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MASS AIR FLOW SENSOR (MAF)
Both of these sensors are in MOST cases what we refer to in the trade as main inputs.
These sensors have more than 1 job.
They are used to tell the ECU IMPORTANT INFORMATION that it requires to be able to process into tune changes,relivant to the values the sensor inputs to the ECU.
Both MAP & MAF SENSORS have more than just an influence to the FUEL strategy employed by the ECU.
They both also play the no.1 most important input in relation to the IGNITION TIMING CURVE.
Also in some cases MAP SENSORS are also BAP SENSORS (BAROMETRIC ABSOLUTE PRESSURE SENSORS)
BAP SENSORS measure BOROMETRIC PRESSURE which is relative to ALTITUDE.
In some cases EG: R33GTSt SKYLINES with standard engine management the MAF is used also for the ECU to control switch on point for OVERBOOST FUEL CUT.
This is why when trying to get more than 9psi out of the RB25DET in an R33GTSt you cant without employing some type of FUEL CUT DEFENDER or AFTERMARKET ECU.
BAROMETRIC PRESSURE in RELATION to ALTITUDE,
Your maximum BAROMETRIC PRESSURE is found at SEA LEVEL.
The higher in ALTITUDE you go the lower the BARO. PRESS. is.
AIR is NOT ONLY DENSER WHEN COLD, but is also DENSER relative to PRESSURE. EG: BOOST.In saying this the higher in altitude you go the colder the air is, TRUE, however the OXYGEN QUANTITY is not as abundant as at SEA LEVEL.
Hence for HIGH ALTITUDE MOUNTAIN CLIMBERS require breathing aparatus supplying them with OXYGEN as the higher you go the LESS OXYGEN CONTENT in the ATMOSPHERE.
The BAP SENSOR takes a sample of BARO.PRESS. to caculate air density in relation to the ambiant pressure of your vehicles location,whether your cruising at sea level or cruising around up the TABLELANDS.
Some factory ECU's employ the MAP SENSOR/BAP SENSOR in the following manner.
Every time the ignition is turned on the pressure applied by the BARO.PRESS. to the diaphram in the MAP SENSOR is read by the ECU for it to calculate air OXYGEN DENSITY.
ALL AUTRONIC ECU's SMC,SM2,SM4 & all AUTRONIC PLUG & PLAY ECU's use the INTERNAL MAP SENSOR to do this BARRO.PRESS. sampling.
This time frame is almost instant. ( we are talking NANOSECONDS here.)
The time it takes you to turn your ignition key from the off position to an imediate engine cranking position the AUTRONIC has already taken its measurement and started to apply this value to all of it's CALIBRATION MAPS.
The BAP input is only applied at each ignition restart cycle.
Without getting to deep in this strategy, the BAP INPUT is only a MINOR TRIMMING INPUT in this type of senario.The ECU looks at its other main inputs to calculate true tune values. EG: TPS, ECT,ACT,MAP,MAF,etc,etc.
However the new AUTRONIC SM4 has the provisions in it's hardware and software to use BAP as a PERMANANT INPUT, influencing the engines state of tune at all times.
The SM4 can be CONFIGURED to be wired up using a BOSCH external MAP SENSOR & then using it's INTERNAL MAP SENSOR as a PERMANANT BAP SENSOR.
It's as simple as telling the ECU in it's set up program whether you wish to use BARO-TPS or BARO-MAP as you MAJOR TUNING INPUTS.
POINTS OF INTEREST REGARDING MAF & MAP SENSORS:
ALL MAF SENSORS DO HAVE A LEVEL OF RESTRICTION TO FLOW.
ANY THING in the inlet tract to the inlet valve IS a RESTRICTION to FLOW.
A couple of ways to get around or to minimise the amount of flow restriction are:
MOST MAF SENSORS have a mesh type guaze (screen) at the inlet side and normally on the engine side of them.
CAREFULLY REMOVE these mesh screens, some cases like early GEN3 ENGINED COMMODORES this screen is ALUMINIUM.
WHEN you remove your screens a simple quick reality check for you to try,COMPRESS,SQUASH, the screen down as flat as you can by hand.
Put the screen back up to the opening in your MAF,this will give you a basic idea of just how much CROSS SECTIONAL AREA you are HINDERING your incoming air flow by.
Remember those of you with 2 screens use both to get a more accurate flow restriction value.
POINT of interest GEN3 owners with early MAF's these guys have roughly a 3rd of there cross sectional flow path restricted by the 2 alloy mesh screens.
NEXT STEP, this one is IMPORTANT MAF OWNERS the screens you have just REMOVED CAREFULLY and i mean CAREFULLY, THROW the F#*KING THINGS IN THE BIN.
MAF's are very prone to CONTAMINATES.
MAF's use either HOTWIRE SENSING or THERMISTORS to calculated : AIR DENSITY,AIR TEMP and AIR QUANTITY being injested.
Whether your MAF uses HOTWIRE or THERMISTORS they both work the same way.
A REFERENCE VOLTAGE is applied to the HOTWIRE or THERMISTOR this voltage is NORMALLY 5V.
The SIGNAL RETURN WIRE to the ECU has a different value than the input(REF.VOLT.) that is applied, dependant on air flow.
EG: your ECU applies 5v to the sensor, your the HOTWIRE or THERMISTOR heats up,this heat is a resistance to voltage flow (amps)
So with 5v applied the sensor wire or probe heats up the return voltage drops,the amps climb.
The resistance causes the voltage to go down so your ECU may for example only see 2.5 volts on its signal return input.
The ECU goes to it's program table,looks for a reference to the input voltage it is getting then takes the appropriate strategy in relation to the fuel and timing table as required.
REMEMBER I AM SIMPLIFYING THESE PROCESSES AS MUCH AS POSSIBLE TO ENABLE THOSE OF YOU WHO DONT KNOW THIS STUFF UNDERSTAND IT. IT IS ALOT MORE COMPLEX THAN MY EXAMPLES ARE.
CONTAMINATES eg: DIRT that gets through your air filter, OIL from those of you running pre oiled type air filters(K&N,BLITZ etc) and ENGINE OIL that is brought back to the inlet piping via the CRANK CASE ROCKER COVER BREATHER HOSES which is forced back towards the MAF on DECELERATION caused by compressor surge.
EVER PULLED OFF YOUR AIR FILTER TO TURBO HOSE AND FOUND OIL IN THER, this is how.
These CONTAMINATES get deposited onto your HOTWIRE or THERMISTORS, which inturn INSULATES them.
When the 5v ref. is applied to the sensor the sensor gets hot causing VD( VOLTAGE DROP) on the signal return wire.
NORMALLY as your engine revs climb and the amount of air injested increases passing the HOTWIRE or THERMISTOR causing it to cool down therefor making the SIGNAL RETURN VOLTAGE get higher.
5V REF. APPLIED,SENSOR heats up,voltage drops.
ENGINE REVS INCREASE, AIR VOLUME INCREASES cooling sensor,voltage climbs.
EG: 5V APPLIED at idle you may have 1.5v on the SIG.RETURN WIRE.
5V APPLIED at 6000 rpm you may have 4.7v.
THE HIGHER THE ENGINE REVS,THE HIGHER THE RETURN VOLTAGE WILL BE.
When the sensor is contaminated it inhibits the ability for the incoming air charge to pull heat away from the sensor.
THE SENSOR IS INSULATED FROM THE AIR CHARGE,therefore staying hot,therefore the SIG.RETURN voltage will be lower than it should be.
THIS inturn causes the engine to run lean and may cause DETONATION as the ref volt. will be telling the ECU to run a perticular timing strategy that in most cases is to ADVANCED.
::::NB. WELL WRX OWNERS:::: THIS PROBLEM is as COMMON ON THESE CARS AS THE CARS THEMSELVES.
I recommend cleaning these sensors every 5000-10000 KM'S.
USE PRINTEDCIRCUIT BOARD CLEANER(avail. DICK SMITH etc)
DO NOT, TOUCH THE HOTWIRE OR THERMISTOR PHYSICALLY.
YOU WILL DAMAGE IT.
YOU CAN USE CARBY CLEANER but PCB CLEANER works better.
HOTWIRE TYPE SENSORS WEAR EARLIER THAN THERMISTOR TYPE SENSORS. VL COMMODORES prone to this problem.
MAP SENSORS.
IN MY OPINION ARE THE ULTIMATE IN LOAD SENSING.
They are "ALMOST" CONTAMINATE FREE.
IF CONTAMINATED THEY DONT OFTEN SUFFER ANY ILL EFFECTS.
THESE SENSORS USE DIAELECTRIC MEMBRANE or DIAPHRAM.
ENGINE VACUUM or BOOST ACTS apon a PIEZIO TYPE CRYSTAL, WHICH IN ITS OPERATION is alot more complex to explain and understand than the MAF SENSOR.
THOSE of you who wish to know can e-mail me on SUPERCHARGED8@aapt.net.au
In a nut shell MOST COMMON CONTAMINATES WILL NOT HAMPER THEIR PERFORMANCE.
AS FOR DAMAGE "ENGINE BACKFIRES" can damage them.
AND a hefty blow with a hammer WILL DAMAGE THEM.
APART from these examples they are pretty much indistructable in a normal operating situation.
IMPORTANT things to mention regarding MAP SENSORS.
REGARDING BIG DURATION,TIGHT LOBE SEPERATION CAMSHAFTS, which gives your engine lower engine vacuum values at idle etc,also on valve overlap you will suffer from a phenonimum called "VALVE REVERSION"
THESE issues will effect your engine tune, however with a decent ENGINE MANAGEMENT SYSTEM you will be able to tune and stablise the engine idle etc, also having good drivability.
CAIRNS PEOPLE have a listen to the idle on ROD'S BLACK B#$@CH, and TROY"s WHITE AU UTE "FRYDAY"
BOTH of these cars are running my engines, both are running what i regard as HEALTHY BUMP STICKS.(CAMSHAFTS)
Both have overlap and reversion issues.
BOTH DRIVE AND RUN SWEET DUE TO GOOD ENGINE MANAGEMENT AND GOOD TUNING.
THINGS NOT TO DO REGARDING THE PLUMBING OF THE VACUUM HOSE FROM THE ENGINE TO THE MAP SENSOR.
"""""""DO NOT PLUMB ANYTHING INTO or IN WITH THE MAP SENSOR HOSE.
ALWAYS HAVE THIS HOSE ON ITS OWN.
HAVING PLUMBED IN WITH FUEL PRESSURE REGULATORS or BLOW OFF VALVES etc WILL GIVE ISSUES.
WHEN the fuel pressure reg. is receiving pulses under the diaphram from the fuel pulses generated by the injectors pulsing on and off & when you have vacuum applied at idle then it goes when throttle is opened this will give pulses to the MAP SENSOR, NORMALLY not bad enough to cause serious engine tune issuse, but can make chasing that nice LIGHT LOAD TUNING and IDLE QUALITY tuning an issue.
BLOW OFF VALVES WILL GIVE SIMILAR PROBLEMS ESPECIALLY IF THE ARE SET TO SOFT, that they open fully with little boost.
The opening & closing,between vac & boost send pulses up the MAP HOSE.
HUNTING during idle with MAP sensors is NORMALLY a VERY GOOD INDICATION THAT THE TUNING IS INCORRECT, SO RECHECK YOUR AIR FUEL RATIO & IGN.TIMING VALUES at that point of HUNTING.
EVEN IF YOUR ARE RUNNING A BIG CAMSHAFT, YOU WILL BE ABLE TO TUNE OUT THE HUNTING.
THOSE OF YOU THAT HAVE EXTRA COMPONENTS PLUMBED IN WITH YOUR MAP HOSE BECAUSE YOU DONT HAVE ENOUGH VACUUM POINTS TO RUN EVERYTHING WITH OUT MULTIPLE HOOK UPS, FIT A RESTRICTOR TO THE MAP ONLY HOSE, TRY 0.8 mm to start and watch your vacuum values on the laptop to see if they have stablised, then retune and so on.
IF you have MAP hose on fuel press. reg hose, once again fit a restrictor and also a FUEL PRESSURE PULSATION DAMPENER.
SKYLINE OWNERS your cars have these standard, looks similar to a pressure reg. but has no vacuum hose port on it.
NORMALLY found on return line on the drivers side of the engine bay near the fuel filter.
WELL I HAVE A SORE FINGER NOW SO I'M FINISHING THIS OFF.
HOPE THIS INFO WAS OF HELP OR INTEREST TO YOUSE.
Edited by david kriedeman, 09 July 2006 - 01:51 PM.
