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Variable Cam Control Tuning: Switched Cam Tables

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Switched Cam Tables

05.32

00:00 - As we discussed in the last module there's a significant difference between MAF based and speed density based ECUs in the way their operation will be affected by cam timing changes.
00:11 There's also a variety of ways that ECU manufacturers use to represent the fuel and ignition tables so that we can manipulate them and it's worth having an understanding of the various options that are common.
00:23 As already discussed, MAF based systems give an advantage with continuously variable cam control systems since they're directly measuring mass airflow.
00:32 We don't necessarily have a fuel table so to speak with this style of ECU either and often the ECU will either have a target air/fuel ratio table or perhaps separate the engine operation between closed loop and open loop where only a simple 2D target air/fuel ratio table is provided for use during open loop operation.
00:54 This makes our life relatively easy as tuners and we should be able to simply ask for our target air/fuel ratio and the ECU will deliver it, even when we move the cam timing.
01:04 If an error does exist in a MAF based ECU and we're not achieving our target air/fuel ratio, this means that the MAF calibration table is incorrect which can be quickly and easily corrected as we demonstrate in our Practical Reflash Tuning course.
01:19 Things get a little trickier with speed density based ECUs though and here we find a significant difference between OE and aftermarket ECUs.
01:30 The aftermarket have universally concluded that the easiest way of operating is to use a single 3D fuel or VE table which needs to be optimised for the current cam position.
01:41 Providing that the cam position for a given combination of load and RPM is always the same, this is a very effective and quite straightforward way of tuning. The problem however is that this assumption isn't always going to hold true.
01:56 Particularly during cold start operation when the oil is thicker it can be tricky to get the cam timing to track the target perfectly and as soon as there are errors between the cam target and the actual cam position, this leads to errors in the fuel and ignition timing.
02:12 As you'll see as we move through the course there are fortunate ways to combat this situation too.
02:18 On the other hand, if we take an engine such as Honda's K20 that incorporates variable timing on the intake cam as well as their mechanically switched VTEC system, the factory engineers chose a slightly more complex method to deal with the calibration.
02:32 In this ECU, you'll find 6 fuel and ignition tables for low speed cam operation which are used below the VTEC changeover point and 6 fuel and ignition tables for the high speed cam operation above the VTEC changeover point.
02:46 To add a little more complexity there's also 2 cam target tables that define the intake cam position target for low speed and high speed operation plus some parameters that define the VTEC changeover.
02:59 On face value this seems incredibly complex but it does make sense when you understand what Honda were thinking.
03:07 Essentially the K20 offers 50° of intake cam movement and the tables are split up into 10° increments.
03:15 In this way, we have fuel and ignition tables at 0, 10, 20, 30, 40 and 50° for both low speed and high speed cam operation.
03:25 When correctly optimised this means that regardless what the cam position is, the ECU simply references the relevant fuel and ignition map and the tuning will be correct.
03:36 This is undoubtedly a thorough solution however it does potentially result in a huge amount of work for the tuner.
03:43 Particularly when there are areas of operation where we know that we won't be using the extremes of cam operation.
03:49 For example we can be reasonably confident that at 7000-8000 RPM and wide open throttle, the intake cam is going to be at or close to the fully retarded position and definitely not at 40-50° of advance.
04:03 If you're wondering what happens when the cam timing happens to be between 2 of the map targets, let's say 25°.
04:10 Then the ECU simply interpolates the values for fuel and ignition from the surrounding maps.
04:16 In this case the fuel and ignition values would be exactly halfway between the values in the 20 and 30° maps for the current operating point.
04:24 Honda's technique isn't the only method used to deal with speed density tuning with variable cam control though and many late model factory ECUs have chosen not to incorporate a conventional 3D VE table like what we may be familiar with in the aftermarket.
04:41 Instead the volumetric efficiency of the engine is constantly calculated through a complex quadratic equation based on a number of coefficients that we have access to adjust.
04:52 This allows the ECU to reference different coefficients to define the current VE based on the current cam position.
04:59 On face value this might sound daunting without the benefit of a normal VE table to manipulate.
05:06 Don't get scared off just yet though because fortunately many of the tuning solutions we rely on for these applications include special tools that convert the coefficients into a visual VE table that we can understand and conversely will convert our VE table adjustments back into new coefficients.