If your Subaru WRX, WRX STI, Forester XT, or other turbocharged model is tuned and you have a Cobb AccessPort, you probably monitor some parameters related to knock, or at least have heard that you should. While many owners are worried when they see any knock, this isn’t always a concern. In this article, we’ll explain how Subaru’s knock control system works, and how the various parameters are related.
While many people may be alarmed to see any knock showing on their AccessPort, some occasional knock is normal and not a cause for concern. The ECU is designed to keep the engine safe under varying conditions that may cause knock. Things like fuel quality and variation in ethanol content, altitude, and heat all affect how the engine runs. Under different conditions, some knock is bound to happen, and the ECU is designed to handle it.
There are three parameters related to knock control that most WRX owners monitor on their AccessPort. These are feedback knock, fine knock learning, and dynamic advance multiplier (otherwise known as DAM). We’ll explain what each of these mean below and how they work together.
On older Subarus, it is helpful to think of these as progressively broader responses. The 2015+ FA20DIT and FA24DIT strategy is not always a strict sequence, however. The ECU can choose among feedback correction, fine learning, and a DAM change based on RPM, engine load, and other operating conditions. COBB explains this difference in its current DIT knock-control documentation.
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How Does a Subaru Knock Sensor Work?
A knock sensor does not directly identify combustion knock. It is a piezoelectric vibration sensor bolted to the engine block. Engine vibration flexes the piezoelectric element inside the sensor, which produces a small electrical signal for the ECU.
The ECU evaluates that signal during defined RPM and load conditions and compares the measured noise with calibrated thresholds. When the signal exceeds the applicable threshold, the ECU treats it as a knock event and may remove ignition timing. This is why the AccessPort reports the ECU’s response to perceived knock, not a direct measurement of cylinder pressure or engine damage.
The sensor also hears normal mechanical vibration. An air-conditioning compressor engaging, a loose heat shield, a rattling bracket, or another impact can resemble knock closely enough to cross the threshold. This is usually called false knock. The load, RPM, throttle position, boost, and repeatability of the event are therefore more useful than one isolated negative number. COBB’s Subaru table documentation describes how sensor noise is compared with cylinder-specific knock thresholds.
Feedback Knock
Feedback knock is the real-time correction most commonly seen when the ECU responds to a detected event.
When the ECU detects knock based on the signal from the knock sensor, you’ll see this registered in feedback knock. The value refers to degrees of ignition timing advance.
The feedback knock correction will often start with a small value, such as -1.41 degrees, and keep retarding timing if the ECU continues to detect knock. The displayed correction is the amount of timing removed, not a direct measurement of knock intensity. Once the ECU no longer detects the event, it returns the correction toward zero in gradual steps.
It’s normal to have occasional knock that doesn’t follow any sort of pattern, and the knock correction system is designed to handle that and keep your engine safe. Generally, seeing occasional feedback knock is nothing to worry about. Sometimes this is false knock caused by the air-conditioning compressor or something loose in the engine bay. Values of -1.41 or -2.82 are common while cruising.
If you see higher values like -4.20 or -8.44, this might be nothing if it’s a very occasional random event. If you see large feedback knock values frequently, there may be a problem. It could be low quality gas, or there could be a problem that needs to be addressed with the car physically or in the tune.
Avoid requesting full boost at low engine speed, especially in a high gear. Low-speed pre-ignition is an abnormal combustion event that can occur before the commanded spark and can be much more damaging than ordinary knock. The knock sensor may react to the resulting pressure and noise, but an Accessport correction does not identify LSPI by itself. Downshift instead of flooring the throttle when the engine is below the useful boost range, and follow the calibrator’s guidance rather than treating one RPM threshold as universal.
When the ECU detects a pattern rather than a one-time event, it may store a correction for that RPM and load range. That’s where fine knock learning comes into play.
Cobb AccessPort mounted in a 2017 Subaru WRX
Fine Knock Learn
If your ECU detects that knock is happening more frequently with some pattern, then it will apply fine knock learning. This value also indicates a number of degrees in ignition timing. The logic is that if knock is likely to happen under some combination of calculated load and RPM, then the ECU should pull some timing before it happens. This is better than waiting until knock happens, and then and using feedback knock correction.
Like feedback knock, it’s not always a concern to see fine knock learn. It can be totally normal depending on the temperature, fuel quality, and your tune. If you’re seeing feedback knock and you don’t normally, first try filling up at a different gas station. It may be fuel quality that’s causing the knock.
It’s common to see fine knock learn, especially with off the shelf tunes, like the ones from Cobb and MAPerformance. While these tunes are a great improvement over the stock one, they’re not tailored specifically to your car. A custom WRX dyno tune or e-tune can account for the individual car and fuel, especially with stage 2 and beyond.
When fine knock learn correction is being applied, the ECU will gradually test if it’s safe to return to normal by gradually advancing the ignition timing.
To clear the learned values, you must drive while hitting each RPM and load range that the ECU has recorded a value for. If it sees feedback knock again, it will keep the fine knock learn values. If you reset the ECU, these values will also be cleared, but some learned correction is normal. If there is a real problem, it should be addressed before resetting the ECU.
The ECU can also reduce the dynamic advance multiplier when its knock-control conditions call for a broader timing adjustment.
Dynamic Advance Multiplier (DAM)
The dynamic advance multiplier is a broader learned part of the ECU’s knock-control strategy. This value is not a number of degrees, but a multiplier. Reducing DAM removes some of the timing available through the dynamic-advance tables across a wider operating range.
The ECU has a table of dynamic advance values that determines how much ignition advance should be used. The current dynamic advance value is multiplied by the DAM. After this value is calculated, feedback knock correction and fine knock learn are added to get the final timing advance value.
With most flashed tunes, the DAM value starts at its max value. For most Subarus, including ones with an EJ25, or ones with an FA20, DAM ranges from 0 to 1. For 2.0 EJ models, it ranges from 0 to 16. (Source: Cobb Tuning). With the stock tune, this value starts below the maximum and works its way up when no knock is detected. On these 2.0 EJ models, DAM is sometimes referred to as IAM (ignition advance multiplier).
If the DAM drops below the default value, then it’s something you should at least investigate. Don’t be too alarmed, this doesn’t mean your car has a serious problem necessarily! You should try to empty your gas tank and fill up with a known good source, and maybe check that all your bolts are tight if you’ve installed aftermarket parts. If you have an intake, then you could check if your AF Learning 1 (fuel trim) is inside of normal ranges and that everything is securely fastened.
Once the DAM has dropped and stabilized, the ECU will start to test if it’s safe to raise back up gradually. You’ll see positive values for your fine knock learn, showing how the ECU testing if advancing the timing is safe. When it does this, after a few attempts, if it doesn’t detect any knock, it will start to raise the DAM up. While an ECU reset will clear DAM, it’s not recommended to do this unless you’ve addressed the reason for the knocking.
Timing Advance Calculations
The final timing advance is calculated based on all of the three parameters we’ve described. Based on the logic described above, here are the equations the ECU uses to calculate how much timing advance should be used.
If your DAM ranges from 0 to 1 (most models), the current timing advance calculated like this:
Timing advance = (current dynamic advance max * DAM) + feedback knock + fine knock learn
For models where DAM ranges from 0 - 16 (2.0 EJ models), it’s slightly different, since DAM is not represented as a decimal value:
Timing advance = (current dynamic advance max * (DAM/16)) + feedback knock + fine knock learn
Closing Thoughts
Do not diagnose the engine from one isolated correction, but do not dismiss a repeatable pattern either. Record the RPM, load, throttle, boost, fuel, temperature, and whether the event repeats under similar conditions. Stop making hard pulls when corrections recur under load, DAM falls unexpectedly, the engine misfires, or the car develops new noise or drivability symptoms. Check the fuel and hardware, then send a complete log to a qualified Subaru technician or tuner. On an EJ255 or EJ257 with oil use, smoke, or a cylinder-specific misfire, follow the ringland-failure testing guide rather than treating AccessPort data as a mechanical diagnosis.
Frequently Asked Questions
What causes feedback knock?
Feedback knock can happen randomly as a one-time event due to varying conditions in the environment. Bad quality gas can also cause knocking. Mechanical issues that may cause knock include bad MAF or front O2 sensors, boost leak, and excessive blow-by.
What does positive fine knock learn mean?
Positive fine knock learn means your car’s ECU is testing to see if it’s safe to raise the DAM. It does this after the DAM has dropped and stabilized. It then tests a more aggressive timing advance, and if no knock is detected, it will raise the DAM.
What should AF Learning 1 be at?
AF Learning 1 should generally be +/- 8%, but the closer to 0, the better. AF Learning 1 measures how much your ECU has learned to compensate for fuel to reach the target air-fuel ratio.
What is ignition timing advance?
Timing advance refers to the number of degrees before the piston reaches top-dead-center that the spark will ignite. Igniting the air-fuel-mixture when the piston is closer to top-dead-center helps to reduce knock at the cost of power.
What does advancing the timing mean?
Advancing the ignition timing means the spark is earlier, further from top-dead-center. This gives more horsepower but makes knock more likely.
What does retarding the timing mean?
Retarding the timing means the spark happens closer to top-dead-center. This reduces the chance of knock but also reduces the power output. When knock is detected, the ECU retards the timing.
What is low speed pre-ignition?
Low speed pre-ignition is an undesired early combustion event that direct-injected engines are susceptible to. It occurs during low engine speeds and high loads. You can reduce the risk by waiting until higher RPM before accelerating heavily. Use an oil with the viscosity and specifications required by Subaru; current API SP oils include an LSPI test and supersede the older SN Plus category. Our WRX oil guide explains the current options.