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Practical Reflash Tuning: Transient Tuning

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Transient Tuning


00:00 - Transient enrichment, accel enrichment or tip in enrichment as it's often referred to is required to ensure stable control over the air fuel ratio as the throttle is either opened or closed rapidly.
00:16 The way this transient enrichment is handled can vary widely from one ECU to the next, and we will deal with specific examples where required within our worked example section.
00:29 In this module, however, we will cover a few key points that are worth keeping in mind.
00:36 First, let's discuss briefly why we need transient enrichment in the first place.
00:42 During normal injector operation, some quantity of the fuel being delivered will end up forming a fuel film or puddle on the port wall.
00:53 The volume of this fuel film changes based on many factors, but airspeed and manifold pressure are two of the more significant.
01:03 In simple terms, when the throttle is opened sharply and manifold pressure rises sharply, the volume of the fuel film increases.
01:14 Conversely, when the throttle is shut quickly and the manifold pressure drops, the fuel film will decrease.
01:22 Under steady state conditions, the fuel is evaporating from the fuel film at the same rate that it's being topped up by the injector.
01:31 In this way, the engine still receives the total mass of fuel that the injector is delivering and hence the target air fuel ratio can be achieved.
01:42 When the throttle is opened sharply though, a portion of the injection pulse momentarily is used to increase the size of the fuel film until equilibrium is once again achieved.
01:55 This means that during the transient event, the engine isn't receiving all of the fuel being supplied by the injector, and hence it will run lean momentarily.
02:07 The opposite situation occurs when the throttle is sharply closed, and in this case, the engine runs rich momentarily.
02:16 Traditionally, transient enrichment is based off rate of change of throttle position, but it's important to understand what's really happening and it's the change in manifold pressure that's really the key to transient enrichment.
02:33 The reason I've just mentioned the fuel film is because some of the newer generation of OE ECUs directly model this fuel film, and hence some understanding of it is necessary.
02:46 It's still quite common however, for many OE ECUs to base transient enrichment off change in throttle position.
02:55 Provided you understand what's happening inside the ECU, you'll be able to deal with either situation.
03:02 In some ECUs, such as Subaru for example, the transient enrichment tables command a specific injector pulse width increase in milliseconds.
03:12 And in this case, if you fit larger injectors, you will need to adjust these tables or the engine will run too rich when the throttle is opened sharply.
03:22 If the transient enrichment tables are handled in this manner, a good place to start is to adjust the tables by accounting for the change in injector size.
03:32 For example, if you've doubled the size of the injector, halving the transient injector pulse width would be a sensible approach.
03:41 To do this, we can simply divide the original injector size by the new injector size and then use this as our multiplication factor.
03:50 So, for example, if we originally had 500 cc injectors, and we'd fitted 1,000 cc injectors, the multiplication factor would be 500 divided by 1,000 which equals 0.5.
04:05 Now we can multiply the transient enrichment pulse width by 0.5.
04:11 Understandably, the tables may still need to be adjusted further from the starting point, but typically, this is going to get us close.
04:21 Since the MAF sensor can respond a little slowly to rapid movements in throttle position, some ECUs will include both a MAF sensor and MAP sensor and momentarily reference the MAP sensor during transient periods to provide faster response.
04:39 In this case, we may have a little more work to do because the ECU will include a volumetric efficiency table that is used during these transient periods.
04:49 Now as well as correcting any error in the MAF calibration, we'll also need to adjust the VE table when we make changes that will affect the engine's volumetric efficiency.
05:01 The process of turning this sort of ECU is to first scale the MAF sensor to ensure accuracy, then force the ECU to switch to the speed density system referencing the speed density table and ignore the MAF sensor.
05:17 You can then optimise the VE table in much the same as you'd go about rescaling a MAF sensor.
05:24 Once both systems are correctly tuned, the MAF sensor can then be enabled again.
05:30 Failing to do this on a heavily modified engine can result in poor drivability during periods of rapid throttle movement.

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