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Author Topic: maf explanation  (Read 2042 times)
wywindsor
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« on: December 29, 2009, 12:05:27 am »

Well not to beat a dead horse but i can't find the answer I'm looking for in layman's terms here.I read in other threads that a maf is not calibrated for injector size but to fool the ecm or programmable logic controller in instrumentation and eletrical work.As I read there is a heating element that is trying to maintain 100 degrees in the maf,this takes a certian amount of voltage and amperage to maintain this as air passes through the maf.Would this be a correct statement.I see in Joel's graphs as air cfm goes up so does the voltage this could keep  the amperage the same or less. linear or non linear scaling?And last but not least why can't a person cal his own maf or ecm. Will a pms tuner do this.



                                                          Thanks Ray
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Joel5.0
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« Reply #1 on: December 29, 2009, 08:37:10 am »

The technical explanation/description of the "hot wire" MAF is correct. As air cools the wire, its voltage draw is then used to calculate mass of air going through, which is then compared to a pre-programmed voltage transfer curve to convert into Kg/hr. IOW.... a voltage reading of 3.60 vdc could be programmed to be correlated to 598.5 kg/hr OR it could be programmed to mean 610.5 kg/hr.

MAF sensors are calibrated to a preprogrammed transfer curve in the ECM logic, what the aftermarket does (the MAF calibrated to xx lbs. injectors) is scale the voltage output through the graph to "misinform" the ECM and fool it into thinking less air is passing through to compensate for the bigger fuel availability. Leave transfer curve program constant, vary the MAF output approach. There are other MAF additions that allow the MAF output to be calibrated to match the ECM transfer curve based on after the fact fuel control results (wideband). Examples are the MAFia, URD, etc.

A Tweecer or Moates QH does allow you to reprogram the whole MAF transfer curve based on the MAF output........ leave the MAF output constant, adjust the MAF transfer curve approach. This one, is considered the best and most accurate way to set A/F control.

MAF calibration = match kg/hr to specific voltage output based on a fix transfer curve.

MAF transfer curve recal = match MAF output to required kg/hr values + manipulation of the the adder tables and functions in the logic.

Hope this helps.
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wywindsor
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« Reply #2 on: December 30, 2009, 12:32:57 am »

Thanks Joel

   Still have a few questions

   Are there variable resistance pots in the maf they use to cal aftermarket mafs ?

   Why would showing less kg/hr , cfm or voltage reading to the ecm,make the ecm compensate for more fuel. I would think by the ecm seeing more air or voltage passing through it would tell the injector to send more fuel.

I would also think that the only reason the aftermarket would cal to an injector size was because the customer sized his injector for his horse power?

This would be difficult not knowing the cfm requirments of your engine. Wouldn't V.E. come into play as well as boosted engines need different cals.
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jon_daniels
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« Reply #3 on: December 30, 2009, 04:38:13 am »

  Why would showing less kg/hr , cfm or voltage reading to the ecm,make the ecm compensate for more fuel. I would think by the ecm seeing more air or voltage passing through it would tell the injector to send more fuel.

right. if the MAF "sees" more air it will keep the injectors open longer... great, if you are using the injectors that the ECM was programmed for. however, with larger injectors you do not want them to stay open longer, rather, you want them to be open less time so that they do not create a rich condition. if a 36 lb/hr injector is opened as long as a 19 lb/hr it will dump much much more fuel in.

after all, the purpose of larger injectors is to reduce the amount of time the injectors are open, ie shortens the injector pulsewidth, and also to allow enough fuel to go through IF the MAF sensor voltage represents an airflow near the maximum end of the transfer curve. as far as i know the aftermarket scaling of the MAF voltage output is not linear. near idle and at part throttle the calibration attempts to have the fuel output of the larger injectors nearly mimic the stock injector output (since the o2 sensor will dictate fuel enrichment in these modes) and at higher airflows it will show a voltage output that results in the injectors supplying a bit more fuel than stock.
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Jon - 89 GT 3.73
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« Reply #4 on: December 30, 2009, 09:23:27 am »

Thanks Joel

   Still have a few questions

   Are there variable resistance pots in the maf they use to cal aftermarket mafs ?

   Why would showing less kg/hr , cfm or voltage reading to the ecm,make the ecm compensate for more fuel. I would think by the ecm seeing more air or voltage passing through it would tell the injector to send more fuel.

I would also think that the only reason the aftermarket would cal to an injector size was because the customer sized his injector for his horse power?

This would be difficult not knowing the cfm requirments of your engine. Wouldn't V.E. come into play as well as boosted engines need different cals.



   I think you have it a little back wards. Think of it like this. Lets say we have a maxed out stock setup. Lets assume we are at 5v on the MAF and 100% duty cycle on the injectors.

  Now we replace them both with a 38lb marched setup MAF and injectors.

  The new MAF will be able to meter about twice as much air as the stocker. Remember same engine same air flow just two different meters.

  Stock meter reads up to about 900 KG/HR at 5V. New meter reads to about 1800 KG/HR.
  Stock meter was maxed out at 5v so we can assume that is 900 KG/HR.

  So what is going to be the new meters voltage output at 900 KG/HR?? It is going to be about 3.8v. Now look up 3.8v on the stock MAF curve in the stock ecu. it is going to be about 450 KG/HR.

  So now the ecu "thinks" half the amount of air is entering the engine. And will command half the amount of fuel from the injectors. But with the 38lb injectors it really isn't. It's the same as be for with the stock 19lbers.
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wywindsor
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« Reply #5 on: December 31, 2009, 12:10:09 am »

Thanks Bender460

This makes sense ,but when they cal a maf to  injector lbs this is what i would call over band  calibration say 0 kg/hr = 0volts and 1800 kg/hr = 5volts. 1800/ 2.08 =865cfm. 2.08 being the divider at sea level 59 degrees at 29.92. for cfm.A 347 ci @ 7000 rpm / 2/1728 =703 cfm.that would be if this engine was 100% v.e. or better.And we used injectors that support 39lbx8cyl=312lb x.80 =250lb per hr. @.50bsfc this should support 500hp.This is all hypothetical but 865 cfm - 703 cfm is 162 cfm off.So I believe that when Joel stated it is better to map the curve in the ecm you would be better off.


 I only came up with this so we could control in a proper band width.Not over or under what is needed.

So does anyone know what they do when they recal a maf.I have heard they change the port size to the sensor.Or do they change the sensor resistance.Knowing this would help me create a way not have to send a maf in all the time for calibration.


                                                                               Thanks Ray

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jon_daniels
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« Reply #6 on: December 31, 2009, 01:24:03 am »

So does anyone know what they do when they recal a maf.I have heard they change the port size to the sensor.Or do they change the sensor resistance.Knowing this would help me create a way not have to send a maf in all the time for calibration.
the easiest way to keep from having to send your MAF in for recalibration is to adjust the transfer curve itself. else, C&L sells meters that utilize the stock electronics with varying sample tube sizes depending on what injectors you use. these tubes are available separately.


on edit:
This makes sense ,but when they cal a maf to  injector lbs this is what i would call over band  calibration say 0 kg/hr = 0volts and 1800 kg/hr = 5volts. 1800/ 2.08 =865cfm. 2.08 being the divider at sea level 59 degrees at 29.92. for cfm.A 347 ci @ 7000 rpm / 2/1728 =703 cfm.that would be if this engine was 100% v.e. or better.And we used injectors that support 39lbx8cyl=312lb x.80 =250lb per hr. @.50bsfc this should support 500hp.This is all hypothetical but 865 cfm - 703 cfm is 162 cfm off.So I believe that when Joel stated it is better to map the curve in the ecm you would be better off.


 I only came up with this so we could control in a proper band width.Not over or under what is needed.
maybe i'm not fully understanding what you're trying to do. i believe what you're saying is that you want to fine tune the MAF output so that at 5 volts your engine is actually ingesting the maximum amount of air possible; this is so that the airflow representation throughout the MAF voltage output range can be more accurately represented. is this correct? wouldn't you want to include some margin of error? at any rate it would surely be more viable to do this by adjusting the maf transfer function in the tune. no resistor is going to return the desired results across the entire range.
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Jon - 89 GT 3.73
wywindsor
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« Reply #7 on: January 01, 2010, 12:09:09 am »

 Let's say a 347 at 7000 rpm ingest 703 cfm.
 A 460 at 6000 rpm ingest  798 cfm

Both producing 500 hp
 

This should use the same injector, why would the maf be calibrated the same?

This is just a theory I have.I'm an  electrician and learning instrumentation. We don't use 0 to 300 inch scaling to control something that works 0 to 80 inch we would use 0 to 100 inch.This gives us a little extra but alot better control.I was not looking to set resistance at one point, but to fine tune for a particular application without having to change a scale in the ecm.We may call this poor boying but if it works what the hell.     





                               thanks Ray


                                                       
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bender460
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« Reply #8 on: January 01, 2010, 12:58:10 am »

    Ok......Calibrated....How are you using that word?

  IMO the word calibrated is ONLY used when you need to trick the ecu in sending a lower pulse width to BIGGER than stock injectors. This is only done when no tuning is involved. But sending it in all the time to be recalibrated?

   Think about this for a minute. Why not change the housing size using the same sensor?


   I understand your theory. I wish there were more than 30 points in a transfer curve. But depending on application one can use those 30 points to there benefit.


 



 
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92lx
Stock block Flat top  347
TFS highprts, CI TLSR, Probe shafts 1.6
super vic, 39lb inj 
Wilson 4500 TB
Ultimate 4800rpm 8" converter
Full Team Z front end, UPR rear
11.19 at 120......D.A. 5500'
jon_daniels
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« Reply #9 on: January 01, 2010, 02:23:26 am »


This is just a theory I have.I'm an  electrician and learning instrumentation. We don't use 0 to 300 inch scaling to control something that works 0 to 80 inch we would use 0 to 100 inch.This gives us a little extra but alot better control.I was not looking to set resistance at one point, but to fine tune for a particular application without having to change a scale in the ecm.We may call this poor boying but if it works what the hell.                  
ok, yeah. that's what i thought you were getting at. i'm not an electrician or an electrical engineer, but i can't see how it would be possible to change the resistance independently at multiple points. there is after all only one output wire coming from the sensor.

you know, a basic moates tuning setup would cost you under $200 new and i'll bet you could find one used for cheap since they just came out with the new moates quarter horse. either way you go you would need a wideband setup as well to fine tune.
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Jon - 89 GT 3.73
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