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Variable Cam Control Tuning: Handling Fuel and Ignition Tables

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Handling Fuel and Ignition Tables

03.16

00:00 - While the requirements in terms of handling fuel and ignition tables aren't as strenuous for a switched cam system as one that offers continuously variable cam control, there are still some considerations here.
00:11 Since we dealt with Honda's K20 in the last module, we'll continue using Honda as our example for now.
00:17 If you're not personally tuning a Honda engine then don't worry as the discussion is relevant to any engine running a switch cam system.
00:25 As discussed, Honda includes a high speed and low speed map for both fuel and ignition.
00:30 This might seem unnecessary if we're just switching the cam at a specific RPM however this allows Honda to incorporate what we refer to as windowing of the switchover point.
00:41 Windowing refers to the ability to vary the switching point across a window of RPM and load ranges.
00:48 For example at wide open throttle, we may want to switch the VTEC on at 4500 RPM however at light load we may actually want to delay the switchover to perhaps 5500 RPM in order to acheive optimal torque.
01:02 In some instances the ECU may choose to not switch to the high cam operation at all as a failsafe.
01:09 Incorporating dual maps means that the ECU can seamlessly switch the VTEC system at any point or not at all and the fuel and ignition will be correct, assuming of course that both maps have been correctly calibrated.
01:22 In the aftermarket we can usually incorporate similar functionality by using dual fuel and ignition maps that change based on the VTEC status.
01:30 However, it's much more common to just use a single map and simply switch the VTEC based on RPM alone.
01:37 This is of course quicker and simpler even if potentially it may be giving away a little performance under part throttle and mid RPM operation.
01:45 Since we don't often spend time in these regions under steady state conditions, we could argue that its not overly relevant.
01:52 Even using just RPM as the switching parameter does raise the potential for some inaccuracies to creep in if we're using a single table since the VTEC system will usually incorporate what is referred to as hysteresis.
02:04 This just means that the VTEC switch on and switch off points aren't identical.
02:09 Perhaps this system switches on when the RPM exceeds 4500 however it won't switch off until the RPM drops below perhaps This is included to prevent the system shuttling on and off repeatedly if we happen to sit right on the changeover RPM.
02:26 It might sound like an unlikely scenario but it can occur.
02:31 This hysteresis then means that there will be some small region of the map where we're essentially operating with incorrect fuel and ignition if we're only using a single map as opposed to the switched map strategy.
02:43 This, in reality is a minor consideration and if your switching points are correctly optimised then you shouldn't be seeing a dramatic change in fuel or ignition requirements across the switchover point but it is still worth understanding.