What exactly does the MAF do?!
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Registered User
Joined: Sep 2001
Posts: 4,152
From: Woodstown, NJ
What exactly does the MAF do?!
Seriously.
I've read pages and pages of how the LT1 PCM calculates fuel delivery and ignition timing, among other things, but nowhere does it provide any physical data direct from the MAF sensor. I always see RPM versus MAP, but no MAF?
I know you can disconnect the MAF and convert the PCM to speed density mode with a little bit of tweaking with LT1_edit, so the MAF can't be THAT important, can it?
Just curious if anyone has any input on this "mystery sensor."
I've read pages and pages of how the LT1 PCM calculates fuel delivery and ignition timing, among other things, but nowhere does it provide any physical data direct from the MAF sensor. I always see RPM versus MAP, but no MAF?
I know you can disconnect the MAF and convert the PCM to speed density mode with a little bit of tweaking with LT1_edit, so the MAF can't be THAT important, can it?
Just curious if anyone has any input on this "mystery sensor."
Registered User
Joined: Dec 1999
Posts: 1,697
From: Petersham, MA
This has been covered before a couple of weeks ago. In a nutshell, the MAF does what speed density does, only more accurate. The MAF reports its voltage reading directly to the PCM based on the mass of the incoming air, so it's definitely not a "mystery" sensor.
Administrator
Joined: Nov 1998
Posts: 71,110
From: Hell was full so they sent me to NJ
You might want to read the air/fuel management section of my ScanMaster writeup.
When the PCM knows the mass air flow into the engine, all it needs is the target A/F ratio, and it can calculate the required mass of fuel required to mix with that mass of air, and then calculates the injector pulse width required to supply that mass of fuel.
In speed density, you are not measuring air mass flow directly. You are multiplying engine displace/2 X volumetric efficiency (VE) to get the volume air flow. Then the PCM uses the inlet air temperature (IAT) and the manifold absolute pressure (MAP) in the perfect gas law to calculate the density of the air, and then volume flow X density = mass flow.
The weak point of the speed-density system is that any time you make a change radical enough to alter volumetric efficiency (e.g. - heads & cam), you need to enter new VE values for each combination of RPM and engine load (MAP). Without the updated VE figures, the PCM can not calculate the mass air flow. In the MAF system, the sensor measures mass air flow directly, and the PCM cn better respond to more radical changes.
MAF (Mass Air Flow):
In a "mass air" engine control system, as used in the 94 and later F-bodies, the PCM only needs a few pieces of information to manage the air and fuel mixture. The most important is the amount of air entering the engine, to be used for combustion with the correct amount of gasoline. More specifically, it needs to know the "mass" of air, or the number of pounds or grams of air that entered the engine to be used to burn the gasoline.
The Mass Air Flow (MAF) sensor measures the mass of air entering the engine, expressed in "grams per second" or "pounds per minute". The ScanMaster displays only the metric units - grams per second (gps).
Note - to convert grams per second to pounds per minute:
grams/second = pounds/minute x 7.56
Example: 30.0 pounds/minute x 7.56 = 226.8 grams/second
At idle, the air mass flow will range from as low as 6gps to as high as 10gps. If you reading is suspiciously low, you may have a vacuum leak. If it is suspiciously high, and the idle speed is correct, it's most likely a "faulty" or "miscalibrated" MAF. This problem is typical of "home ported" units.
At WOT, the peak air flow in a stock, normally aspirated engine, with a 5,800rpm rev limit will be about 240gps.
The MAF sensor works by internally measuring the temperature of the air coming into the system. It then uses the wires to heat the small amount of air that is actually touching the wires, a fixed number of degrees above the incoming air. By measuring the electrical power required to heat the air, and knowing the specific heat of air, the MAF sensor can calculated the "mass" (or roughly "pounds") of air entering the engine. It then converts this signal to a variable frequency output. Inside the PCM, there is a calibration table that converts the frequency to a "mass air flow" rate.
In a "mass air" engine control system, as used in the 94 and later F-bodies, the PCM only needs a few pieces of information to manage the air and fuel mixture. The most important is the amount of air entering the engine, to be used for combustion with the correct amount of gasoline. More specifically, it needs to know the "mass" of air, or the number of pounds or grams of air that entered the engine to be used to burn the gasoline.
The Mass Air Flow (MAF) sensor measures the mass of air entering the engine, expressed in "grams per second" or "pounds per minute". The ScanMaster displays only the metric units - grams per second (gps).
Note - to convert grams per second to pounds per minute:
grams/second = pounds/minute x 7.56
Example: 30.0 pounds/minute x 7.56 = 226.8 grams/second
At idle, the air mass flow will range from as low as 6gps to as high as 10gps. If you reading is suspiciously low, you may have a vacuum leak. If it is suspiciously high, and the idle speed is correct, it's most likely a "faulty" or "miscalibrated" MAF. This problem is typical of "home ported" units.
At WOT, the peak air flow in a stock, normally aspirated engine, with a 5,800rpm rev limit will be about 240gps.
The MAF sensor works by internally measuring the temperature of the air coming into the system. It then uses the wires to heat the small amount of air that is actually touching the wires, a fixed number of degrees above the incoming air. By measuring the electrical power required to heat the air, and knowing the specific heat of air, the MAF sensor can calculated the "mass" (or roughly "pounds") of air entering the engine. It then converts this signal to a variable frequency output. Inside the PCM, there is a calibration table that converts the frequency to a "mass air flow" rate.
In speed density, you are not measuring air mass flow directly. You are multiplying engine displace/2 X volumetric efficiency (VE) to get the volume air flow. Then the PCM uses the inlet air temperature (IAT) and the manifold absolute pressure (MAP) in the perfect gas law to calculate the density of the air, and then volume flow X density = mass flow.
The weak point of the speed-density system is that any time you make a change radical enough to alter volumetric efficiency (e.g. - heads & cam), you need to enter new VE values for each combination of RPM and engine load (MAP). Without the updated VE figures, the PCM can not calculate the mass air flow. In the MAF system, the sensor measures mass air flow directly, and the PCM cn better respond to more radical changes.
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