What does a MAF really read?

[ol boy]

Here goes... Helped a buddy today flow a throttle body with a custom bell with a ford 2005 MAF plumbed in the side. We had the whole set up on a flow bench sucking air through the TBI and air bell. We then adjusted the air flow to get to a specfic voltage output from the MAF. After hitting 20 points we ran out of air on the flow bench. Final cfm at only 3.5 volts was 1200 which is near 2500kg/hr. All the points got calculated into kg/hour for the binary file in an eec4 ford ecu.

Form this I'm thinking that a MAF really only reads the velocity of the air flowing past it and not really the mass of anything. Normally in an OEM set up the only constant is the diameter of the MAF sensor housing. A 2.5 inch dia housing will need a higher velocity to pass the same amount of air per minute then a 3.5 inch housing. If the same sensor was mounted in both housings and passing the same 500cfm the one in the 2.5 in housing will have a higher analog output reading then the 3.5 in housing.

If MAF truely measured the mass of air moving through it then there would be no need for measuring the temperature. The mass would change with temperature... Every MAF sensor has a built in thermistor to measure air temp. I'm beginning to think that the OEMs calculate the mass of air that the MAF housing can pass based on the diameter of the housing and measuring the velocity of the air flowing through it. IMO we can put just about any MAF sensor into any size housing as long as we know the velocity range to volt output.

Tell me what you think....

Later Ryan

[billr]

I'll take a stab at it:

A MAF sensor does measure the mass, by seeing how much the hot-wire in there is getting cooled. That depends on the air mass, of course, but also the air temperature because a pound of cold air is going to cool the MAF sensor wire more than a pound of hot air. There is probably also a difference in the heat-capacity of air (maybe same effect I'm already trying to describe) as the air temperature changes

[ol boy]

So at 2500 rpm at the same load does this engine move the same amount of cfm of air if the temp is 30 degrees or 100 degrees? If it does then won't the cooler air have a slightly higher velocity than the less dense hotter air?

I'm not sure any MAF element is calibrated to any specific g/s or kg/hr flow rate as that will change with the diameter of the housing its installed into. As density changes for a given engine load so will the velocity of the air through a fixed orifice.

[subwoofer]

It measures actual mass, not velocity. As temperature goes up, density goes down and velocity up for a given mass. Air mass is what actually matters, since oxygen mass vs fuel mass is what we are trying to control.

[ol boy]

If that is true then moving the element from a 2.5 dia housing to a 3.5 on the same engine would result in the same analog output, but I'm sure it won't.

I tested this yesterday with a 2005 ford MAF element which was installed into a 4 inch intake throat. Output voltage at 2500kg/hr was only 3.2 volts.

[jsmcortina]

If that is true then moving the element from a 2.5 dia housing to a 3.5 on the same engine would result in the same analog output, but I'm sure it won't

The mass passing through the sensing element is reduced when the tube diameter is increased.

James

[ol boy]

Is that from a velocity change or % change of the area the sensor takes up within the housing?

[jsmcortina]

Is that from a velocity change or % change of the area the sensor takes up within the housing?

Area.

James

[elaw]

Just to clarify a little...

The sensing element measures the mass of air that is flowing over it... that is, the air flowing over the element itself. The air flowing over the element is typically just a small portion of the air flowing through the sensor housing but it's a known portion, so that is taken into account when translating the sensor's output voltage or frequency into mass airflow rate.

What that means is that you can increase the range of a MAF sensor by moving the sensing element to a larger housing (assuming the element is a separate part), because the element will then be exposed to a lesser percentage of the overall airflow. But the element is still measuring mass, it will just be scaled differently.

[Matt Cramer]

Just to clarify a little...

The sensing element measures the mass of air that is flowing over it... that is, the air flowing over the element itself. The air flowing over the element is typically just a small portion of the air flowing through the sensor housing but it's a known portion, so that is taken into account when translating the sensor's output voltage or frequency into mass airflow rate.

What that means is that you can increase the range of a MAF sensor by moving the sensing element to a larger housing (assuming the element is a separate part), because the element will then be exposed to a lesser percentage of the overall airflow. But the element is still measuring mass, it will just be scaled differently.

And that also means that if you put a bend right upstream (or sometimes even downstream) of the MAF, the air flow distribution will change and can cause you to need a different MAF calibration.