MAF or MAP know your sensors

Pinkfluffybunnys · Post by **Pinkfluffybunnys** » Wed Jun 25, 2008 2:23 pm

(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.)
•
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
•
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:
------------------------------------------------------------

* 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:
--------------------------------------------------------------

* 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:
--------------------------------------------------

*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:
-----------------------------------------------------

* 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

Gary57 · Post by **Gary57** » Wed Jun 25, 2008 2:30 pm

Not as accurate to tune – The sensors lack the full range accuracy of the MAF sensors.

Why not?? Mine works to 3 decimal places...

Pinkfluffybunnys · Post by **Pinkfluffybunnys** » Wed Jun 25, 2008 2:57 pm

I am not really attracted to any sensor, but I need to source the link to that info.
It’s not the physical resolution of the sensor but the location and how the calculation is done to get the speed-density.

It seems to lack the low down resolution in low load areas and finite changes in Air Mass that can be picked up by a MAF .

none of these sensors are perfect

Gary57 · Post by **Gary57** » Wed Jun 25, 2008 3:02 pm

Yeah but changes in air mass are determind by the IAT sensor.

I would say a MAP sensor being closer to the valves can compensate for all the changes made to the air tracked where as the MAF cannot. But yeah I agree non are perfect.

Pinkfluffybunnys · Post by **Pinkfluffybunnys** » Wed Jun 25, 2008 3:16 pm

Cool makes sense’s.

The low load area's are where the MAP sensor does fall below the MAF, the MAF sensor can meter better also don’t for get the BAP that’s calculated on start up unless you have a BAP sensor on you ECU.

Doctor G · Post by **Doctor G** » Wed Jun 25, 2008 3:19 pm

That's why I won't be running a MAP or a MAF on my built engine!

Gary57 · Post by **Gary57** » Wed Jun 25, 2008 4:00 pm

Still dont get the low load areas?? The Sensor is calibrated to atmospheric pressure so no need for a BAP sensor.

DcG what will you be using??

Pinkfluffybunnys · Post by **Pinkfluffybunnys** » Thu Jun 26, 2008 10:08 am

This is just info I have picked up with the research on these sensors.
If you have more info then we could add it to the thread to get a better understanding.

The MAP sensor can also be used to measure the barometric absolute pressure (BAP sensor) when the engine is not turning, i.e., when the intake manifold is not being evacuated by the engine's pumping action.

When the engine is running, the difference between the BAP and the MAP is the engine's manifold vacuum. As atmospheric pressure decreases with increasing altitude, so must vacuum to maintain the same MAP but unless you have a dedicated BAP sensor there is a small variation in the Speed – density calculation. Standard atmospheric pressure changes as the weather changes but it’s not a huge variation.

BAP = MAP + Vacuum

I guess the same variation could bee seen on hot wire AFM another thing that people have been saying on tuning forums (not very reliable, I know) is the MAP sensor because of how it works is very sensitive to the location where it is installed and the design of the manifold it’s placed on. There was one guy that picked up two different vacuum readings when he changed the location of his MAP sensor on the manifold boost was still seen at the same values though

Gary57 · Post by **Gary57** » Thu Jun 26, 2008 12:51 pm

Yeah but the ECU doesnt need to know BAP to do any calculations. All it cares about is the pressure/vacuum there is going into the cylinders, BAP has nothing to do with it.

The MAP when my car is idling is the same here as it would be in JHB, these sensors are calibrated to absolute pressure not atmospheric.

veecee · Post by **veecee** » Thu Jun 26, 2008 4:41 pm

ok now im confused...

pink fluff says it matters whether the ECU calibrates itself for atmospheric pressure (BAP).

gary says not. i suppose it depends on the ECU???

but this worries me - i am going to run Gotech on my SR20 right. it uses a MAP sensor. does that mean that i will have issues if i drive my car to durbs????

i remember fly200sx came up to JHB last december to come to the wesbank drifting competition. his car struggled to run nicely because of the change in altitude (well, thats what they said it was!!!

) i also dont remember what management he was running...

But i'm not interested in having issues like that!

Gary57 · Post by **Gary57** » Fri Jun 27, 2008 9:15 am

Pressure is pressure I dont care where you are. You will just have less pressure in JHB than we have in DBN.

veecee · Post by **veecee** » Fri Jun 27, 2008 9:24 am

pressure this week was hectic here in JHB. i done 6 days worth of work in 4 days!!! still gotto work today and be on call for saturday!!! bastards!!!

now this back to atmospheric pressure thing - why did fly's car struggle in JHB??? i've heard of this happening to other tuner cars too!!!

does it mean i should get the car dyno'd in jhb on map1, and down in durbs on map2??? then swap out half way between the 2 if i am driving down???

Gary57 · Post by **Gary57** » Fri Jun 27, 2008 9:33 am

His car must have had a shit tune.

Your car is tuned according to how much pressure there is in the manifold. Now if you go to Holland you are below sea level so you have more pressure so the ECU compensates for that, same thing as you in JHB.

veecee · Post by **veecee** » Fri Jun 27, 2008 9:37 am

the ECU compensates for that by measuring BAP like pinkfluff said?
so you agreeing with him now???

or does the word manifold ABSOLUTE pressure have something to do with it???

Gary57 · Post by **Gary57** » Fri Jun 27, 2008 9:41 am

No VC you tune your car according to ABSOLUTE pressure so you dont have this exact issue. If you tuned according to BAP then yes you would have the issue between DBN and JHB.

Absolute pressure is the key word here.