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# EFI Tuning Fundamentals: The Speed Density Principle

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## The Speed Density Principle

### 02.10

 00:00 - When it comes to supplying the correct amount of fuel, the ECU needs to first know what mass of air is entering the engine. 00:07 We've discussed the common techniques the ECU uses for measuring airflow and these can be broken down into measuring the airflow directly by way of the mass airflow sensor or calculating airflow using the speed density principle. 00:19 If the ECU is measuring mass airflow directly, as is the case with the majority of factory ECUs, then the process of calculating the required injector pulse width to achieve a certain air/fuel ratio target is quite simple. 00:33 The ECU knows the mass of air entering the engine, it knows what mass of fuel the injectors can supply for a given pulse width and it knows the target air/fuel ratio so it becomes an easy job to calculate how long to open the injectors for in order to achieve that target. 00:48 The process the ECU goes through is a little more complicated when the ECU is calculating airflow using the speed density principle though and in order to get a really solid understanding of what the ECU is doing, we're going to work through an example of how the ECU calculates the required injector pulse width in order to achieve a specific target air/fuel ratio. 01:10 This section is going to require a little basic maths but I don't want to scare you off. 01:15 Firstly, this isn't a process you're going to need to go through every time you're on the dyno and you definitely don't need to be a maths whizz in order to tune an engine. 01:25 Understanding what's going on inside the ECU however makes the difference between an average tuner and an excellent tuner and there are some really important points to take away from this process. 01:37 We can break it up into 4 parts, first we need to calculate the mass of air entering the engine. 01:43 Now this is going to be straightforward as we've already done this earlier in the course. 01:48 The next step is to calculate the required fuel mass in order to achieve our desired air/fuel ratio. 01:54 Then we need to calculate the available cycle time and finally we can calculate the injector pulse width needed to deliver this amount of fuel.