Quote: Originally Posted by TJ1376
Quote: Originally Posted by blackbird_R/T
The manifold absolute pressure (MAP) sensor is what the PCM uses to calculate the amount of air going into the engine and determine when you're going into "overboost".
Hehehe..
I get to KINDA correct Blackbird...
Map/Tip sensors are not calculating the AMOUNT (volume) of air going into the engine... They are calculating the PRESSURE of air going into the engine.
Thats your two big differences between the MAP sensor and the MAF sensor (used on other cars). One measures pressure... 1psi, 2psi, 3psi etc.. The other measures flow or volume of air...
TJ
Maybe "kind-of" if you read it wrong, but not really.
Key word = calculate, not measure
But the MAP sensor is used in part to "measure" the amount of air going into the engine via the PCM. The PCM does that by
measuring pressure and then
calculating the
mass of air from that.
I'd say it's more of a clarification for someone who has minimal knowledge of how the MAP sensor works.
And since it was brought up, the SRT-4 does use a speed-density system where it uses the
manifold absolute pressure (MAP) sensor to measure pressure at the intake manifold and use that in calculations that include rpm and other sensor data like the intake air temp sensor (IAT) along with the known engine volume to determine the amount of air in the combustion chamber and therefore calculate the fueling requirements.
The
mass air flow (MAF) sensor used on other engines can actually directly
measure the amount of air physically entering the engine. Inside the most common MAF sensors are a sensing element comprised of a heated wire or film surface. As more air crosses the wire/surface it starts to cool them down, but due to design of the heated element the resistance of the wire/surface also decreases allowing more current to flow and keep it at the same temperature. You can then measure that amount of current being used to hold a constant temperature. I also believe some may now use a sensing circuit to directly measure the wire/surface temp and the internal controller adjusts current to keep it at a constant temperature.
In either case the control circuit in the MAF then uses that current required to keep it at the same temperature to determine airflow (since it cools down proportionate to airflow) in conjunction with an IAT that helps the PCM determine the amount of air (since the density of air changes with temperature). The MAF sensor then outputs a signal to the PCM telling it how much air is coming in. On a scan tool, with the GM's for example, you can usually watch this calculated value as grams/sec of air.
The MAF sensor can be placed anywhere with smooth airflow (not near intake pipe bends, etc) in the intake tract between the air filter and throttle body. Some turbocharged cars use them before the compressor's inlet and others use them in-between the turbo outlet and throttle body. In any case a MAF car like the WRX from the factory can't use an external BOV that vents to atmopshere because the MAF sensor would think there's more air flowing in when it vents. Also more trivia was when GM was first doing development work to add fuel injection to their 2.8L OHV V6 they used both MAP and MAF. For example the Citation/Celebrity got a MAF setup and the Fiero got a MAP system. If there were long-term emission problems with either system they could have switched to using one system exclusively.
Both sides have their advantages and disadvantages to the tuner. BTW I also own more than a few other cars that use MAF sensors so I'm a little familiar with their operation.
