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Subaru Knock Correction

Did you see some unusual knock on your AccessPort? 


Don't panic!  Read through this page to understand more about how the knock correction system on your Subaru works and how your Boosted Performance custom tune works in conjunction with this system.

The Subaru knock control system uses a three part approach to react to and avoid detonation (knock).  Below is an outline of each part of the system and how it works when the ECU detects possible knock.  We say "possible" because the ECU has no way to distinguish real knock from false knock because it simple listens for anything that makes a sound in the frequency range of knock but other things can be heard and assumed to be knock.

Feedback Knock Correction (FBKC) - This is generally the first approach the ECU takes to the detection of possible knock event.  The ECU responds with an immediate but temporary reduction of ignition advance that presents as a negative number indicating the amount of ignition advance being removed.  If the ECU stops detecting knock it will stop reducing the ignition advance and will eventually start removing the reduction which in turn advances the timing back to the base target.  This is the most common type of correction to see and it is completely normal and appropriate to see at a reasonable level.  Our opinion of a reasonable level is up to -2 FBKC seen randomly and generally at lighter loads but it is acceptable to see at high loads, such as full throttle, on occasion.  What is not normal is seeing FBKC build up past -2, especially during full throttle loads or seeing it constantly during full throttle or high loads.  So in other words, if you begin a full throttle pull and the FBKC continues to increase from -1, to -2, to -3 and this continues through the pull or if every time you do a full throttle pull you see -2 or more in the same rpm area, this could be real knock that should be addressed which we will go into more detail on under the causes of knock.  If you are just noticing up to -1 to -2 FBKC on your AccessPort randomly; this is normal and nothing to worry about.

Fine Learning Knock Correction (FLKC) - This type of knock correction is similar to FBKC but is a long term stored reduction of ignition advance for a given load range.  This type or correction can happen when the ECU detects consistent knock in a particular load range or if the intensity of a detected knock event is high enough the ECU can sometimes skip FBKC and immediately apply FLKC.  This type of knock, like FBKC presents as negative numbers indicating the reduction of ignition advance.  The difference is that it will continue to show up in the same load range it has been stored into until the ECU is no longer detecting knock in that range at which point it will reduce the FLKC until it is back to zero.  Seeing up to -1 to -2 randomly is acceptable but more than this should be addressed. Doing an ECU reset will clear this learned knock as well.

Dynamic Advance Multiplier (DAM) - This type of knock correction is very difference from FBKC or FLKC.  To understand this correction, it is important to know that the Subaru ECU has the ignition advance target broken up into two parts.  The first part is a base advance table and the second part is a dynamic additive table.  The majority of the target ignition advance is in the base table while the dynamic additive table holds the rest.  The ECU adds the additive values to the base values to determine a final ignition advance (before compensations) but first it multiplies the additive ignition advance by the DAM.  This is the important part. Under normal conditions, the DAM should remain at 1 (16 on an 02-05 WRX) so the additive timing is multiplied by 1 and the full amount is used.  If the DAM is reduced, then the whole additive table's values are reduced before being added.  A DAM of .5 (or 8 on the 02-05 WRX) reduces the additive timing by half.  The point of this is for the ECU to do a very broad reduction of ignition advance when it detect a heavy amount of detonation that is not able to be safely controlled using FBKC, FLKC or even both.  Generally the ECU will attempt to use FBKC and FLKC first but if it detects a high enough intensity of knock, it can go straight to a DAM reduction.  Over time the ECU will move the DAM back up to the maximum value so long as it does not continue to detect knock.  An ECU reset will put the DAM back to its base value which on a BPT custom tune is 1 (16 on the 02-05 WRX ECU).  With a BPT custom tune, your DAM should typically always reman at the max value.  If you experience a small reduction, you should be able to do an ECU reset and should not see it happen again.  If you do see your DAM lowering consistently, refer to the common causes of knock correction to look for solution. 


The Boosted Performance Custom Tune Approach

Over years of testing, development and tuning Subarus, we have developed our own unique strategy for tuning the knock control system which is different that the OEM approach and different from many other tuners, some of which just rely on the OEM approach.  The OEM approach attempts to be dynamic, meaning it leans on the knock control system to find the appropriate amount of ignition advance under all conditions.  The problem with this is that it requires exposing the engine to detonation more frequently and we are all aware that Subaru engines/pistons do not tolerate this well.  Our approach is to set every custom tune with ignition advance mapping, including compensations, that requires as little intervention from the knock control system as possible while at the same time, allowing the knock control system to have the full authority to intervene if the need arises.  As a result, our custom tune should not see the DAM being lowered or see more than -1 to -2 FBKC on occasion or see more than -1 to -2 FLKC show up that repeatedly comes back in the same area if cleared.  If you are seeing what we consider normal then you have nothing to worry about.  If you are seeing more than that, then it is time to look into the possible causes below.  If one of these does not seem to be the cause or resolve the issue if addressed, then we want to see logs so we can help determine what the issue might be or attempt to solve it with tuning changes.

Common causes of knock correction

Bad Fuel - Low quality fuel is a very common cause of seeing more knock correction that normal.  If fuel quality is low enough, seeing the DAM lower is very common.  Try a full tank of fresh fuel from a different, highly trafficked station to see if this solves the issue.

Lean fueling - A large number of things can cause the fueling to get leaner than we target on a Subaru.  EJ powered Subarus do not have a good fuel control system or even a good ability to monitor high load fueling with the AccessPort.  This is all due to the fact that the front o2 sensor is placed between the engine and turbo which results in a skewed reading at higher loads due to the effect of exhaust back pressure on o2 readings.  As a result, we cannot see the engine is running leaner than intended nor does the ECU have any fuel control at these higher loads.  This is why we recommend a wideband AFR gauge be added on all EJ powered Subarus.  It is valuable to be able to monitor fueling and ensure it is always in the targeted range. If you are able to confirm lean fueling (higher values than around 10.9-11.2 AFR during high loads) or we suspect lean fueling, these are the common causes to investigate.  Fuel pressure issues from a bad or old fuel pump.  Dirty and clogged fuel filters.  Dirty MAF sensor.  Vacuum leaks.  Missing or damaged vacuum reference to the fuel pressure regulator.

False knock caused by noise near the engine - Examples of this are things like engine mounts, transmission mounts, pitch-stop mounts that are going bad or being loose and letting the engine rock around and make noise in the subframe.  Other common causes are things like intercooler brackets or other miscellaneous brackets, head shield rattling, etc.  This can be anything near the engine that can move, rattle or make a metallic noise.  This noise can be in the frequency range of real knock and the ECU can assume it is real knock.  If bad fuel or lean fueling have been ruled out, then this is the most common cause if you are continuing to see unusual knock correction.

Internal engine noise causing false knock - This is the worst case scenario and the thing we hope is never the issue but if you have bearing wear or issues in the valvetrain, these things can cause noise that is picked up as knock.  If everything else seems to be ruled out, take a good listen to engine or take it to a professional to be listened to.  Also consider doing an oil analysis which can often detect metals that are present when engine wear levels are too high.

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