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.

AND MOST IMPORTANTLY IF ANY OF THIS IS GIBBERISH TO YOU TRUST YOUR TUNER YOU HIRED TO DIAL IN THE CAR TO THE BEST OF THEIR ABILITY.

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  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
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