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Practical Reflash Tuning: Knock Detection

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Knock Detection

04.32

00:00 - Easily the biggest killer of any performance engine is knock, also known as detonation.
00:07 I'm not going to discuss knock in detail here, and for a complete description of what knock is, and what causes it, refer to our EFI Tuning Fundamentals course.
00:18 Knock detection and control strategies have been employed by OE ECU's for many years, and in my own experience, the current generation of these systems are usually excellent.
00:30 The principle behind knock detection, is to use a block mounted knock sensor, to detect the vibrations in the block that are associated with knock occurring.
00:41 Often these knock detection systems will employ sophisticated digital signal processing techniques, that help to focus on the known frequency that knock occurs at, as well as the window during the combustion event when knock is likely to occur.
00:58 This all improves the system's ability to accurately detect knock, while ignoring background noise associated with normal engine operation.
01:08 This, in itself, is an impressive feat, given the noisy mechanical nature of the engine's operation.
01:15 Once the ECU has a signal that defines whether or not the engine is knocking, it needs to do something with that signal.
01:23 The exact techniques will depend on the particular ECU you're tuning.
01:29 However, the most common approach is to include a high octane, and a low octane ignition timing map.
01:35 The ECU will instantaneously retard the timing, based on a knock event, and depending on the amount of activity from the knock sensor, the ECU will bias the timing between the high and low octane maps.
01:50 In this way, the ECU provides some learning, based on how prone to knock the engine is.
01:56 Another approach that's used by Subaru in particular, is to use a base ignition timing map, and an ignition advance map.
02:05 These are both complete 3-D tables, and the base timing map defines what could be considered very safe timing values that should result in knock free operation.
02:17 The advance map then defines how much additional advance that the ECU may add to the values in the base ignition table.
02:26 In basic terms, the ECU then decides how much advance to add from the advance map, based on the activity from the knock sensor.
02:35 Often, as well as modifying the ignition timing to help remove knock, the ECU may also modify the fueling, temporarily richening the air fuel ratio, and in an attempt to cool the combustion charge temperature, and hence reduce the likelihood of knock continuing to occur.
02:54 This is why some ECU's will also contain a high octane and low octane fuel map.
03:01 It's becoming commonplace for tuners specialising in reflashing, to rely solely on the knock control system and data logging, to aid them with optimising the ignition timing, in deciding when the engine is knocking.
03:16 While in 95% of situations this works out fine, it's not always guaranteed.
03:23 As I've mentioned previously in the course, I've had many instances, particularly with modified engines with forged pistons and solid lifters, where the ECU is picking up false knock, which is where the ECU thinks the engine is knocking, but in reality it isn't.
03:40 While this isn't likely to damage the engine, it can result in a very conservative ignition table, that leaves a lot of potential power on the table.
03:51 The more concerning situation though, is where the ECU is unable to detect knock that can be heard audibly.
03:58 Obviously, with no way of detecting knock, and correcting for it, this could quickly result in damage to the engine.
04:07 I'm a strong advocate for confirming that the knock control system is able to adequately and accurately detect knock, before I'm happy to trust it exclusively.
04:19 I do this by using audio knock detection equipment to listen for knock.