Variable Cam Control Tuning: Implications of MAF vs SD systems
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Implications of MAF vs SD systems
05.27
| 00:00 | - While cam control is a fairly mature technology at this point, there still isn't an overwhelming agreement between OE manufacturers about how best to implement the system when dealing with fuel and ignition delivery. |
| 00:13 | There are also quite a few considerations you'll need to keep in mind while tuning a cam control system depending on the style of ECU you're tuning. |
| 00:21 | In this set of modules we'll cover some of the common strategies you'll come across so that you'll have a good understanding of how to approach them. |
| 00:29 | We'll start by discussing MAF based ECUs and the differences compared to those which use the speed density operating principle since this will have a significant effect on what you can expect to see in terms of fuel and ignition changes as the cam timing is adjusted. |
| 00:45 | A mass airflow sensor or MAF sensor for short is common in the OE world and it's a sensor that sits in the inlet track and as its name implies measures the mass of air entering the engine. |
| 00:57 | This is favoured by many OE ECUs since providing the MAF calibration table which converts the frequency or voltage output from the sensor into an actual mass airflow reading is accurate the ECU can do a very good job of maintaining a specific target air/fuel ratio. |
| 01:14 | The speed density system on the other hand is what you'll almost exclusively find in the aftermarket standalone ECU world however a select number of OE manufacturers also tend to favour this technique. |
| 01:27 | Speed density eliminates the MAF sensor and instead of measuring the mass airflow directly it's calculated using a manfold absolute pressure sensor and the ideal gas law equation. |
| 01:39 | We're not going to get too deep into the background operation in this course so if you want a better understanding we do cover this in our EFI Tuning Fundamentals course. |
| 01:49 | The important part to understand for our purposes here is that the basis for the speed density system is a volumetric efficiency table or fuel table which ultimately defines how much fuel the ECU will deliver at each combination of load and RPM. |
| 02:06 | Provided these tables are accurately calibrated then the speed density system can do a great job of controlling fuelling. |
| 02:13 | A key consideration with a speed density based ECU is that the VE or fuel table is only correct for a specific cam angle. |
| 02:22 | This gets tricky when we're able to move the cam timing because as soon as we do this the VE or fuel table is no longer correct and we'll find that our air/fuel ratio can vary wildly from our target, requiring the VE table to be adjusted to get us back on track. |
| 02:39 | The same situation occurs for our ignition table as well since the correct or optimal ignition angle is a function of the cylinder filling which is affected by the cam timing. |
| 02:48 | However at least as far as smooth operation is concerned, the engine is more sensitive to errors in the air/fuel ratio than to ignition timing that perhaps isn't quite optimised. |
| 03:00 | On the other hand, moving the cam timing becomes less of an issue on a MAF based ECU because as the cam angle moves and the engine's volumetric efficiency changes, so does the airflow through the mass airflow sensor. |
| 03:12 | The sensor therefore reports this change to the ECU and the air/fuel ratio should still track our target quite nicely. |
| 03:20 | Since the load axis for the ignition table on a mass airflow sensor based ECU will also be derived from the MAF sensor, perhaps expressed as grams per cylinder or grams per revolution, we'll actually also end up referencing different ignition timing values as the cam angle is adjusted. |
| 03:38 | This doesn't mean that we have no work to do with a MAF based ECU though and even if the fuelling remains largely perfect, we're almost certainly going to require some work on our ignition tables in order to optimise them. |
| 03:51 | The other little trick here is that even on many MAF based factory ECUs, the primary load input for fuel and ignition will be the mass airflow sensor however it's common to also include a speed density based sub system as well. |
| 04:06 | This is done because of the latency or lag that exists on a purely MAF based system during transient conditions such as throttle tip in. |
| 04:15 | Under these conditions a speed density based system can respond to the fast changing conditions quicker than the mass airflow sensor so the ECU momentarily reverts to the speed density system until steady state operation is resumed at which time it references the MAF again. |
| 04:32 | This means that even when tuning some factory cam control engines that use a MAF sensor you may still need to adjust the speed density sub system as well. |
| 04:41 | Admittedly many tuners either don't understand the operation of the speed density subsystem or perhaps don't even realise it's there and in lightly modified vehicles, often the existing calibration will be close enough to achieve acceptable drivability. |
| 04:56 | While this technically isn't the correct way of tuning, it will often get you through. |
| 05:00 | On the other hand, as we make more significant changes to the engine configuration, we'll find the drivability suffers significantly if we don't do the job properly. |
| 05:10 | If this is all sounding a little too complex at this point then I don't want you to worry. |
| 05:16 | All these concepts will become a lot clearer once we move into the practical sections of the course and see these aspects being demonstrated on the dyno. |
