What is a MAF sensor? What does it do? Why does it matter if I put a bigger air intake on my car? Why do I care?
The MAF sensor is the engine ECU’s way of measuring the mass flow rate of air into the engine that is used to calculate load and ultimately deciding how much fuel to inject as well as where to land on the numerous maps in the ECU’s tune. When a MAF sensor is used alongside an o2 sensor providing real-time feedback on the fueling mixture, the result is a very predictable and smooth running engine that responds well to different conditions. MAF sensors do not measure airflow directly. Instead they rely on a change in an element that is used by the ECU along with other sensors to calculate the mass flow rate. There are two types of common MAF sensors, the “Hot Wire” and the “Vane” type. For the sake of this article, I am going to specifically talk about “Hot Wire” MAF sensors like you see in your typical Subaru and other modern cars.
Subaru MAF sensor (pic from iwsti.com)
A Hot Wire MAF sensor relies on the air density to change the electrical properties of a conductor that is much like a toaster wire. This conductor is heated and as it is cooled by the air charge, the resistance on that wire changes and the ECU can use this information for it’s calculation. What, you say? How exactly does that tell the ECU anything? If you’re not familiar with Ohm’s law, I will let wikipedia explain it really quickly.
“Ohm’s law states that the current through a conductor between two points is directly proportional to the voltage across the two point. Introducing the constant of proportionality, the resistance, one arrives at the usual mathematical equation that describes this relationship.
Where I is the current through the conductor in units of amperes, V is the voltage measured across the conductor in units of volts, and R is the resistance of the conductor in ohms. More specifically, Ohm’s law states that the R in this relation is constant, independent of the current.”
That’s a mouth full. But basically in normal speak, if you fix 2 of those variables, the 3rd variable changes.
The Hot Wire MAF sensor’s wire is heated (duh). The temperature of a conductor changes it’s resistance. The resistance decreases as the temperature decreases. When air flows past the conductor, it cools it. The more air that flows past it, the more it is cooled. So now we see that have one of our variables in the equation above, R, and it changes as the actual air flow changes. Let’s look at I, current. That’s how we are heating the wire with the ECU and can be assumed to be a fixed value. Then we have V, voltage. That is the golden ticket for the ECU. Voltage is something that the ECU can watch easily and safely without burning up components. So now we see how the ECU converts this change to the property of a conductor to a value it can use. It applies current to the wire, the resistance changes as air enters and then the voltage returned to the ECU changes. BOOM. There’s your reference to “airflow.” As the resistance is lowered, the returned voltage to the ECU rises.
But wait! What can the ECU do with voltage? How does that tell it how much airflow is entering the engine? The engine’s ECU uses an electrical circuit for every sensor on the car to determine what is going on with that circuit by watching the change in voltage or resistance. It changes what it wants to do as the voltage or resistance across that circuit changes. The maps in the ECU’s ROM are calibrated to convert that voltage into something it can use in it’s calculations.
For a MAF sensor, voltage is generally converted to grams per second (g/s). This is the amount of air (in grams) flowing per second past the MAF sensor.
Here we have a stock FRS MAF scaling table. On the left is the voltage reference. To the right of that is the Mass Airflow in g/s. Then we have a 2D graph of the MAF scaling curve.
So air flows past the MAF sensor’s conductor, it’s resistance changes, so voltage returned to the ECU changes, the ECU looks at this map, and finally converts the value into grams per second. Now that it knows how much air is entering, it moves on to other variables and calculations to arrive at the final fuel/timing settings. It truly is amazing how quickly this stuff happens.
Why your intake needs a tune
The manufacturer (or tuner) has calibrated the MAF scaling for the car in the condition that the car was in at that time. The reason that MAF scaling needs to be performed on a car with a different air intake is really obvious when you exaggerate the scenario. Now I humor you with my MS Paint skills.
Here we have the same MAF sensor placed in two different sized pipes. Lets just assume that the small one is 3″ and the large one is 4″.
The area of the 3″ pipe is 7.07″. The area of the 4″ pipe is 12.57″. That’s 78% more area than the 3″ pipe. That’s also 78% more airflow that is not directly affecting our heated MAF sensor conductor.
In our MAF scaling picture above, we see that the ECU expects 7.06 g/s total air flow through the pipe at 1.64 volts. Now the MAF sensor is going to read a lower voltage because less air is actually flowing past it, even though the engine is still pulling the same amount of actual air into the engine. So the ECU sees a voltage of 1.45v instead of the 1.64v and injects fuel for 4.08 g/s instead of the 7.06 g/s from before. The result is a lean mixture because the ECU thought it was receiving less air.
The design of the intake system can also skew the readings. This is because air flows past the sensor in a different manner than it was tuned for. Maybe it is more smooth or maybe is worse. Either way, if it’s different than what the car was tuned for, the engine can benefit from MAF Scaling.
The very first thing I do when tuning a car is touch up the MAF scaling to bring fuel trims close to zero. Even on a 100% stock car, there is room to make it better. This is simply because every car is in fact different and the manufacturers calibrate for the best safe average. Starting with dialing in the MAF scaling ensures that you are starting out of the right foot with proper info being fed into the ECU. The info from the MAF sensor is used throughout the ECU’s routines to determine load which in turns affects literally everything.
During closed loop conditions where the ECU is watching the o2 sensor and providing real-time corrections, this may not be much of an issue as long as the necessary correction isn’t outside of the ECU’s allowable correction limit. But when you enter open loop conditions (WOT), then you’ll just get a lean condition if there are no stored corrections affecting those airflow ranges.
Your MAF sensor is a very important piece of the load calculation and fueling puzzle that your car is constantly crunching numbers trying to figure out. MAF sensors are very good at adapting to changes in airflow into and out of the engine as well as changes in environment. But anything that affects HOW air flows past the sensor needs to be addressed. It is also important that the sensor is clean so that the conductor reacts properly to the air flowing past it. A dirty conductor will not react to the change in air density as accurately or quickly as a clean one. Open element oiled air filters are the worst for gumming up MAF sensors. At the end of the day, the MAF sensor is the first and likely most important device that the engine’s ECU uses to make it work properly. Take care of it and think about what changes happen to it when you make modifications.