MAF vs MAP?

[jayjr1105]

I often hear these two terms but what's the difference? I know in general they measure incoming airflow and reports it to the computer for air/fuel mixture.

 

[Quattro Pete]

MAF measures air flow.
MAP measures pressure and then calculates air flow from it.

https://mechanics.stackexchange.com...ifference-between-a-map-and-a-maf-sensor

 

[jayjr1105]

So MAP is the more sophisticated piece with the ability to adapt simply from the car's altitude? pretty cool.

 

[OVERKILL]

Originally Posted by jayjr1105
So MAP is the more sophisticated piece with the ability to adapt simply from the car's altitude? pretty cool.


MAP is the less complicated of the two solutions, typically referred to as Speed Density. MAF systems also have a MAP sensor, but it is typically used to measure AP. MAP infers air volume based on vacuum and a table, whilst MAF measures air volume directly and corrects for barometric pressure. Ergo, MAF is the more precise system, and can adapt to engine modifications, whilst MAP, if you make changes that impact vacuum, you royally screw things up.

 

[LDM]

MAP measures pressure. MAF measures air flow. Both can be used to determine fueling. As OVERKILL said, just using a MAP sensor is typically called Speed Density, it uses the reading from the MAP sensor (as well as usually an air temp sensor and coolant temp sensor) and then calculates fuel needed from a table in the ECM. MAF tends to be the more accurate system and is what is used in pretty much all modern vehicles, as it is also more capable of variation.

But back in the late 80s/early 90s, MAF sensors were not anywhere near as accurate as they are today. In fact, many were very inaccurate, as in the case of the Bosch MAF used in my 89 Camaro. I ended up doing an ECM conversion and going to the 90-92 version that uses Speed Density so the car would run better and be easier to tune. Today MAP is mostly there as a backup in case the MAF fails. If that happens, the PCM will move over to the backup Speed Density mode, turn on the SES light, and at least keep going until you can get the MAF fixed. A lot better than the terrible limp mode of the older computer controlled cars that pretty much did nothing.

 

[Tman220]

Here is an example of a VE table from my heavily modified LT1
The horizontal axis is the pressure in Kpa inside the intake manifold which is what the MAP (Manifold Absolute Pressure) sensor measures. The vertical axis is obviously RPM and the numbers in the cells are % Volumetric efficiency.

The camshaft, heads, intake, exhaust, and more all affect volumetric efficiency and this table needs to be pretty close so that the engine will run half way decent before closed loop operation, and still then it needs to be close enough so that the fuel trims are within a correctable range.

 

[Tman220]

Here is an example of a MAF table from my engine PCM. The MAF measures air flow by heating a wire and the current required to keep it a certain temp, the sensor output is a frequency in Hertz and the calibration is in Grams per second

 

[Y_K]

And they also 'age', but not at the same rate as O2 sensors.

 

[Rand]

Originally Posted by LDM
MAF tends to be the more accurate system and is what is used in pretty much all modern vehicles,


Surprisingly MAP is still used, the widely available pentastar engines use MAP 3.2L and 3.6L

 

[Surestick]

I've never really understood the use of MAF. They seem to have frequent issues getting dirty and causing problems whereas MAP sensors rarely seem to have issues.

Any difference in accuracy due to one measuring the airflow directly, the other inferring it from pressure is rendered irrelevant when you add an O2 sensor in the exhaust. Additionally, a MAP doesn't introduce any restriction in the intake.

Even if they are more accurate that's only until they (seemingly inevitably) get dirty and then they aren't accurate and seem to lead to a host of issues all of which have got to have an adverse effect on emissions.