MAFs 101 - Modular Fords

  • MAFs 101

    First off you need to understand a mass air system. A mass air system measures the MASS of air flowing into the engine. There are several ways that this can be done, none of them are important for this discussion. Air-Fuel ratio is a ratio of the MASS of air the MASS of fuel. So if you are commanding a given air-fuel ratio and you know the mass of air coming into the engine, you know the mass of fuel that needs to be injected as well. If you know the total mass of fuel you want injected and the injector flow rate and the number of injectors, you then know how long to hold the injectors open for to get that desired amount of fuel.

    Notice how boost or location of the MAF and things like that didnıt come into that explanation? Because itıs necessary, thatıs why. Putting on a supercharger just increases the mass of air flowing into the engine. More boost normally means more air mass and hence it will calculate more fuel needed. (Here is a good place to mention that the reason for needing a "chip" on an ı03 Cobra, once youıve changed the pulley, relates to some logic that, above a certain air flow relative to a lookup table, the MAF is ignored. By changing this one value, this problem goes away and it works just like described above.)

    So, as long as your mass air meter does not saturate (meaning the output of the meter exceeds the 5 volts that the EEC will acknowledge), and the transfer function within the ECU is correct (meaning at that given voltage the air flow that it thinks it has at that voltage is correct) the ECU will automatically calculate the correct amount of fuel to inject and deliver that fuel. So, if you run more boost, the EEC will automatically put in more fuel at any given RPM based on the air mass increase. Now, if the air meter saturates, you have a huge problem. Then means the ECU no longer knows how much air is flowing into the engine so it no longer knows how much fuel to inject. In this situation, the engine then starts to drift lean; pretty much any KenneBell car with a stock MAF does this. Yes, many people will "rape" the calibration and do things like command a richer and richer air-fuel ratio to maintain a constant air-fuel ratio. This is a failure waiting to happen. If you set up the table to give you 12:1 air-fuel ratio with the MAF pegged, and now you get a day with better air, the engine WILL run leaner. Five percent better air is 5% leaner than when it was on the dyno. Same thing applies if you try to use the volumetric efficiency table when the MAF pegs. This specific table gets heavily modified based on a bunch of things, most of which no longer apply correctly due to the mods on the vehicle, and the motor will NOT be right as air density and air temp change.

    Let me explain boost. Boost is nothing more than the measure of backpressure in a system, not the measure of flow. Rather than get into a textbook explanation of this, Iıll give an example.

    You have a totally stock 4.6L motor. You add a Vortech Supercharger and power pipe and now it makes 10#ıs of boost with a 3.48 pulley. Well, you didnıt upgrade the fuel system correctly and now hurt the motor. While itıs out you decide to perform a few upgrades. You add a set of ported heads, better camshafts, ported Bullitt manifold, headers, all the good parts. But you decide to leave the 3.48" pulley on the blower. When you get the motor back together and run it, now you only have 7.5#ıs of boost instead of 10. Why? Iım sure most of you know why, itıs because by opening up the induction and exhaust system increased the flow through the engine so much that there is less restriction that the blower is pushing against, so it has less boost. But, it definitely makes more power, a lot more power. The blower doesnıt have to work as hard because there is less boost (since the power to spin any positive displacement pump is a function of the pressure drop from the inlet to the outlet), and at a constant compressor speed, the flow of the blower increases if you reduce the pressure drop across the blower. If you donıt believe me, check out a compressor map on Vortechıs site. So you have more airflow, more power, but less boost.

    Now, let me talk briefly (Ha!) about an internal combustion engines spark requirements. Volumetric efficiency is the measure of actual air flowing into the engine, divided by the theoretical air flowing into the engine. So on a 4.6L if, in two engine revs, you flowed 2.3Lıs of air, youıd have a voleff of 50% (2.3/4.6). If you had 6.9Lıs of air flowing into the engine (this means air is being forced in by some power adder) then your voleff would be 150% (6.9/4.6). At a constant voleff, as engine RPM goes up, ignition timing needs to go up (more advanced). This is, in part, to compensate for the faster spinning engine needing to start the combustion process sooner so that itıs at the "right" point when the piston starts traveling down. At a constant engine RPM, as voleff goes up, the spark requirements of an engine go down. This is because the higher cylinder pressure, from more air being stuffed into the cylinder, (due to higher voleff) will result in a faster combustion process, so you need less timing for complete combustion. The above example does NOT take into account spark limits for octane or air temp, it is basically the spark needed to make peak power.

    Ok, here is why a MAP sensor is actually bad to use for spark requirements. Remember the above example of the Vortechıd 4.6L? Well, boost went down when the motor was modded but airflow went up and it made more power. If airflow went up, then voleff had to go up. With a system that changes timing based on MAP, the above car would end up with more timing since there was less boost. Anyone that knows that knows anything about engines and tuning knows this would be bad. You just added a bunch of power, increased air flow, increased cylinder pressure, and your system just added more timing when it should have taken timing away. The PMS supporters will say that you just go in and change timing. Well, if the system is so good, then why do you have to adjust it for things like that? You donıt on a correctly setup mass air system.

    Now, with a mass air system, it calculates volumetric efficiency. If you know the air flow going into the motor, the engine RPM and the engine displacement, you can calculate voleff very, very accurately. The Y-axis on the spark tables in the Ford software is voleff. In the above example you would see a higher voleff. If you set up the spark table correctly, you will now go in there and look up a spark value with a higher voleff, which is set up for less spark at this higher voleff. No need for boost reference, itıs based on how much air is flowing into the motor at that RPM.

    Now a positive of the PMS is this; In the 1993 and older Mustangs, their spark at WOT is just a function of RPM, no input for voleff or airflow. Yes, if you run boost you can change this function to retard timing as RPM goes up but there is no compensation for anything other than RPM. The PMS will adjust spark, not ideally, for boost. In this dinosaur example, a boost sensor is better than nothing if spark if just a function RPM and nothing else. I personally donıt care what the shop manual says, they are wrong. The Cobra does NOT use itıs MAP sensor for anything fuel related other than compensating for quick, transient changes in manifold pressure. Beyond this itıs only used for EGR stuff and barometric pressure calculation (no the BP is not used in the fuel calculation). I doubt I cleared everything up, but I should have at least given you something to think about. .

    (Note: this article was written by SCT)
    Comments 1 Comment
    1. MF/Ken's Avatar
      MF/Ken -
      Just a quick FYI, there are a couple of images missing from this article. We'll be adding them in a couple of days, sorry for the inconvenience.
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