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Practical Reflash Tuning: MAF-Based ECUs

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MAF-Based ECUs

04.50

00:00 - By far the most common way of measuring air flow into the engine when it comes to OE ECUs is with a mass air flow sensor or MAF sensor for short.
00:12 In many ways this is the perfect option since as its name implies, it can directly measure the mass of air entering the engine.
00:21 This is important because when the ECU is calculating how much fuel to supply, it needs to know the mass of air entering the cylinder.
00:31 When we're talking about air/fuel ratio, we're discussing a mass ratio.
00:37 For example, an air/fuel ratio of 12.5/1 means we're supplying 1 kilogram of fuel for every 12.5 kilograms of air entering the engine.
00:49 The measurement units aren't important as we could equally use units of grams or pounds, but the important aspect is the fact that the ECU needs to know the air mass.
01:02 Now if the ECU knows what mass of air is entering the engine, it becomes very easy for it to accurately calculate the required injector pulse width in order to achieve a specific air/fuel ratio.
01:17 In order for the ECU to make these calculations though, it must also know what the injector flow is, or to put it another way, how much fuel will be supplied for a given injector pulse width, and then of course what our desired air/fuel ratio is.
01:37 For this reason you'll typically find a target air/fuel ratio map in this sort of ECU, along with some scaling data that defines the size of the injectors.
01:50 The accuracy of this sort of ECU's fuel control relies on both the MAF sensor calibration and the injector scaling information to be accurate.
02:01 While this should be the case for a stock vehicle, consideration needs to be given to both of these if the engine is being modified.
02:10 While the MAF sensor in many ways is the perfect option for accurate mass air flow measurement, there are also some downsides.
02:19 First up is the measurement range of the MAF sensor.
02:23 A MAF sensor will be chosen by the manufacturer to suit the expected airflow for a particular engine, and there may not be a huge amount of headroom left when we start modifying the engine and increasing the airflow.
02:38 If we exceed the measurement range of the MAF sensor, its output will flat line and the engine will be receiving more air than the ECU thinks.
02:48 This makes it impossible for the ECU to properly control the fuel and ignition.
02:54 Of course it's always possible to fit a larger MAF sensor, or, alternatively fit the existing MAF sensor into a larger housing to increase it's scale, but these options will also affect our tuning.
03:08 A larger MAF sensor can also reduce resolution at low load, and this in turn can affect the tuning in these areas.
03:18 In older vehicles, the MAF sensor could actually present a reasonable restriction to airflow, and hence reduce engine power.
03:28 The MAF sensor needs to physically sit in the airflow, and some are quite bulky in their design, so it's easy to understand that they can reduce airflow.
03:39 For example, the MAF sensor fitted to a GM LS1 has proven to strangle the intake airflow slightly, and removing it can offer up to an additional 10 kilowatts or more, at the rear wheels.
03:53 On the later LS2 and LS3 however, the MAF sensor was larger and much less restrictive.
04:00 Possibly one of the largest drawbacks with a MAF sensor is that it can be upset by reversion air flow.
04:08 What I mean here is that it doesn't matter which way the air is flowing through the MAF sensor, the air mass will still be measured.
04:16 This becomes an issue with large cams with a lot of overlap, where we can potentially see air reversion occurring at idle, and essentially the same air can be measured multiple times as it moves through the MAF sensor and is then pulsed back again.
04:33 This can make it difficult or impossible to maintain accurate control of the tuning at idle for example.