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Practical Reflash Tuning: Speed Density-Based ECUs

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Speed Density-Based ECUs


00:00 - We've now discussed MAF Sensors, which directly measure mass airflow, but another option is for the ECU to calculate the mass airflow using a Manifold Absolute Pressure Sensor or MAP Sensor for short.
00:14 The MAP Sensor measures the air pressure in the intake plenum, and the ECU can then calculate the mass airflow by using the ideal gas law.
00:25 This is often referred to as the speed density principle.
00:29 We don't need to understand this calculation in order to reflash the ECU, and it's enough to simply understand that the key difference between a MAF Sensor and a MAP Sensor is that one measures mass airflow directly, while the other calculates it.
00:47 The speed density principle is the predominate operating principle in the world of aftermarket ECUs, however in O.E applications, it's actually quite unpopular.
00:59 This mainly comes down to the requirement for the O.E engineers to very accurately control the air fuel ratio in order to ensure emissions compliance.
01:10 If the ECU is directly measuring mass airflow, there's less chance of error creeping in than if the mass airflow is calculated.
01:19 This can often result in finer control.
01:23 It's not all bad news when it comes to MAP Sensors though, and despite the MAF Sensors offering some slight accuracy advantages, the reality is that a speed density based ECU can provide excellent control of fuel delivery and can offer some significant advantages over a MAF Sensor, so let's discuss those now.
01:46 You'll remember that one of the limitations of the MAF Sensor was the measurement range.
01:52 MAP Sensors also have a defined measurement range, however, it's very easy to select a MAP Sensor to suit the pressure range your engine needs to work across.
02:03 MAP Sensors are available in a wide variety of measurement ranges, including 1 bar, 2 bar, 3 bar, and 5 bar for example.
02:12 All we need to do is select a MAP Sensor that's capable of reading a little higher than our expected working pressure range.
02:21 When selecting a MAP Sensor though, we do need to remember that MAP stands for Manifold Absolute Pressure, and that's what the sensor reads.
02:30 Absolute pressure, not gauge.
02:33 This means that if we want to be able to measure 1.5 bar of boost pressure, we would actually need a 3 bar MAP Sensor since we're wanting to measure 1.5 bar above atmospheric pressure, which is 2.5 bar absolute pressure.
02:52 Unlike a MAF sensor calibration, that's quite complex and will be dependent on the entire intake tract, a MAP sensor calibration is very straightforward and is very simple to swap from one sensor to another without affecting your tune.
03:08 A MAP Sensor is not affected by revision pulses like a MAF Sensor, and this can make the MAP Sensor a better option in engines fitted with large cams.
03:18 Lastly, a MAP Sensor needs to be able to measure the air pressure in the intake plenum and will normally be either directly fitted to the plenum or connected to the plenum via a length of vacuum hose.
03:33 This means that the MAP Sensor offers zero restriction to intake airflow ensuring maximum engine performance, as well as freeing up your options for intake plumbing.