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Work in progress: Air Metering Simplified

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  • Work in progress: Air Metering Simplified

    I am going to elaborate on this basis for an addition here. I typed it up during a lab section I had to be in, but was already done with. Maybe we can get a good "FAQ you" type thing going or something?

    ----------------------------------------------------------

    Most MAF's display their airflow as simple voltage curve. The voltage starts out low and gets higher as airflow increases. This voltage change is not at a linear rate to the change in airflow.

    Most MAF's have voltage output that can reach 6.5volts DC or even higher with enough airflow. The AFM also needs a reasonable minimum voltage to meter air at idle. Our ECU can read a maximum of 5.12volts DC, which depending on the airflow metering device used could be moving 300CFM or 1000+CFM of air.

    The translation a stock ECU (or any ECU/PCM really) uses is referred to as a Transfer Function or a VQ Map. This is simply a lookup table that the ECU uses to determine how much air is being pulled through the MAF based on the voltage it receives from the MAF. There are additional tables on top of this for things like cold start, etc. The K value is stacked on to adjust for injector size, the dwell (void) time is also added to account for the time it takes the injectors to open. All of this is used to determine injector pulse-width (how much fuel is injected). This is a well-coordinated system capable of doing hundreds of calculation in a second. The system works so fast, it can almost act as if the engine were sitting still.

    Finding a MAF capable of metering the amount of air your engine is capable of moving can be hard. There are many options available to both upgrade the MAF (or upgrade away from MAF entirely), and extend the metering of a MAF you have, or both.

    An engine does not 'care' what kind of engine management system it is running. If required timing and fuel requirements are met and all other variables are exactly equal, the engine will perform almost exactly the same from system to system. The only difference is going to be inlet restriction based on the maf used, and depending on where it is located. Generally speaking, MAF is superior to Speed-Density (SD) when performance modifications are considered on factory cars. To put it simply SD has a "preconceived notion" of how much air the engine is getting based on its factory volumetric efficiency. Do anything to change that (exhaust, cams, etc) and you can throw your whole system off and lose power, or not gain any power at all. From an aftermarket standpoint, speed-density can be tailored specifically to the modified engine the system is being run on and does not have the drawback of having to have a sometimes bulky sensor in the airstream. For any typical aftermarket system to even match the performance across the board compared to a factory system takes hours and hours of time to develop a tune that works in all conditions (something the factory did for you already, but obviously they designed it for a stock engine).

    A MAF, MAS, and AFM are not always the same thing. A MAF (Mass Air Flow) meter is the same thing as a MAS (Mass Air Sensor) but not the same thing as an AFM (Air Flow Meter). Though these terms are used interchangeably you should know they are in fact not the same.

    Methods for getting the most out of your air metering:


    1. Voltage Drop - commonly used to obtain a lower MAF voltage read by the ECU. The voltage of the MAF must extend beyond the measuring range of the ECU (most do) for this to work. You can use a Potentiometer to adjust the voltage (with one terminal going to ground, one to the maf and one to the ECU to "ground out" some of the signal voltage), and a modified VQmap or modified fuel map to account for the difference.
    2. Air Bypass
    - Simple and probably the most effective means. Ashz's DoolZ and the JWT dual-pop both work on the same principal. Not all air is metered, but the difference is accounted for in the VQmap or fuel map. It can be a pain to tune correctly, but once established it is a good method to both releve the restriction of a maf, and the limitations of its metering ability.
    3. Larger MAF sample tube – Cut apart the MAF and put it in a bigger housing. This reduces the airflow past the sensor itself by the amount equal to the increase in cross-sectional area of the tube.
    3. Upgraded MAF From another car- Easy to do if you have either a VQmap for the new MAF, or a way to make one.
    4. Custom made MAF - A few shops exist that custom make MAF's for a given application. Fine adjustments to a MAF may allow it to work as a blow-through (where the maf could be located by the throttle body in a pressurized system), be less restrictive, and offer greater metering ability. The recently defunct PRO-M was one of the most popular shops for making this type of custom MAF.
    5. AFC or similar device
    - Plug and play voltage trimming, very user-friendly, but more or less a band-aid fix for making a new VQmap.
    6. MAF Translator
    - Allows use of a GM MAF, which outputs a square-wave frequency instead of a voltage signal. The translator can be set to scale the frequency and output many different voltage curves for your ECU to read. Hence it "translates the airflow" from the seemingly limitless GM sensors into a format your ECU can read.
    7. MAP ECU or similar
    - Uses a Manifold Absolute Pressure (MAP) sensor, engine RPM and air temperature to figure the amount of air the engine is moving, effectively converting your MAF system to speed-density. The MAP ECU feeds your ECU the correct voltage based on internal functions that you can program.

  • #2
    great read!!

    i want more!!! :P

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    • #3
      1. Voltage Drop - commonly used to obtain a lower MAF voltage read by the ECU. The voltage of the MAF must extend beyond the measuring range of the ECU (most do) for this to work. You can use a Potentiometer to adjust the voltage (with one terminal going to ground, one to the maf and one to the ECU to "ground out" some of the signal voltage), and a modified VQmap or modified fuel map to account for the difference.
      I've never torn apart a stock z31 maf. But I wonder if it would be worth while to have a look at the pcb. Maybe a simple method to scale down the output voltage internally. That would eliminate the possibility of a pot getting corroded or bumped off its setting.
      http://z31performance.com/forum/viewtopic.php?f=5&t=147

      Comment


      • #4
        I have one to tear apart for testing purposes on tehpopas car. I don't know if it will be able to meter the amount of air his new engine will move even with like a 4" sample tube, but we'll see.

        My car has been put on the back burner, so I don't reall feel like messing around with it. I plan to install the new clutch, tune it a little better to break the 450rwhp bearrier on the dyno so it should have a usable 4-7K powerband and then leave it be to focus on his car and my 87T. It would be nice if he could do SOME of the work himself, but at least he's paying me a little for all my time.

        Comment


        • #5
          It would be awesome to be able to use a larger tube with stock electronics. Would reduce the amount of pcb modifications. Not that you arent already going beyond hobbyist to socket an ecu.

          Or treat it like the ford guys do: big tube and larger injectors accordingly. ECU thinks all is well and nothing has changed. I have mixed feelings on that method but it works.
          http://z31performance.com/forum/viewtopic.php?f=5&t=147

          Comment


          • #6
            Well, like I said (and meant to elaborate on). a larger sample tube is not going to make all of the 'adjustment' needed for larger injectors or higher fuel pressure because the voltage does not correspond to airflow in a linear fashion. If it did, then you could just bypass the correct percentage of air (or in this case increase sample tube size) by the same amount as the increase in fuel injector flow rate. Unfortunately you can't; it would run like ass. It requires more tuning (fuel map or VQmap changes) or a trim of the MAF element itself. This was part of the point I was trying to make, if it makes any sense?

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            • #7
              voltage does not correspond to airflow in a linear fashion
              I wasnt paying attention there. Shame it isnt linear. All electronics should be linear.
              http://z31performance.com/forum/viewtopic.php?f=5&t=147

              Comment


              • #8
                reducing the output voltage of the afm is the most economical way, say someone were to spend the time with a stock car using romulator with a stock program, reduce the afm voltage and make a new vqmap...

                this should be something we could all use, a known drop with a known map, we could get 400hp out of the stock afm this way

                Comment


                • #9
                  G-E wrote: reducing the output voltage of the afm is the most economical way, say someone were to spend the time with a stock car using romulator with a stock program, reduce the afm voltage and make a new vqmap...

                  this should be something we could all use, a known drop with a known map, we could get 400hp out of the stock afm this way
                  Bernard's bin's on my website use a voltage drop with the stock maf; he brings it to around 1.0v (from the maf, not at the ECU) at idle and claims it extends the metering to meet that of the cobra maf. I have not tried it, but I see no reason to doubt his claim. The only problem with the voltage drop is that you lose some of the resolution that you had when a stock voltage curve was being used. It slides the while curve "down" in voltage. Big change in airflow = very very small change in voltage.

                  Comment


                  • #10
                    Since you guys with turbos are worried about pushing too much air through the MAF and running out of metering ability, I want to get more air through. I read a thread somewhere about removing the wire mesh at the ends of the MAF tube and increasing the flow by about 30%. In discussing this with a Mustang tuner he suggested that the screen on the front end might help in smoothing out the flow as it came from the filter. If that is the case you might still get a 10% increase in flow by removing the back end.
                    What things would have to be adjusted to utilize the added flow?
                    ...and how much HP do you have at 0 boost

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                    • #11
                      /me leaves this one alone. Flaming not needed here.
                      http://z31performance.com/forum/viewtopic.php?f=5&t=147

                      Comment


                      • #12
                        Russ84na wrote: Since you guys with turbos are worried about pushing too much air through the MAF and running out of metering ability, I want to get more air through. I read a thread somewhere about removing the wire mesh at the ends of the MAF tube and increasing the flow by about 30%. In discussing this with a Mustang tuner he suggested that the screen on the front end might help in smoothing out the flow as it came from the filter. If that is the case you might still get a 10% increase in flow by removing the back end.
                        What things would have to be adjusted to utilize the added flow?

                        Well, the changes it makes are interesting actually. I know MKIV supra owners commonly do this and claim it does make a HP difference. I never noticed a difference myself, but 5hp on a ~330hp (at the time) car is hard to notice.

                        If the airstream is turbulent to begin with, the screens do "straighten" the air through the sensor and give it a more accurate reading. As long as you don't have the filter right on the maf (IE there is a good 6" or so of piping before the maf) its metering accuracy and voltage/airflow should not change substantially. That is, you should not have to retune the fuel maps much if at all
                        after removing the screens. I removed the screens on a stock maf and ran it for a few weeks before I switched to the cobra maf I believe I noted a very very small AFR change under boost but it's hard to remember back that far.

                        At any rate, it will absolutely flow better with the screens removed, but how much better is the question. I'd wager it makes a 'substantial' difference, but you would have to put it on a flow bench to find out.

                        Give it a shot and let us know! Dyno results would be very interesting to see as well.

                        Comment


                        • #13
                          I have a question

                          what if instead of reducing the afm metered voltage output, what about dropping the voltage into the afm? like when you run a 12v fan at 9v it spins slower....

                          could we keep the existing "linearity" with a lower voltage output without actually impeding the signal ?

                          I'm not an EE so this is only a guess....

                          Comment


                          • #14
                            I don't think the sensors are designed to run at a lower voltage, but I'm not really sure at all about altering the input voltage. I've never seen it done, so there has to be SOME reason why.

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                            • #15
                              What I would suspect there is it would not regulate properly at lower voltages. Most all voltage regulators are a pulse width type. At lower/higher voltages the ramp speed of the transistor is different and they are less accurate/reliable. If a component is using a proper regulated power supply then it will try to adapt for lowered voltage by increasing current to maintain its set output voltage.
                              http://z31performance.com/forum/viewtopic.php?f=5&t=147

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