MAF wire is a "positive k" thermistor, the hotter it gets, the higher its resistance. The colder it gets, the lower its resistance, and the more current that passes through the wire. However, the more current flowing through the wire, the hotter it gets (similar to an electric heater). This sets an upper stable limit of current, since as the resistance of the wire increases, the less current that flows through it. This "stable" point changes depending on how much air is flowing past the wire, providing cooling. The more air flowing past the wire, the more current the wire can allow through before it heats up enough to offset the current. Therefore, at any given time, the amount of air flowing through the MAF is proportional to the amount of current flowing through the wire in the sensor.

Hot film MAF
A hot film MAF is comprised of a diaphragm with a heating zone and two symmetrically arranged temperature sensors on either side of the heating zone. A control circuit maintains the heating zone at a constant overtemperature above ambient such that the downstream sensor reads a slightly lower temperature. The more air that blows past the upstream sensor, the cooler it is when compared to the downstream sensor. Thus, the difference in temperature between the upstream and downstream temperature sensor is proportional to the amount (and direction) of air mass flowing over the membrane. A thin film MAF can also detect a reverse flow of air, which may happen during high load factors. Reverse flow pulses from the pistion generate waves in the air upstream of the throttle body, and should not be allowed to affect the meter's readings.

S4 specific information
The B5 S4's Bosch Motronic ME7.1 ECU uses a mass air flow based fuel injection system. During open loop (engine warm-up and wide open throttle) operation, the MAF tells the ECU how much fuel to deliver to keep the air fuel ratio at an optimal level (also known as stoichiometric).

Unlike wideband O2 sensor equipped cars, the ME7.1's narrow band 02 readings are ignored during open loop operation. The O2 sensors (along with MAF readings) are used to determine fueling only during closed loop operation (idle and partial throttle). Note that since MAF readings are also used during closed loop operation, long term fuel trims (or LTFTs) may be affected by faulty or incorrectly tuned MAF setups, including the use of blow off valves (or BOVs).

BOV effect on MAF readings
BOVs can release measured air from the intake system, and when they do, they make the ECU "think" there is more air in the combustion chamber than there really is. This will cause the car to run rich during closed loop operation, and to compensate, LTFTs will go down. However the car may still run rich (or lean) during closed loop operation, since the LTFTs only reflect recent condions, and if the BOVs are venting more (or less) often than they were recently, the LTFT will be wrong. To make matters worse, the car can also run rich open loop, again depending on how much air was lost while the BOVs were venting.

This fudge factor (both in partial and wide open throttle operation) is almost impossible to properly correct for, since it varies wildly depending on driving condtions and driver behavior.

Temperature (IAT) effect on MAF readings
Because of their design, both the Bosch and Hitachi MAF correct for ambient temperature internally, so the ECU does not use the IAT sensor in its load calculations.

IAT effect on timing and fueling
However, IAT does play a role in adjusting the base timing and fueling maps. Unfortunately, those adjustments are done before any correction factor (ignition retard) that comes from the knock control function, so IAT related timing adjustment values are not visible to tools like VAGCOM or ECUx. In general, the higher the IAT is above approximately 50&deg;C (~120&deg;F), the more timing will be pulled and the more fuel will be added.

Bosch vs. Hitachi
B5 S4s came with two different types of MAF sensors - Hitachi (06C 133 471A) and Bosch (part number?); both are film-type MAFs. Each has totally different signal characteristics and are not compatible, even if properly rewired. A-Box/B-Box cars (MY2000) came with Bosch MAFs. H-Box/J-Box (MY2001), M-Box/L-Box (MY2001.5), and T-Box/AA-Box (MY2002) cars came with Hitachi MAFs. Go here for more information.

In order to switch between the two, you must have the correct version of ECU. Changing from one "box" to another is as easy as unplugging one ECU and plugging in another. Some tuners offer "flash" upgrades where an ECU can be reprogrammed via the OBD-II port. However, changing ECU boxes may also require a cluster recode.

Trouble shooting/tuning
(work in progress)

Logging

 * max min values?

Bad MAF

 * unplugging -> always closed loop, ECU ignores maf signal, and uses O2 and MAP only
 * Audi 2.7t self study guide page 50 says "If the air mass meter fails, the air mass is calculated on the basis of a characteristic curve (throttle valve angle and engine speed)"

How MAF housing affects readings

 * larger housing -> smaller value
 * ECU has fixed top end reading, so have to scale MAF if we want more HP (since more HP means more air will be passing through MAF)
 * more air means we need more fuel - larger injectors, higher pressure FPR
 * how to scale MAF housing diameter with injector size and fuel pressure.

MAF and fueling

 * how lemmi fueling (pri/second/accell/decel)
 * affects open loop fueling (MAF vs injector duty)
 * affects closed loop fueling (LTFTs)
 * see also Lemmiwinks.