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Launch Control: RPM Limit Configuration

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RPM Limit Configuration

02.59

00:00 - Since the launch control system will rely on the ECU's engine rev limit function this is the very first place to start our discussion as we need to know how the limit will affect the launch control system.
00:11 The way the rev limit works is very dependent on the particular ECU so we'll look at more specifics of the system in our worked examples.
00:20 For now, though, we'll discuss the basics.
00:23 Generally, when defining the rev limit the ECU will use we may have the option of a fuel cut, ignition cut, or both.
00:31 This will define what the ECU will cut in order to control the engine speed.
00:37 In most instances I find I get the best results using an ignition cut, which will randomly cut spark to different cylinders.
00:45 As we'll see later, this is essential if you want to employ ignition retard to help build boost while stationary.
00:52 However, it does require a little caution.
00:56 An ignition cut results in complete intact charge of fuel and air passing through the engine and into the exhaust.
01:03 There is the possibility of this igniting which can be problematic for some engines with poor valve train designs.
01:09 Nissan's SR20DET is a great example where an ignition cut rev limit may result in the rockers being thrown off the valves.
01:18 If your engine is known to have these sort of valve train issues I'd recommend using a fuel cut type rev limit.
01:26 Most rev limit functions will also include a soft cut and a hard cut and we can usually define an RPM range across which these will function.
01:35 The soft cut will start applying a percentage of ignition or fuel cut to control the engine RPM.
01:42 A low powered engine may only require a moderate percentage cut while a more powerful engine will require a higher percentage of cut to control the RPM and the ECU will constantly adjust the amount of cut to maintain a stable RPM.
01:58 If the RPM reaches the hard cut limit, however, the ECU may apply a 100% cut to prevent further increases in RPM.
02:08 The aim with these parameters is to achieve smooth RPM limiting at our target RPM where we're operating in the soft cut area.
02:17 If we simply had a 100% cut at our RPM limit the limit function would be very aggressive and the result would be large RPM oscillations that would make it hard to launch the car effectively.
02:31 The specific configuration of the RPM limit will depend on your engine, so it needs to be tested and adjusted to suit.
02:39 In particular, the number of cylinders and the specific power level of the engine will influence how the limit should be set up.
02:46 There isn't a single configuration that you can apply to all engines.