Maf Sensor
02-20-2003, 04:34 PM
#1
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Join Date: Jun 2002
Location: Longview, TX, USA
Posts: 60
Maf Sensor
came across this and thought I'd pass it along....
MAF Sensor
The MAF Sensor uses hot wires inserted in a known cross sectional area to measure airflow. The MAF Sensor and the IAT Sensor output signals to the PCM determine the injector pulse time during Open Loop Condition. The MAF output signal is dependant on the resistance in the wire due to the wire temperature. The temperature is a function of the air velocity, as the faster the velocity the lower (cooler) the wire temperature. So, as the velocity of the air increases (acceleration) the wire cools and sends a signal to the PCM to lengthen the pulse time of the injectors to deliver more fuel in order to maintain the desired A/F ratio of 14.7:1.
In front of the heated wire is a screen. Many people are suggesting that improved airflow will be gained by cutting the screen out of the MAF. It is not recommend that this be done on an F-body. Most people mistake this screens purpose as preventing debris from hitting the hot wires in the MAF (that is what the air filter is for). Actually the screen is a bank of straightening vanes used to straighten the airflow before passing by the hot wires. Eddies and vortices (turbulence) in the MAF will cause inaccurate readings. Straightening vanes are used in most mass flow measurement situations where 10 diameters of straight upstream conduit are not available. The F-body intake does not meet this requirement.
An accurate MAF sensor is critical for the engine's fuel/air ratio adjustments. So, what happens when the screen is removed? The flow (mass) is the same, BUT without the screen (which is a restriction) the velocity is lower. Lower velocity means a hotter wire temperature which equates to a LOWER flow! So the PCM adjusts the injectors to provide LESS fuel. Since the HO2 sensors do not provide input when the MAF is online the PCM does not adjust for the LEAN condition, until it goes back into Closed Loop Condition.
MAF Sensor
The MAF Sensor uses hot wires inserted in a known cross sectional area to measure airflow. The MAF Sensor and the IAT Sensor output signals to the PCM determine the injector pulse time during Open Loop Condition. The MAF output signal is dependant on the resistance in the wire due to the wire temperature. The temperature is a function of the air velocity, as the faster the velocity the lower (cooler) the wire temperature. So, as the velocity of the air increases (acceleration) the wire cools and sends a signal to the PCM to lengthen the pulse time of the injectors to deliver more fuel in order to maintain the desired A/F ratio of 14.7:1.
In front of the heated wire is a screen. Many people are suggesting that improved airflow will be gained by cutting the screen out of the MAF. It is not recommend that this be done on an F-body. Most people mistake this screens purpose as preventing debris from hitting the hot wires in the MAF (that is what the air filter is for). Actually the screen is a bank of straightening vanes used to straighten the airflow before passing by the hot wires. Eddies and vortices (turbulence) in the MAF will cause inaccurate readings. Straightening vanes are used in most mass flow measurement situations where 10 diameters of straight upstream conduit are not available. The F-body intake does not meet this requirement.
An accurate MAF sensor is critical for the engine's fuel/air ratio adjustments. So, what happens when the screen is removed? The flow (mass) is the same, BUT without the screen (which is a restriction) the velocity is lower. Lower velocity means a hotter wire temperature which equates to a LOWER flow! So the PCM adjusts the injectors to provide LESS fuel. Since the HO2 sensors do not provide input when the MAF is online the PCM does not adjust for the LEAN condition, until it goes back into Closed Loop Condition.
06-04-2004, 10:38 AM
#2
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Join Date: May 2004
Location: Miami Beach, FL
Posts: 9
Sounds very logical to me. Anyone else have thoughts on this. I have never removed the screen in mine because of that very reason. I read this before. I would still like to see more debate on this issue. Thanks.
06-04-2004, 11:23 AM
#3
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Join Date: Nov 1998
Location: Hell was full so they sent me to NJ
Posts: 70,735
While in "concept" that writeup is headed in the correct direction, it has some errors.....
The wire do not run cooler or hotter as a function of airflow. The elements including the resistance of the wires are arranged in a Wheatstone bridge, and the sensor calculates the energy (BTU's) required to keep the wire as a constant temperature relative to the incoming air temperature.
While removing the "screen" ruins the calibration of the meter, it is not because the velocity of the air is reduced when the screen is removed. It is because the distribution of flow velocity across the open area of the meter is altered. The meter is calibrated on the basis that the small sample of air that is acually touching the wires and removing heat, is representative of the flow distribution across the entire face of the meter. Removing the screen makes the flow distribution less predictable, and less uniform, and destroys the calibration.
Bottom line.... same result.... the screen serves a purpose. Interestingly, GM deleted the screen from the newer versions of the Z06 MAF sensor, because the symetrical layout of the air ducting provides a fairly unifoprm flow distribution without the screen. They found 5HP, but of course they took the time to verify and program into the Z06 computer the correct calibration curve for the meter.
The wire do not run cooler or hotter as a function of airflow. The elements including the resistance of the wires are arranged in a Wheatstone bridge, and the sensor calculates the energy (BTU's) required to keep the wire as a constant temperature relative to the incoming air temperature.
While removing the "screen" ruins the calibration of the meter, it is not because the velocity of the air is reduced when the screen is removed. It is because the distribution of flow velocity across the open area of the meter is altered. The meter is calibrated on the basis that the small sample of air that is acually touching the wires and removing heat, is representative of the flow distribution across the entire face of the meter. Removing the screen makes the flow distribution less predictable, and less uniform, and destroys the calibration.
Bottom line.... same result.... the screen serves a purpose. Interestingly, GM deleted the screen from the newer versions of the Z06 MAF sensor, because the symetrical layout of the air ducting provides a fairly unifoprm flow distribution without the screen. They found 5HP, but of course they took the time to verify and program into the Z06 computer the correct calibration curve for the meter.
06-04-2004, 05:55 PM
#5
Registered User
Join Date: Mar 2001
Location: SLC, UT
Posts: 2,668
Originally posted by Injuneer
While in "concept" that writeup is headed in the correct direction, it has some errors.....
The wire do not run cooler or hotter as a function of airflow. The elements including the resistance of the wires are arranged in a Wheatstone bridge, and the sensor calculates the energy (BTU's) required to keep the wire as a constant temperature relative to the incoming air temperature.
While removing the "screen" ruins the calibration of the meter, it is not because the velocity of the air is reduced when the screen is removed. It is because the distribution of flow velocity across the open area of the meter is altered. The meter is calibrated on the basis that the small sample of air that is acually touching the wires and removing heat, is representative of the flow distribution across the entire face of the meter. Removing the screen makes the flow distribution less predictable, and less uniform, and destroys the calibration.
Bottom line.... same result.... the screen serves a purpose. Interestingly, GM deleted the screen from the newer versions of the Z06 MAF sensor, because the symetrical layout of the air ducting provides a fairly unifoprm flow distribution without the screen. They found 5HP, but of course they took the time to verify and program into the Z06 computer the correct calibration curve for the meter.
While in "concept" that writeup is headed in the correct direction, it has some errors.....
The wire do not run cooler or hotter as a function of airflow. The elements including the resistance of the wires are arranged in a Wheatstone bridge, and the sensor calculates the energy (BTU's) required to keep the wire as a constant temperature relative to the incoming air temperature.
While removing the "screen" ruins the calibration of the meter, it is not because the velocity of the air is reduced when the screen is removed. It is because the distribution of flow velocity across the open area of the meter is altered. The meter is calibrated on the basis that the small sample of air that is acually touching the wires and removing heat, is representative of the flow distribution across the entire face of the meter. Removing the screen makes the flow distribution less predictable, and less uniform, and destroys the calibration.
Bottom line.... same result.... the screen serves a purpose. Interestingly, GM deleted the screen from the newer versions of the Z06 MAF sensor, because the symetrical layout of the air ducting provides a fairly unifoprm flow distribution without the screen. They found 5HP, but of course they took the time to verify and program into the Z06 computer the correct calibration curve for the meter.
*side-note*
That is very correct about the Z06's MAF... Thus the only place I could get good Freq Vs Flow information was a '02 Z06 Vette, the year they removed the screens from the MAF (Luckily my buddy bought one right before my stocker crapped out
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