Practical Reflash Tuning: Step 5: Optimising the Tune
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Step 5: Optimising the Tune
07.51
| 00:00 | - At this point in the process we should have a car that starts and runs relatively close to stock. |
| 00:06 | And we can now begin the actual tuning process. |
| 00:10 | As we've found, in most cases we're going to be able to focus our attention solely on the wide-open throttle open loop area of the engine operation. |
| 00:19 | And with an accurately scaled MAF, this can be accomplished very quickly. |
| 00:25 | The first task is to complete a base run on the dyno in order to establish a starting point. |
| 00:32 | Depending on the level of modifications, we may not be able to safely completely a full ramp run throughout the rev range on our very first attempt. |
| 00:42 | It's quite possible that either the air/fuel ratio will be unsafe or the engine may experience detonation so we always want to be ready to abort the run if something is outside our safe operating parameters. |
| 00:57 | Any time we're running the engine on the dyno, we want to also be scanning or logging from the ECU so that we can look at the data log after the run's completed. |
| 01:07 | It's a good habit to get into to make sure the log is running before we begin any ramp run on the dyno. |
| 01:14 | We don't necessarily need to save every data log, but it's hard to find out what happened if we forgot to press record. |
| 01:23 | Assuming we've been able to run the engine through the entire rev range, the task now comes down to optimising the tune to get the most power and torque while ensuring safety and reliability. |
| 01:36 | Again, the process will vary a little depending on the specifics of the tuning task. |
| 01:42 | But the first step is to confirm that no knock was present during the ramp run. |
| 01:48 | Knock can be very damaging, as we've discussed, so if knock was occurring, we need to deal with this right away by retarding the timing in the areas of the ignition map that knock was noticed. |
| 02:02 | Once we have the engine running in a knock free state at full throttle, I'd begin with adjusting the fuel delivery until you can achieve your desired target air/fuel ratio. |
| 02:14 | If the MAF is scaled accurately, our measured air/fuel ratio should straightaway be very close or identical to the numbers in the air/fuel ratio map. |
| 02:25 | If we're still seeing a discrepancy of a few percent, rather than adjusting the MAF scaling further it's acceptable to make these corrections in the AFR target table instead. |
| 02:38 | To do this, we can utilise the correction factor we learned in the EFI training fundamentals course. |
| 02:44 | The correction factor is simply our measured air/fuel ratio divided by our target air/fuel ratio. |
| 02:52 | Let's say we had a target air/fuel ratio of 12.0:1, but the measured air/fuel ratio ends up at 11.9:1. |
| 03:02 | If we put these numbers into the equation, we end up with a result of 0.99. |
| 03:09 | This simply means that the measured air/fuel ratio was one percent richer than our target. |
| 03:16 | If we target an air/fuel ratio that is one percent leaner, this will correct the error we're seeing and achieve our actual desired target of 12.0:1. |
| 03:28 | To do this, we want to take the correction factor we calculated of 0.99 and now we want to use the inverse function on our calculator. |
| 03:39 | Alternatively, you can divide one by your correction factor to achieve the same aim. |
| 03:46 | This gives us a result of 1.008 and if we now multiply this by the number in our AFR target table, which was 12.0, we get a new target of 12.1. |
| 04:00 | If we enter the new target of 12.1, the measured air/fuel ratio should now meet our real aim of 12.0. |
| 04:08 | Hence, correcting the error. |
| 04:11 | The reason why we need to use the inverse function is because of the way AFR targets work. |
| 04:18 | In our example, we're initially too rich. |
| 04:22 | The correction factor of 0.99 shows that we need to reduce the fuel delivery by one percent to achieve our target. |
| 04:31 | This would be fine if we were directly commanding the injector pulse width since we could simply multiply it by the correction factor of 0.99 and the pulse width would be reduced which would have the effect of leaning out the air/fuel ratio. |
| 04:47 | When we have an AFR target map though, in order to target a leaner mixture we actually need to increase the AFR target number, not reduce it. |
| 04:58 | The inverse function achieves this for us. |
| 05:02 | In the perfect world, the measured air/fuel ratio would perfectly match the commanded air/fuel ratio. |
| 05:09 | And certainly, in many situations, this does hold true. |
| 05:14 | In some ECUs though, even with a well calibrated MAF sensor, you'll still see some small discrepancy throughout the rev range. |
| 05:23 | If I'm seeing an error greater than about plus or minus five percent between my commanded air/fuel ratio and the measured air/fuel ratio, I'll tend to address this in the MAF scaling. |
| 05:36 | However, if the error is within these bounds, I'll adjust my target AFR to suit. |
| 05:41 | This is one reason why we can't just blindly put our desired targets into the target AFR table and then assume the result will be perfect. |
| 05:52 | Once the fueling is accurate, we can then optimise the ignition timing. |
| 05:58 | This is simply a process of adding timing and testing to see the result. |
| 06:04 | Provided there's no knock occurring, I'll typically add timing in two degree increments initially. |
| 06:11 | Once we get close to MBT timing, you can start making finer adjustments to suit. |
| 06:18 | As I described in the ignition tuning module, the process is to advance the timing until we either reach MBT or we find the knock threshold of the engine. |
| 06:30 | If knock does occur, it can be damaging, particularly on engines producing very high specific power levels. |
| 06:38 | For this reason, if we detect knock it's always best to abort the dyno run, retard the timing where the knock occurred, and then try again. |
| 06:49 | It's worth pointing out that once we have the fuel and ignition close, there can often be benefit in trying different air/fuel ratios to see if this benefits the engine power. |
| 07:02 | On GM's LS V8 engines, for example, our richer air/fuel ratio at high load and high RPM can cool the combustion charge which in turn can allow a little more timing to be added. |
| 07:16 | On pump fuel particularly, this can be worth a modest improvement in engine torque and power. |
| 07:25 | If the engine is turbocharged, we need to repeat the fuel and ignition tuning across the range of boost pressure we expect the engine to operate at. |
| 07:35 | In a turbocharged engine, I'll begin the tuning process at the minimum, or wastegate boost pressure, and then slowly increase the boost while continuing to adjust the fuel and timing to suit. |
