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Practical Reflash Tuning: Fuel Delivery

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Fuel Delivery

06.28

00:00 - Tuning the fuel delivery in an OE ECU in many ways is very simple in comparison to a stand-alone ECU, but it does require the correct approach to get the right results.
00:13 The approach we need to take will depend on exactly how modified the particular engine is but in many cases it's actually much simpler and faster than tuning a standalone ECU.
00:26 The key is understanding that the ECU is most frequently going to be directly measuring mass airflow via a MAF sensor.
00:36 Provided the MAF sensor calibration and the injector data in the ECU are accurate, then our job of adjusting the fuel delivery is as simple as asking for a new target air/fuel ratio.
00:50 Of course if you've changed the MAF sensor or injectors, then these will need to be dealt with prior to tuning the fuel, and we'll deal with both aspects shortly in separate modules.
01:03 The other aspect that makes our job incredibly simple when it comes to adjusting the fuel delivery is that the ECU will spend the majority of its life operating in closed loop mode, where it's targeting a stoichiometric air/fuel ratio, and the ECU is trimming the fuel delivery to maintain this.
01:22 Now we can't quite ignore this area of the engine's operation as if we find the fuel trims are excessive then it would indicate that either the MAF calibration or perhaps the injector calibration is inaccurate.
01:36 While in a perfect world we would like to have the short term and long term fuel trim sitting at zero, this isn't realistic and even a 100% stock factory car will always see some minor trims present.
01:53 As a general guide I like to see the sum of the fuel trims at any point sit between plus and minus 5%.
02:01 Obviousy the closer the better, but if we're within 5%, then the ECU is doing its job well.
02:08 If we're seeing trims exceeding this range, then before we move on to the open loop tuning, we'd need to address the MAF scaling, or perhaps injector scaling if the injectors have been swapped.
02:21 As we'll find out shortly, it's not uncommon for what may seem to be minor changes to the intake system to end up having some large impact on the accuracy of the MAF scaling.
02:34 Once we're comfortable that the closed loop area of operation is under control, and the fuel trims are deemed to be acceptable, we can move on to the open loop tuning.
02:45 The approach will depend a little on the particular ECU tuning, and whether the engine is naturally aspirated or forced induction.
02:54 In the simplest case, which is demonstrated by some GM ECUs, notably on the LS V8 series, the open loop air/fuel ratio is defined by a single two dimensional table called a power enrichment table.
03:09 This table defines target air/fuel ratio versus engine RPM anytime the engine is operating in open loop mode.
03:18 This table works in conjunction with a separate table that defines at what throttle position the ECU will transition from closed loop to open loop.
03:28 In this situation all we need to do is choose our desired air/fuel ratio targets, and put them into the table.
03:37 Provided the MAF scale is correct, we should find that the measured air/fuel ratio straight away tracks this new target.
03:45 When we're making these changes in the ECU and measuring the results, it's always useful to log commanded air/fuel ratio where possible.
03:55 This parameter is the air/fuel ratio that the ECU is actually requesting and while the main table that you're tuning is important, there may be other compensations being applied that affect the ECU's final decision on air/fuel ratio.
04:12 Logging this can often highlight the reason why you're not reaching the air/fuel ratio target you entered into the map, and it's important to know this rather than try and fix the discrepancy by tweaking your targets.
04:27 If there is a discrepancy between the measured and target air/fuel ratio, it would point to the MAF scaling being a little out, and while it's possible to correct it in the MAF scaling as we'll see, typically if the error is only a few percent, you can simply make adjustments to the power enrichment table.
04:47 With the power enrichment table as described you might also want to adjust the point in the engine load when the ECU transitions from closed loop to open loop.
04:58 Often this transition is set by the manufacturer to aid emissions and fuel economy at the sacrifice of power, and transitioning to open loop earlier can improve part throttle performance.
05:12 The other technique that's common for forced induction vehicles and some naturally aspirated vehicles is to include a three dimensional table of target air/fuel ratios.
05:24 This has normal axis of load and engine RPM and allows the air/fuel ratio to be tailored more specifically to the current load and RPM.
05:34 This is essential if you want for example, to target richer air/fuel ratios as boost increases on a turbo car.
05:42 The principle of tuning the table's the same, however, it's important to datalog the engine load so that you can accurately determine where in the table the ECU is accessing at any point.
05:55 If the ECU uses a MAP sensor to calculate load rather than directly measuring it, you're going to be faced with one more task which is to calibrate the volumetric efficiency table.
06:08 This table is used by the ECU in it's load calculation and you could liken this to the MAF sensor calibration.