(Hot wire sensor (MAF) (this is your common AFM in our beloved nissans)
(Some Info was taken from Nissan Silvia and Wikipidia and other sites)
A hot wire mass airflow sensor determines the mass of air flowing into the engine’s air intake system. This is achieved by heating a wire with an electric current that is suspended in the engine’s air stream, not unlike a toaster wire. The wire's electrical resistance increases as the wire’s temperature increases, which limits electrical current flowing through the circuit. When air flows past the wire, the wire cools decreasing its resistance, which in turn allows more current to flow through the circuit. As more current flows, the wire’s temperature increases until the resistance reaches equilibrium again. The amount of current required to maintain the wire’s temperature is directly proportional to the mass of air flowing past the wire. The integrated electronic circuit converts the measurement of current into a voltage signal which is sent to the ECU.
If air density increases due to pressure increase or temperature drop, but the air volume remains constant, the denser air will remove more heat from the wire indicating a higher mass airflow. Unlike the vane meter's paddle sensing element, the hot wire responds directly to air density. This sensor's capabilities are well suited to support the gasoline combustion process which fundamentally responds to air mass, not air volume. (See stoichiometry.)
Manifold Absolute Pressure sensor (MAP) (As seen in PFC D-jetro and haltech amongst others)
The manifold absolute pressure sensor provides instantaneous manifold pressure information to the engine's electronic control unit (ECU). This is necessary to calculate air density and determine the engine's air mass flow rate, which in turn is used to calculate the appropriate fuel flow. (See stoichiometry.)
An engine control system that uses manifold absolute pressure to calculate air mass, is using the speed-density method. Engine speed (RPM) and air temperature are also necessary to complete the speed-density calculation.
The MAP sensor can be used to directly measure the BAP (barometric absolute pressure).
BAP = MAP (When either of the following conditions are true.)
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o When the engine is not turning.
o When operating at WOT (nearly equal to the barometric pressure)
Once the BAP is known, the MAP sensor can be used to calculate intake manifold vacuum.
BAP - MAP = Manifold Vacuum
or
BAP = MAP + Manifold Vacuum
or
MAP = BAP - Manifold Vacuum
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o When the engine is running, the difference between the BAP and the MAP is known as intake manifold vacuum. The ECU learns the BAP just before cranking the engine, i.e., when MAP equals BAP.
As atmospheric pressure decreases with increasing altitude, vacuum must also decrease to maintain the same MAP in order to maintain the same torque output. This is accomplished by opening the engine's throttle more as altitude increases. However, the BAP learned at the beginning of the trip becomes obsolete as altitude changes.
Sometimes an engine control system will use both a BAP sensor and a MAP sensor to continuously maintain an accurate barometer and manifold vacuum. However, neither vacuum nor barometer are necessary for fuel determination, although they are helpful for other engine functions. The critical information is the air's density in the intake manifold, and the speed of the engine, i.e., the speed-density method.
The BAP sensor is often located within the ECU, and the MAP sensor is usually located near the intake manifold.
Benefits of a hot-wire MAF sensors are:
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* Responds very quickly to changes in air flow
* Separate temperature and pressure sensors are not required (to determine air mass)
* No retune required for changes in setup – Example: Adding cams, changing the engines VE
* Gives better resolution for more accurate tuning
*Better resolution for tuning off Boost \ low load \ vacuum area’s
*Better fuel economy more stable low boost \ Low load AFR’s
Drawbacks of a hot-wire MAF sensors are:
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* Dirt and oil can contaminate the hot-wire deteriorating its accuracy
* Installation requires a laminar flow across the hot-wire
* larger overall package
*More sensitive to mounting location - Example: If mounted to close to the turbo or using an open air BOV
*More expensive if being replaced
**Restriction on the air intake, This only applies to smaller diameter MAF’s most 80mm MAF’s are bigger then the turbo inlets **
Benefits of a MAP sensor are:
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*High Boost pressures are supported by the sensor – Example: The limits of the sensor are not commonly reached
*Less expensive
*Smaller overall package
*Easier installation in cramped engine bays
*Improved throttle response – Example: Sensor is closer to the intake quicker response to change
**Less restrictive on the air intake, This only applies to smaller diameter MAF’s most 80mm MAF’s are bigger then the turbo inlets**
Drawbacks of a MAP sensor are:
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* Requires retune for any change to the engines VE (eg change exhaust, cams, even something as minor as a dirty air filter can stuff the tune)
*Requires a IAT to be Accurate
Now a little off the facts from what I have been able to pick up in the stuff I am reading is MAP sensor cars have a more conservative tune. The MAP sensor by design using the speed-density method of calculation does not respond as well to atmospheric or climate changes because of it first start BAP reading.
The MAP sensor was developed primarily as an alternative to the more expensive and larger MAF sensor and is normally found in budget engines although they have been seen in more expensive motors as of late like the Australian LS1 the sensor is cheap and be coming the stable input for aftermarket management systems the world over.
MAP sensors are normally used in conjunction with MAF sensors the MAF being the primary input and the MAP sensor monitoring EGR operation. Without MAF, monitoring of the airflow is very tough, and so the chances on reproducibility of the desired performance in the field is difficult ,EGR with the feedback system and exhaust sensors are necessary to lower the chances of engine to engine variation of performance another small disadvantage is as the engine wears, the mean vacuum is decreased. This 'fools' the MAP sensor into overfuelling. This is only a problem on very high mileage engines