Mass Air Flow Sensor Facts and Findings

By: Richard MacCutcheon
Introduction: The following is a compilation of information about MAF sensors and their function from mostly WWW and mail list information. It is biased towards the Ford style two wire MAF used on mostly any Ford since the early 1990s gasoline engines.

The MAF sensors used on many of today's cars primarily use the two-wire, hot/cold wire, setup to detect the mass of the air passing through it. These wires are designed to give a feedback voltage to the engine control system (EEC, PCM, ECM, whatever) using an electronic function wired into the sensor. Many misconceptions have arisen in the realm of modifying the MAF to gain power on the vehicle. Here are some facts and information to help you better understand the use and function of the system.

The MAF is a sensor. It cannot take an active role in the operation of your vehicle, which is what the PCM does. The PCM uses all of the various inputs and runs the engine according to its program. The MAF transfer function not only tells the computer how much air is coming into the motor, but the function is used to calculate load using other inputs which profiles spark and fuel. As far as sensors go, it is a somewhat complex unit. It basically uses two wires within its body to create a current draw and output based on the different characteristics of the wires. But the way these wires act in the sensor body itself can be somewhat problematic. First, there is the flow of air entering the sensor. The air stream can have many influencing characteristics that can affect the sensor's ability to give accurate output. Turbulence and velocity variations can give false output based on the original flow profile of the sensor. So let's talk about the design for a second. When the engineers at the factory incorporate the sensor into an intake system, they must not assume it to be a perfect flow of air. The air filter, air box, sensor placement and many other factors affect the true sensing capability of the sensor. This is why when a MAF transfer function, the data stored in the computer about the sensor, is derived for one vehicle it may not be identical to the transfer function for a similar vehicle with the same exact sensor. The load calculations will also vary. Likewise, the downstream inputs that can affect the sensor vary from one design to another.

A running engine creates impulses that flow upstream, intake growl noise, that are waves of air that are now impinging on the sensor from behind. In an effort to eliminate the effect of various intake feedbacks the designers use a backflow-preventor in the form of a bar across the MAF sensor cavity using a backing plate to deflect these impulses away from the sensor elements. When the engineers develop the transfer function for a system they measure the flow of the sensor in a real-life situation. Designers actually use a laser Doppler system to precisely measure the flow characteristics. Designing a single basic design that can be used across many applications is the goal so costs can be minimized. Suffice it to say that a MAF from identical cars will not be identical. The variations in the electronics that drive the sensor have some influence on the way the car runs. As a model year progresses other variables, like part lot numbers and such can cause one car to run much better than the next. If all of the variation of the sensors and devices like injectors are taken into account, you could have a marginal car to begin with. This is why some people see great gains with slight mods and others see nothing or it gets worse.

Modifying the MAF has risks as well as potential benefits. Changing the flow characteristics can cause great  problems. First, let's look at the post removal issue. Removing the post does increase the flow capacity of the meter, but now has the potential for noise from the intake causing other problems. This is more important at idle and low throttle settings.
Why? The MAF transfer function is not linear. The function is flatter at low flows and increases at an increasing rate as flow increases. If there is airflow noise at the flatter portion of the curve, the noise voltage is a much greater percentage of the total voltage being sent to the computer and the computer may balk. Thus you end up with rough or poor idle. At high flows the noise can be greater but the curve is exponential and the noise is less influential. Also, the velocity of the air in the sensor with no post has dropped at the same load. This affects the load calculation that causes the injector pulses to be off the intended design. They may be better they may be worse. Cars with closed loop operation capabilities can detect the lean or rich condition caused by the change and make adjustments on the fly. Car may start rough but smooth out as you drive. As you drive in the Wide open throttle condition, open loop, the car is now only relying on the data stored in the computer and modifications, which are not "good" or beneficial, will now affect the performance of the motor. The one perceived increase in performance has to do with the fact that the original programming of the PCM is on the rich side, extra fuel for reduced wear and tear on the motor. This modification effectively leans out the mixture and provides more efficient combustion. Over the long run though, the PCM will use it's adaptive capabilities to make the mixture correct as read by the Oxygen sensors. But having changed the flow characteristics of the sensor, the flow across the elements is also changed, possible reduced. Thus the voltage would be lower and the effective flow would cause even leaner conditions. Also, load calculations will probably be off their original curve and injector pulses may be affected. One trick to provide for the change in flow of the no-post MAF is to make a proportional change in the sample tube that contains the sensor wires. Assuming you get the hole size just right, the transfer function of the MAF is still not likely to match that of the original function map in the computer. The curve could be steeper sooner or flatter later or shifted completely.

At a given MAF voltage the air coming into the meter is known through the transfer function table. Knowing that most people drive around in closed loop situations these variations are quenched by other inputs. Just unplug the MAF all together and you will see that it will run however poorly, but it is not dead. The problem with the sample tube issue is that the overall flow characteristics of the MAF are just different than before. If you were to compare the two curves against each other, they will be very close. And it seems for the most part that the increased airflow, especially at WOT, is of greater benefit than the slight miscalibration of the sensor. What has to take place is the voltage should still represent a given flow, the modification just shifts the curve to provide flows at lower throttle levels. Again this can affect load.

Other modifications can affect even a stock sensor. The airbox, inlet tube and intake tube to the throttle body will cause the system as a whole to be slightly different. Let's not get too freaked out though. Again, the benefits of these mods are often marginally good. One key thing to remember is that the closer the MAF is to the throttle body the higher the effect the intake feedback has on the meter's accuracy. The fewer restrictions you have between these parts also can cause problems, usually rough idle and poor performance at low throttle positions. Ideally get the MAF farther away from the throttle body.  For the greatest benefit from any modification to the MAF, its flow characteristics must be input into the transfer function table in the PCM. Recalibrating the MAF for larger injectors works somewhat but load calculations will be wrong. Most MAFs are limited by their size as to how much they can flow, but even then you can peg the electronics.
Adding a supercharger can cause the meter to not function to its full potential if flows above the transfer function table range occur. The sample tube size can be adjusted to give a broader range than the .5v to 5.0v the current Ford sensors have. Below are two side by side lists of MAF transfer function for two different setups on the 1994 Mustang GT. The first number in the parenthesis is the voltage and the second is the airflow in KG/hr. Notice that there is a maximum flow of 932 kg/hr on the first and 882 kg/hr on the second. Model year changes or different options in the system caused the difference. Also, the curve for the first one is very similar to the second all the way up to 4.0v and then the first one climbs rapidly.

So even though these are the same model and year, just a MAF swap will cause a discrepancy in the way WOT is computed. This swap is a full swap. The sensor bodies have been very standardized. Swapping just the bodies wouldn't make as much a difference as swapping the electronics. Put the first one in the second car and at the high end the car will get a voltage that represents a flowrate less than the actual flow which is higher, it will run leaner and possibly cause detonation. Now these are just examples and to specifically say what will really happen depends on the other variables. Dirty sensor elements gives less voltage at same flow.
Dirty injectors may cause a lean condition as well as fuel pressure and volume.

U4P0                                             J4J1
# Mass Air Transfer Function
( 15.9998, 932.145 )                       ( 15.9998, 882.085 )
( 5, 932.145 )                                  ( 5, 882.085 )
( 4.75, 808.577 )                             ( 4.6001, 717.327 )
( 4.5, 697.683 )                               ( 4.19995, 568.729 )
( 4.25, 598.512 )                             ( 3.80005, 443.577 )
( 4, 510.114 )                                  ( 3.5, 362.466 )
( 3.80005, 446.745 )                       ( 3.30005, 313.989 )
( 3.6001, 389.397 )                         ( 3.1001, 270.265 )
( 3.3999, 337.118 )                         ( 2.8999, 230.66 )
( 3.19995, 290.86 )                         ( 2.69995, 195.491 )
( 3, 249.037 )                                  ( 2.5, 163.173 )
( 2.80005, 211.65 )                         ( 2.3999, 148.281 )
( 2.6001, 178.381 )                         ( 2.30005, 134.974 )
( 2.3999, 149.232 )                         ( 2.19995, 122.301 )
( 2.19995, 123.568 )                       ( 2.1001, 110.894 )
( 2.1001, 112.162 )                         ( 2, 100.122 )
( 2, 101.389 )                                  ( 1.8999, 89.9828 )
( 1.8999, 91.2501 )                         ( 1.80005, 80.7944 )
( 1.80005, 82.0617 )                       ( 1.69995, 72.2397 )
( 1.6001, 65.2692 )                         ( 1.6001, 64.3187 )
( 1.5, 57.9818 )                               ( 1.5, 57.0313 )
( 1.30005, 44.9914 )                       ( 1.3999, 50.3777 )
( 1.19995, 39.2882 )                       ( 1.30005, 44.3577 )
( 1, 29.4662 )                                  ( 1.19995, 38.9714 )
( 0.75, 19.961 )                               ( 1, 29.1493 )
( 0.600098, 15.2084 )                     ( 0.899902, 25.0304 )
( 0.399902, 10.4557 )                     ( 0.800049, 21.2283 )
( 0, 8.87154 )                                  ( 0.600098, 14.5747 )
                                                       ( 0.5, 11.7231 )
                                                       ( 0, 11.7231 )

To modify the MAF sensor is in all practicality a bad move without telling the computer that the flow has increased via the transfer function table. Also, modified intakes will not be as effective as presumed unless the flow changes are calculated also. The best way to increase the flow to the motor and let it know is to get the system tested or get a system that has been flow profiled and includes the ability to program the PCM. You can't just "recalibrate" the MAF. Load will be incorrect. This can effect the durability of the motor especially when WOT driving is done.

These are issues that you must take into account when debating a change to the system. There are some slight risks involved and you'll have to decide.

I hope this information proves useful. Any discrepancies are my own and you can e-mail me about it. These are only the facts presented as I see them with the information I have gathered. Richard MacCutcheon

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