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EFI Tuning Fundamentals: Tuning Fuel Delivery

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Tuning Fuel Delivery

05.44

00:00 - At this point in the course, we've already discussed, at length, what we're trying to achieve when we're tuning the fuel tables in the ECU.
00:06 And, we've discussed how and why we need to vary our air fuel ratio targets as the engine load changes.
00:12 Remember, that under full throttle operation, we're going to be targeting a richer air fuel ratio than we would at cruise and idle.
00:19 And, the reasons we do this, are to ensure that we're combusting all of the available oxygen in the cylinder to ensure maximum power, while using some additional fuel to help control combustion chamber temperature.
00:31 There are a few aspects to do with the shape of the fuel curve that I want to address in this module, that can help you out when you are tuning.
00:39 One of the key elements is to understand what the fuel table represents.
00:44 Obviously, it's defining ultimately, the amount of fuel that the injectors are delivering.
00:48 However, there are a few more subtle aspects to consider.
00:52 Since we're matching the fuel delivery to the amount of air the engine is consuming, we'd expect the fuel table to increase and decrease as the air flow into the engine increases and decreases too.
01:03 Since we already know that the amount of air the engine consumes is defined by its volumetric efficiency, it stands to reason that the shape of the fuel curve will follow the shape of the engine's VE curve.
01:15 When we're talking about the shape of the fuel table, it's easier to view it graphically rather than numerically.
01:21 This makes it much easier to visualise the overall shape rather than trying to deduce this by staring at a 3D table of numerical values.
01:30 It's also really easy, if you're only viewing the numerical values, to accidentally miss a single number and perhaps end up with a value such as 7% instead of 70%.
01:40 Numerically, this can be hard to pick up but if you're viewing the table graphically it will be immediately obvious When we're tuning on a dyno, we won't know specifically what the engine's VE actually is.
01:53 However, we can measure the engine torque and display a plot of the torque curve.
01:58 Since the engine torque is influenced by air flow, the shape of the torque curve, and of the VE curve, should be very similar.
02:05 With this in mind, when we're tuning the engine, particularly under wide open throttle ramp runs, we would expect the general shape of the fuel table to be similar to the shape of the torque curve.
02:14 That's to say that it will start low, increase and peak, and then begin dropping as the engine RPM increases.
02:21 If you understand this concept, it can help you diagnose certain problems with the fuel system, in particular.
02:27 One of the most common tuning problems you'll come across while tuning, and particularly with a modified engine, is a fuel system that can't keep up with the required fuel demands of the engine.
02:37 This generally begins to show up as an engine with an air fuel ratio that gradually tapers leaner as the RPM increases when you're performing a ramp run.
02:46 Of course, the natural course of action here, when we see the situation on the Dyno, is to increase the numbers in the fuel table in the locations where the air fuel ratio is leaner than we want, and then perform another run.
02:58 If the fuel system is beyond its capability though, we can often get to a situation where no amount of fuel in the table is going to get our air fuel ratio on track, as the injectors or the fuel pump just can't cope.
03:10 This can waste time and potentially be dangerous for the engine.
03:14 If we notice, however, that the engine torque is beginning to drop as the RPM increases, but the numbers in the fuel table are continuing to climb, this is a clear a warning sign that something isn't right.
03:26 In general, we'd expect the shape of a fuel table to be relatively smooth and consistent, and this is usually a sign of a properly tuned engine.
03:34 There are some situations, however, where we might expect a more irregular shape to the fuel curve.
03:40 In particular, if you're tuning a naturally aspirated engine that's fitted with a large cam profile, featuring a lot of overlap and duration, we may find that at low RPM, the fuel table will be quite peaky.
03:52 Normally as we get higher in the RPM though, the engine's VE will smooth out and the fuel table will consequently become more regular and consistent.
04:00 In these situations the peaky nature of the fuel table will also be displayed by peaks and troughs in the torque curve on the dyno.
04:07 So again, the shape of the torque curve should be a guideline as to what the fuel table is likely to look like.
04:14 Since we're typically expecting a fuel table that has the relatively smooth and consistent shape, any time we find one or two zones in the table that seem to require a lot more, or a lot less fuel than those surrounding cells, this may also be a clue that something's not right and it can be worth revisiting these sites to ensure that we do in fact have the correct air fuel ratio at those points.
04:38 Here's an example of what I'd expect to see from a normal well-tuned engine, We can see the numbers in the table increase smoothly with load on the Y axis.
04:46 And, at the same time, the shape of the fuel table follows what we could expect in terms of a sensible torque curve.
04:53 This is another example of what we might expect to see from an engine with a more aggressive cam profile.
04:58 And, you can see the more erratic nature of the fuel table, at low RPM.
05:03 If we're tuning an engine like this, where the engine's VE is changing rapidly over a narrow RPM range, it can be helpful to add additional sites to the fuel table to better control the fuel delivery and hence better control the air fuel ratio.
05:17 So, to summarise this module, remember that the shape of the engine's fuel table should be relative to the air flow into the engine and the air flow is relative to the amount of torque the engine is producing and hence the shape of the fuel table should closely approximate the shape of the torque curve.