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EFI Tuning Fundamentals: Short Pulse Performance

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Short Pulse Performance

05.12

00:00 - Over the last few years we've seen some marked improvements in the quality, and capability of the injectors, that are available for us in the aftermarket.
00:07 At the same time, the knowledge surrounding how the injector operates has also improved greatly, and this has allowed us, as tuners, to do our jobs better.
00:16 Both OE, and aftermarket ECU manufacturers have also drastically improved the technology of their ECUs, to do a more accurate job of controlling the injector.
00:27 As far as the ECU is concerned, it needs to know what pulse width it needs to provide to the injector, in order to result in a certain mass of fuel being delivered.
00:35 Given that the ECU is either directly measuring the mass air flow with an air flow meter, or calculating it via the speed density principle, if it knows the mass air flow of the engine, and the desired air fuel ratio, it can very easily calculate the required fuel mass.
00:51 We've already discussed injector dead time and how that affects the pulse width the ECU needs to supply, in order to achieve a specific amount of fuel delivery, however there's still a little bit more to consider.
01:04 Traditionally, an ECU just assumes that the flow from the injector is linear in relation to pulse width.
01:11 That's to say if we double the pulse width, the injector will provide twice as much fuel, and this assumption is what the ECU uses to decide what pulse width to deliver.
01:21 Let's now have a look at a graph of injector flow versus pulse width, courtesy of Injector Dynamics.
01:27 Note that this plot has been corrected for injector dead time or offset, and the grey line represents the theoretical flow from the injector.
01:35 What you can see, is that above two milliseconds, the actual injector flow very closely matches the theoretical flow, and this is known as the linear operating region.
01:45 Below two milliseconds though, we can see that the flow deviates from the theoretical flow.
01:50 This becomes a lot more obvious if we zoom in on this particular area.
01:55 This is called the lower non-linear operating region.
01:58 If the ECU can't account for this deviation between the theoretical flow and the actual flow, then we're going to end up with the engine running leaner in this area, than the ECU would calculate.
02:09 Traditionally, this error would be accounted for by simply adding more fuel to the fuel table in these areas, but many ECUs now give us the ability to account for this flow discrepancy, by way of a two-dimensional table, known as the short pulse width adder table.
02:24 This table defines the additional time that the ECU will need to add to the injector pulse width, in order to achieve the theoretical flow amount.
02:33 This way of dealing with injector characterisation, comes from the OE manufacturers, as it's an essential part of achieving very accurate control over the fuel delivery, and hence the air fuel ratio.
02:44 For any of you that have delved into reflashing of factory GM ECUs, you may have already seen a short pulse width adder table, particularly if you've ever swapped injectors.
02:55 It's now becoming more common for aftermarket ECU manufacturers to also include a short pulse width adder table, and now you know what it's for.
03:03 The next obvious question though, is where do you get this data in the first place? It's a valid question, and finding the answer often, isn't easy.
03:11 My suggestion is that if you're buying aftermarket injectors, ensure that you buy from a supplier who can provide this data that you'll need.
03:20 As tuners, we can't generate this data on the dyno, it needs to be done with specialised test equipment, in a laboratory style environment.
03:28 If you're dealing with an OE ECU, and you're planning to swap injectors, then it's essential to swap to another injector, where this data is available.
03:37 The data you can see here, is the GM characterisation data that Injector Dynamics supply with their ID1000 injector.
03:44 They've done all of the hard work in testing, so that changing your stock injectors to a set of ID1000s, is as simple as copying the data into the relevant tables in your ECU.
03:55 Assuming that injector dead time or short pulse width adder data will be close enough between two different injectors is futile, and will result in no end of niggly problems when it's time to hit the dyno.
04:07 If on the other hand, you're dealing with an aftermarket ECU that has a short pulse width adder table, then you can get away with leaving those tables set to zero.
04:15 It's not the perfect solution though and it's going to result in fueling inaccuracies at low pulse widths, that you will be needing to correct by manipulating the main fuel table.
04:24 It is important to remember that there are still a huge number of ECUs out there in the market, that completely ignore this non-linear operating area, so setting this table to zero, will just have the ECU respond the same as one of these brands.
04:38 In summary, these days we're seeing the addition of short pulse width adder tables to both OE and aftermarket ECUs and this allows more accurate control over what's called the lower non linear operating region which has traditionally been masked by simply altering the fuel table in these areas to compensate.