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 lower (but constant) 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 (ΔT) 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).

Temperature (IAT) effect on MAF readings

Because of their design, both the Bosch and Hitachi MAF correct for ambient temperature, so the ECU does not use the IAT sensor in its load calculations.

Both types of thin film MAF keep the heating element at a constant overtemperature above ambient, so the ΔT between the upstream and the downstream sensor will always give a "cooling capacity" of the mass of air passing over the film. By holding the overtemperature constant, the "cooling capacity" is linear with the air mass passing over the film, regardless of IAT.

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°C (~120°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 - Bosch (078 906 461B) and Hitachi (06C 133 471A); 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.

	A-Box/B-Box/S8/RS4 aka Bosch		H/J/M/L/T/AA-Box aka Hitachi
pin	color	wire	color	wire
1	blank/unused	none	violet/grey	+5V
2	green/yellow	+12V (ign/bat)	green/yellow	+12V (ign/bat)
3	black/green	ground	black/green	ground
4	violet/grey	+5V	green	signal
5	green	signal	blank/unused	none

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.

Logging/tuning/trouble shooting

Signal conditioning

• For the Hitachi MAF
  • 0-0.5V means negative air flow?
  • 0.5V is zero air flow?
  • 5.0V is maximum reading.
  • the ECU reads roughly $16*(V-0.5)^{2}$ g/sec. Although the voltage to grams/sec conversion is independant of IAT, it appears as though the ECU has a correction factor that goes up with RPM.
• can "tweak" measured maf values by using a piggyback signal conditioner
• difference in polarity between bosch/hitachi?
• Bosch is 0V -> max air flow, 4.5V -> zero air flow, 4.5V-5V -> negative air flow?

MAF housing diameter

• MAF signal has an absolute top end reading of 5 volts. So have to scale MAF if we want more HP (since more HP means more air will be passing through MAF).
• To keep the signal under 5 volts for the maximum air flow, we have to make the diameter larger. This will make the signal smaller for a given air mass flow.
• For a given mass air flow:
  • air velocity goes down with cross sectional area
  • cooling capacity goes down with cross sectional area or diameter?
  • voltage goes down with diameter?
  • MAF reading therefore down with square of diameter and with crossectional area
• more air means we need more fuel - larger injectors, higher pressure FPR
• how to determine MAF housing diameter vs injector size and fuel pressure (fuel flow).

Effect on Airflow

One way to determine how big a housing to use for a MAF value is by simply using percentages.

$X=\pi (A/2)^{2}$

where (A) is the new MAF Housing diameter in mm. (ex.: 88mm, 90mm, etc.)

$Y=\pi (73/2)^{2}$

73mm being the stock MAF Housing size in mm.

Increase in MAF (g/s) in % = $((D^{2}/73^{2})-1)100$

Various modded MAF housing diameters

MAF	Diameter	Deviation	Measured airmass	True airmass
D-Flo	83mm	+29.3%	364 g/sec	471 g/sec
AWE	82-84mm	+30%		473 g/sec
ASP	85mm	+35.6%		493 g/sec
Custom	88mm	+45.3%		528 g/sec
ASP	90mm	+52.0%		553 g/sec

Effect on Velocity

Velocity is a good way to approximate the maximum airflow that can be measured by a sensor, because it is independent of the housing's cross sectional area.

A strong S4 running stock K03s, MAF, and fueling uses approximately 260 g/sec of air.

To calculate velocity, we convert the g/sec into $m^{3}/sec$ (volume of air per second) by dividing by 1225 $g/m^{3}$ (density of air at sea level).

We then divide by the cross sectional area of the housing ($(D\pi /2)^{2}$).

So we get $(((260/1225)(m^{3}))/sec)/(pi*(((73/2)*mm)^{2}))=113.437141mph$

Various stock MAF housing diameters

	MAF	Diameter	Max airflow	Velocity
Stock	S4	73mm	220 g/sec	96 mph
Stock	RS4	83mm	320 g/sec	108 mph
Chipped	S4	73mm	260 g/sec	113 mph
Stage 2+	S4	73mm	280 g/sec	122 mph
VAGCOM limit	RS4	83mm	364.08 g/sec	122 mph
Stage 3-	S4	73mm	310 g/sec	135 mph
5V at sensor	S4	73mm	364? g/sec	159 mph
5V at sensor	RS4	83mm	499 g/sec	168 mph

• VAGCOM limit seems to be 318ish on M/T-box, 335ish for M-box, high 320s for A box.
• Sensor value absolute maximums are 364.08 on VAGCOM and 499 on ECUx.
• Note that some programs scale g/sec readings, so what ECUx/VAG-COM reports can't always be trusted.

Using the maximum velocity

As you can see, the maximum velocity measured by both the Hitachi and Bosch sensors is approximately 160mph. We can use this value to determine what size housing we need to measure a given maximum amount of air mass with a given sensor.

Various modded MAF housing diameters using a Hitachi MAF

MAF	Diameter	Velocity	Measured airmass	True airmass
D-Flo	83mm	160mph	367 g/sec	474 g/sec
AWE	82-84mm*			480 g/sec
ASP	85mm			497 g/sec
Custom	88mm			533 g/sec
ASP	90mm			558 g/sec

• AWE actual dimensions are 77mm (inlet) to 100mm at sensor. It reads roughly equivalent to an 82mm-84mm MAF, but the response isn't exactly equivalent at all velocities.

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.

Bad MAF

• Unplugging the MAF will result in the ECU thinking it is broken. Some think this will prevent the ECU from ever going open loop, some think that the ECU can go open loop, but the MAF signal is simply ignored.
• The 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)"
• Is the MAP used in "characteristic curve" calculations?
• If so, what does the ECU do in the case where it thinks the MAP is dead?