Subaru Knock Control Strategy — What you need to know
Subaru Knock Control Strategy — What you need to know 10/10/17

Subaru Knock Control Strategy From Cobb with BrenTuning Addendum.

Knock! Knock! Who’s there? It could be nothing at all, but, it may be cause for concern. Knowing the difference can give you peace of mind, or could be an early indicator of a mechanical issue ticking away on your Subaru. Datalogging and monitoring with the Accessport can provide useful data on these knock events. Couple that with some basic knowledge and you’re on your way to quickly and easily diagnosing any knock events at any given time.

Subaru knock detection is like using a large net to catch fish. You’ll definitely catch what you want to catch (actual knock) but you will also catch what you don’t want (false knock). If you used a small net, you would get less false knock but might actually miss actual knock. It’s a compromise. Be sure to look at the entire picture when trying to determine whether there is an issue before you bother the tuner. Tuners spend 1-2hrs per day answering the same questions in this attachment, so please read before asking.

First, let’s talk about what exactly “knock” is and how it’s detected by your car’s ECU. Sometimes knock is also called detonation. Detonation is characterized by the air/fuel mixture spontaneously igniting during the compression stroke. The force of this early, unintended explosion is counter to the direction of the piston and results in a sharp increase in cylinder pressure. The increased pressure can exceed the design limits and potentially cause damage. These events typically result in an audible noise that can sound like a “ping”, “knock” or “marbles in a can”. The ECU picks up on this noise with a microphone hard mounted to the engine block and listens for specific frequencies that indicate a knock event. Any number of conditions or factors can induce detonation. The sources are varied but are generally due to increased cylinder pressure, high temperatures, or reductions in effective octane level.

Worried? Don’t be! Detonation events are inevitable and will occur from time to time on any modern vehicle running on pump gas. Your car is built to recognize these events and take the appropriate action to defend against them causing any damage. The Subaru knock detection system tends to err on the side of caution. This makes it common to see “false knock”. False knock occurs when the ECU corrects for a knock event but the noise registered is due to other noises that aren’t necessarily detonation or harmful to the engine. This often happens accelerating from a stop while letting the clutch out, during gear shifts (more so if grunting while shifting), accelerating at low RPM in a high gear, under cruise on the freeway, and during abrupt throttle changes. There are some known situations where it is very common to see timing corrections in response to perceived noise that is 100% normal. In general, these events are not of concern and should not be particularly alarming. Remember, the knock sensor works as a microphone and can also pick up unintended “knock” events, these outside noises sound like knock to the sensor and thus register on your AP. The corrections made during these scenarios are often due to shifting or rapid movements within the drive train and not necessarily to harmful noises generated within the engine:

• On from-stop takeoffs while clutching out
• During gear shifts (this can be exacerbated by “power shifting” or using FFS)
• While attempting acceleration at a low RPM in a high gear
• Under cruising-type conditions at freeway speeds
• After sharply pressing/depressing the accelerator pedal

Knowing which parameters to monitor with the Accessport can help determine if that’s the case. Generally, Dynamic Advance Multiplier (DAM), Feedback Knock Correction, and Fine Knock Learning are relevant when monitoring knock.

DAM represents a global adjustment to ignition timing. If this value is anything less than ideal, it’s a quick indicator something isn’t right (unless immediately following an ECU reset/reflash). On an 02-05 WRX the values will range from 1 to 16, 16 being ideal. On all other Subarus, the value is expressed as a decimal from 0 to 1, the ideal value being 1. HOWEVER, it is normal from time to time in a perfectly good running car to see lower values. No worries if the value is extremely low the car is designed to remove the boost control and go into failsafe.
Feedback Knock Correction is a real-time timing correction the ECU makes based on a perceived noise. The ECU immediately pulls timing (the amount of the Feedback Knock Correction value) and adds timing back (value rises to 0) assuming no further noise is detected.
Fine Knock Learning represents minor learned corrections based on historical Feedback Knock. If the ECU routinely registers knock in a certain RPM and load range, it will apply timing adjustments which can be seen with this parameter.

In general, under high load ONLY we consider Feedback Knock Correction ~-2.8 degrees or less to be “normal” and not of major concern. If DAM is below the ideal value and there are corrections beyond -2.8 consistently, this is good reason for further inspection. If these corrections occur during full boost and wide open throttle it is especially concerning. MIN/MAX MONITORS on an Accessport are not a means of deciding whether the car is running well or not and should never be emailed. A cruise feedback correction of -4.92 off throttle is a lot different than a consistent -4.92 throughout the whole rev range at full throttle. Make sure you know the difference!

Poor quality fuel is a common cause for knock.  Use fuel from top tier retailers in your area to ensure your effective octane is as high as its rating. We recommend Shell V power exclusively. A perfectly running car on Shell V power can pull knock values higher than normal when cheap fuel is introduced (BJ’s/Costco fuel for example). Make a commitment to the fuel from the get go and understand when you switch you may see variation in the tunes knock values until you get back to Shell/Top Tier fuel. Do not bug your tuner for a retune when you make this decision to use cheap fuel. If your fuel is very inconsistent let your tuner know so they build you two maps or a softer map.
Common mechanical causes of knock worthy of inspection are worn MAF or front O2 sensors, boost/intake tract leaks, excessive blow-by, worn plugs and bad fuel. This should be your go-to if the car was running well for a while and then decides to act up. But should be taken care of clearly before any modifying.

2014+ FXT AND 2015+ WRX DIT DAM Strategy Changes

The newer DIT car’s (direct injection turbo – 2015+ WRX and 2014+ FXT) strategy is a bit more reactive. This is intentional and is expected since “premium” fuel is not required on the DIT vehicles.  The strategy has changed and is comparable to what may have been seen as Fine Knock Learning in the earlier non-DIT cars may now result in a drop in DAM in the DIT cars. It shouldn’t be surprising to see a drop in DAM on the DIT vehicles (as it is on early cars where DAM being less than ideal is an indicator there is an issue) especially running lower octane fuels. How long the DAM takes to return to 1 is not indicative of how serious the knock event was and does not mean the car is still knocking. In order to advance the DAM on DIT vehicles, you have to drive in specific load/RPM ranges without knock. Determining whether there is a a potential issue on a DIT vehicle require considering how low the DAM has dropped and what other knock responses (Knock Feedback Correction and Fine Knock Learning) look like overall. It is also common for the DAM and feedback to be high on cruise range on the DIT cars, especially on CVT cars. They were designed in mind to handle the minor knock and don’t need to be close to 1.0 and “0” as much as the EJ2x cars do. If you logged a stock DIT WRX on 91 you would understand what they can take! These cars are very sensitive to fuel quality more so then the EJ so make sure you run top tier fuel all the time, it’s imperative! You will not see a “no knock” perfect DIT tune on pump gas, unless the tuner is giving you 200hp OR desensitizing the knock system in the tune.


Your email is never published nor shared. Required fields are marked *


I'm not spambot *

  1. Great article — wish I had seen this a couple years ago, would have saved myself some stressing.

    Josh Pelland
    February 22, 2018 | 9:59 pm


  • “ Professional, knowledgeable and friendly. The BrenTuning team delivered excellent work and answered all of my questions. Their E-Tune service was a great experience and I would do it again. ”

    Chris Cioffari May 12, 2017
  • “ I highly recommend Bren Tuning. I've gone to see them twice with my 15' STI, first time after getting the access port and installing an intake and last week for GS downpipe, N1 cat back and of course another tune. Couldn't be happier with the results, car runs and sounds even better that I expected. ”

    Troy Cregger May 12, 2017
  • “ Had my 2010 335xi serviced here and I have nothing but good things to say about these guys. I had a lot of work done at once and it all came out perfect. Definitely recommend and you are guaranteed to be pleased with the results! ”

    Dariush Vafaee May 12, 2017
  • “ Just finished getting a e-tune on my 15 wrx and its amazing! Car runs so smooth and has so much power! I highly recommend them for anything you need for your car. I will be taking the 3 hour trip to get more work done. Customer service was great and speedy! ”

    Chris Hernandez May 12, 2017
  • “ I've been working with Bren for over a year now on my 2015 STi. It's not often I find a shop that shares my perspective when it comes to my car. Down to earth, "REAL' car-guys. They treat my car as if it were their own, and do their best to exceed my expectations every time. I also use them for maintenance on my car. Its where I prefer to bring my car, because I know nothing will be overlooked. And when track days come around , they'll be there too, to help and advise me every step of the way. I cant ask for anything more than that. ”

    Mar Salisbury May 12, 2017
  • “ Bren seriously took his time with my tune and it was absolutely amazing. Really worked hard to get it right and when I saw that dyno sheet I almost wanted to cry from joy, power curves were amazing and he ended up getting me perfectly to 337hp 337tq. I couldn't be more happy with numbers like that. Not only was the tune amazing, but the customer service. I had a 330 spot on a Friday, ran into some trouble getting there and wasn't able to arrive until 45 minutes after my appointment. I simply called them, and was told exactly this "hey man no worries we understand, get here whenever you can and we'll make sure we take care of you tonight." To me... That says it all about Bren right there. Excellent work, you've got a lifetime customer here. ”

    Jamie Pereira May 12, 2017
  • “ Had an install and protune done with Bren and the team on my 2015 WRX. Car put out absolutely stunning results! Bren really took time and patience getting the tune right. After the dyno tune he sat with me for a ride along to make sure everything was to my liking. Great team, very friendly, down to earth and professional! ”

    Vahe Keshishian May 12, 2017
  • “ Got my 2015 Wrx e-tuned by Brentuning and could not have been any better. All my questions and concerns were answered in a timely fashion along with the tuning process itself. Car is now so much more drivable with the rev hang removed, pulls much smoother, and pulls way harder all the way to redline. ”

    Perry Alpert May 12, 2017
  • “ Hands Down...The BEST Customer Service Ever. I have a Stage 2+ 2012 STi and worked with BrenTuning to have a Protune added. The car feels completely different. There is significantly less delay in response time. The boost holds much better and is a much smoother pull. I can also feel a significant gain in tongue. Their response time was incredible though. Communication was done through email and the turn around time was instantaneous, literally felt like texting. I would highly recommend BrenTuning for great work and outstanding customer service. ”

    Ben Weber May 09, 2017
Competition use only parts are clearly labeled as competition use only in the product listing page and may only be used according to the guidelines posted on our Emissions Notice to Customers in California page