WinOLS Mastery: Map Identification & Editing: PIDs and Logging Rates
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PIDs and Logging Rates
05.14
| 00:00 | - The actual parameters that are available to log will depend on the ECU you're working with and not all parameters are going to be available on all ECUs. |
| 00:09 | These parameters are often defined by what's referred to as a PID which stands for parameter ID and these PIDs are used by the datalogger to request data from the vehicle. |
| 00:20 | While all manufacturers are required to make available certain PIDs, mainly for the purposes of checking emissions compliance for example, many other PIDs that can be helpful for our purposes with tuning may be available depending on the manufacturer and the complexity of the controller. |
| 00:37 | This is why we can see some differences in the available PIDs on logging tools that are closely developed for certain platforms compared to generic tools that only support logging of the standard OBD2 PIDs. |
| 00:50 | The PIDs we might be interested in to help us with tuning our boost control or our fuel and ignition delivery are likely to be quite different to those we'll be interested in if we're optimising the idle control or variable cam control for example. |
| 01:05 | It can therefore be tempting to just log every PID available to the logger as this way we're covered and we'll have access to all of the data we require, irrespective what we're trying to do as part of our current tuning. |
| 01:19 | While on face value, this theory does make sense, the reality is often a little different. |
| 01:24 | This is very dependent on the specific controller you're working with, however in many cases, the more PIDs we log, the slower the logging rate becomes, to the point that the data is essentially useless to us. |
| 01:37 | Understanding this requires a little knowledge of what the logging rate is and why that's important. |
| 01:43 | For most of the fast changing parameters we're interested in, we're going to need data at a rate of at least 20 Hz which means that we're sampling the parameter 20 times every second. |
| 01:54 | This gives us a complete understanding of how that parameter is changing over time, even if the parameter is changing quite quickly. |
| 02:02 | Let's look at what happens if our logging rate is too low. |
| 02:06 | If we consider a PID such as boost pressure and our logging rate is only 1 Hz for example, this means we're only sampling the boost pressure once a second. |
| 02:16 | This is still going to give us some data and the analysis software will still draw us a nice smooth graph that on face value might look useful however it's going to lose a lot of important detail that we need. |
| 02:29 | For example, as the engine comes up onto boost initially, the boost pressure is going to ramp up very quickly, it could quite likely change by more than one bar in the course of one second, depending on the specifics of the turbo, engine and ramp rate. |
| 02:44 | Let's assume we get our first sample of the boost just as the boost pressure starts to climb and it's 0.1 bar. |
| 02:51 | Our next sample is a second later and the boost is at our target of 1.0 bar and a sample a second later, still shows us boost sitting at 1 bar and so on. |
| 03:02 | On face value now, everything is looking great. |
| 03:05 | Now let's look at the reality of what might have gone on if we'd been able to log that same PID at a higher rate of 20 Hz. |
| 03:12 | Now we're recording and graphing the boost pressure 20 times per second. |
| 03:16 | What we can now see is that the boost actually overshoots our target, reaching a peak of 1.2 bar, before coming back through our target, oscillating slightly under our target and finally stabilising. |
| 03:29 | This would now suggest that our boost control actually needs a little bit of work. |
| 03:34 | You can now hopefully understand that the first log at only 1 Hz doesn't provide enough useful information. |
| 03:41 | That's not to say that everything needs to be logged at higher rates though and slower moving inputs such as inlet air temperature or engine coolant temperature don't change quickly and hence a slower logging rate of perhaps 1-5 Hz is totally acceptable for these parameters. |
| 03:58 | The amount of control over the logged parameters and the logging rate will depend on the vehicle you're tuning as well as the logging software that you're using. |
| 04:07 | While it's not possible to list every parameter that may be useful to log, some of the key ones would include engine speed, mass airflow, engine load, throttle position, ignition timing, knock feedback, fuel trims and commanded air/fuel ratio or lambda. |
| 04:24 | In particular, logging engine load and engine speed will allow you to see where in a particular map the ECU is accessing at a certain point in the datalog. |
| 04:34 | Ignition timing and knock feedback are valuable parameters to aid optimising the ignition timing. |
| 04:40 | Although I'll point out that how knock feedback or knock detection is handled and logged will vary dramatically from one ECU to another. |
| 04:49 | Let's have a quick recap. |
| 04:51 | Remember that the logging rate can be affected by the number of parameters we're logging so it's good practice to focus our logging on just the parameters we need for a particular aspect of our tuning. |
| 05:03 | If the logging rate drops too low, we lose a lot of detail from the logs that can hide the information that we want or even potentially be dangerous. |
