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EFI Tuning Fundamentals: Load Calculation

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Load Calculation

09.01

00:00 - Once the ECU knows the engine speed and engine position, the next input it needs is engine load.
00:06 This input tells the ECU what mass of air is entering the engine, which is essential to supplying the correct amount of fuel and ignition advance.
00:14 There are several ways of providing the ECU with the load signal, and this will depend to a degree on what type of ECU you're tuning.
00:22 By far the most common method of measuring load in OEM installations is with a mass airflow sensor or MAF sensor for short.
00:30 If you're reflashing a factory ECU, then this is the type of load sensor you're most often going to be using.
00:36 There are a variety of different styles when it comes to these sensors but the hot wire MAF sensors are what you're most likely to find measuring airflow into a modern engine.
00:46 This type of sensor uses a wire that's placed in the airflow which is heated by passing a current through it.
00:53 As air flows across the wire, the wire is cooled so more current is needed to maintain the wire at a specific temperature.
01:01 The amount of current required is therefore relative to the amount of air flowing through the sensor.
01:06 This type of sensor accounts for air temperature with a built in air temperature sensor so it's actually measuring the mass of air entering the engine.
01:14 The ECU will contain a calibration table that converts the output from the MAF sensor into mass airflow that the ECU can then use for its calculations.
01:24 These calibration or scaling tables, are often referred to as a MAF translation table.
01:30 Since air mass is what the ECU needs to know in order to provide the correct amount of fuel for a specific air fuel ratio target, a correctly calibrated mass airflow sensor can allow the ECU to very accurately control the fuel delivery.
01:45 While the MAF sensor is great for factory engineers, due to it accurately providing a direct measure of mass airflow, it can present a few headaches to aftermarket tuners.
01:54 Firstly, a MAF sensor can be upset by the air reversion effect that you can see when fitting a large camshaft with a lot of overlap.
02:02 The MAF sensor isn't particularly worried about which way air is moving past the measuring element as it will still measure the flow.
02:09 This means that if idle, where we can have a lot of reversion occurring, the MAF sensor can end up measuring the same air multiple times.
02:17 Once when it comes through the mass airflow sensor, and a second time when the reversion pulse moves back out through the MAF sensor.
02:25 This can make it very difficult to achieve a stable idle mixture.
02:29 We can get the same problems in a turbocharged engines when the throttle is closed quickly from high RPM and high boost.
02:36 If there is no blowoff valve fitted, or the valve is too small to cope, quite often the result can be a reversion pulse as the air backs up against the throttle plate and then makes its way back through the intake system and out through the compressor wheel.
02:50 This can have the same effect on the MAF and can result in stalling, and poor running or rich mixtures when the throttle is closed rapidly.
02:58 The other turbo-related issue that is common with a MAF sensor occurs when an atmospheric venting blowoff valve is fitted.
03:06 In this instance, all of the air entering the engine has already passed through the MAF sensor and has been measured.
03:12 When the throttle is closed rapidly, the blowoff valve opens and vents excess air pressure to atmosphere.
03:18 The result of this is that the ECU still injects the fuel required to suit the amount of air measured by the MAF sensor, but now, some of that air is not making it into the engine.
03:29 This causes the engine to run rich momentarily.
03:33 MAF sensors can also create some problems when we start modifying the engine and making more power than the factory intended.
03:41 MAF sensors are designed to measure a specific amount of airflow based on the amount of power that the stock engine is expected to produce.
03:49 This can become a problem if we start flowing more air than the sensor can actually measure.
03:54 In this case, the output of the sensor will flatline, making it impossible to accurately tune the engine.
04:00 There are a few solutions to this problem, including fitting a MAF sensor with a different scaling or fitting the MAF sensor element into a larger diameter MAF housing.
04:10 Both of these solutions will require the MAF output to be rescaled so that the sensor correctly represents the actual airflow.
04:19 The downside to fitting a larger MAF sensor is that we can end up losing some turning resolution at low airflow such as the idle area of the mapping.
04:27 In extreme cases, this can make it very difficult or impossible to achieve a stable idle quality.
04:34 In order to provide an accurate measurement, the MAF sensor must be located so that all the air entering the engine passes through it.
04:41 This means that it must be located somewhere in the intake track, between the air filter and the throttle body.
04:47 In a cramped, modern engine bay it can be a challenge to actually find sufficient room, and particularly with some engine swaps, it can almost be impossible.
04:57 Before we move on, it's important to understand that the MAF sensor's accuracy can be affected as we start modifying the engine, particularly anything in the inlet tract.
05:07 The MAF sensor is normally calibrated by the factory engineers along with all of the intake plumbing, the air filter, and even the air box.
05:15 This means that anytime we make changes to the intake plumbing, fit a freer flowing air filter, or a new airbox, the accuracy of the MAF sensor calibration may be affected and this may require the calibration to be adjusted.
05:29 With aftermarket ECUs, we're much more likely to use a MAP sensor as load signal instead of a MAF sensor.
05:36 MAP stands for manifold absolute pressure and it measures the air pressure in the intake manifold as opposed to the airflow coming into the manifold.
05:45 From this pressure signal, the ECU can then calculate the airflow rather than directly measuring it.
05:51 The MAP sensor is the backbone of the speed density operating principle which we'll investigate in more detail shortly.
05:59 While the speed density principle is by far the most common choice in standalone ECUs, some factory ECUs also work on this principle.
06:07 With many of the reflashing packages that are popular now, you may also have the opportunity to modify a factory ECU that originally used a MAF sensor, and instead use a MAP sensor or a hybrid where both the MAF and MAP sensors are utilised.
06:22 MAP sensors come in a wide variety of types and designs including sensors that mount directly on the intake manifold or sensors that can be mounted remotely in the engine bay and connected to the intake manifold via a length of vacuum hose.
06:37 The MAP sensor works by outputting a zero to five volt signal in relation to the vacuum or pressure it's subjected to.
06:44 There are a variety of advantages to using a MAP sensor in place of a MAF sensor, so let's take a look.
06:51 Firstly, since the MAP sensor doesn't mount in the path of the intake airflow, it provides no restriction to airflow.
06:58 In some instances with a MAF sensor, these will provide some level of restriction which can negatively affect the power produced.
07:05 Second, with the small physical size of the MAP sensor it's easy to mount in a tightly packed modern engine bay.
07:12 Thirdly, MAP sensors are available in a range of different outputs to suit your intended pressure range, so matching the MAP sensor to the level of boost pressure you're expecting to use is very easy.
07:23 Lastly, MAP sensors are not affected by reversion pulsing from large camshafts or turbochargers, and they're compatible with atmospheric venting blowoff valves.
07:33 One downside with using a MAP sensor is that the speed density calculation assumes that the relationship between airflow and pressure remains constant.
07:42 What this means is that if an engine is correctly tuned using a MAP sensor and then some mechanical aspect of the engine is changed resulting in increased airflow there's no way for the ECU to know this and the result could be an inaccurate air fuel ratio.
07:57 In comparison, the MAF sensor is able to directly measure the increased airflow and could account for this to some degree by adjusting the fuel delivery as required.
08:07 This isn't typically a problem, but what it does mean is that if you make any changes to an engine tuned via a MAP sensor, it's important to confirm or adjust the tune as required.
08:18 In summary, there are a few different methods of providing the ECU with a load signal, most commonly in factory vehicles this will be via a mass airflow sensor which directly measures airflow.
08:30 These are well suited for their purpose and will often be what you're working with when reflashing a stock ECU but they do have some limitations once you start heavily modifying your engine.
08:40 This is where a MAP sensor can be an advantage which is what you'll usually come across when you start working with aftermarket ECUs.