Understanding the knock control system- 2015+ wrx fa20
Understanding Knock in the 2015+ WRX: Common Questions Answered
Knock Is Inevitable – Your Car Will Never Be Completely Knock-Free
Every engine experiences knock to some degree, which is why modern vehicles are equipped with knock control systems. If knock could be completely eliminated through tuning alone, manufacturers wouldn’t invest in knock sensors—they’d simply program it out. However, factors like fuel quality, environmental conditions, and driving style are always changing. The ECU is designed to adapt to these fluctuations, adjusting knock control parameters accordingly. As a result, the car is constantly learning and relearning knock values, which may sometimes result in the Dynamic Advance Multiplier (DAM) dropping as a precaution.
Knock Sensors Don’t Detect Knock Directly – They Detect Noise
Contrary to popular belief, knock sensors aren’t specifically tuned to detect knock itself; instead, they act as broadband frequency detectors that monitor noise levels. The ECU is programmed to interpret these noise patterns, and if a certain threshold is exceeded, it responds by pulling ignition timing.
Common non-knock-related noises that can trigger the sensor include:
• A loose exhaust or rattling components
• Hitting potholes or bumps in the road
• The air conditioning system cycling on or off
• Engine bay connectors (such as TGV/EGR connectors) bouncing around
Because of this, occasional knock readings on a data log don’t always indicate an actual problem.
How the ECU Manages Knock: DAM, FBKC, and FLKC
The ECU relies on three main strategies to control knock:
1. Dynamic Advance Multiplier (DAM)
2. Feedback Knock Correction (FBKC)
3. Fine Learning Knock Correction (FLKC)
The choice of which method to use depends primarily on engine load and RPM, rather than the severity of the knock event itself.
• DAM (Dynamic Advance Multiplier): Acts as a global timing adjuster primarily for wide-open throttle (WOT) conditions, particularly in the 2,000-4,500 RPM range as the car begins to build boost. If the ECU detects knock during this phase, the DAM may drop as a preventative measure to globally reduce timing under high load conditions.
• FBKC (Feedback Knock Correction): Typically addresses real-time knock events. If knock is detected, FBKC temporarily reduces timing to mitigate the issue.
• FLKC (Fine Learning Knock Correction): Functions as a long-term learning tool. If the ECU records knock at a specific RPM and load range, it stores that adjustment and references it each time the car passes through those same conditions.
Interpreting Knock Data on an Accessport or Log
• If DAM drops, it means the ECU has detected knock under higher load conditions and is globally reducing timing as a safeguard.
• FBKC values appear when knock occurs at a given moment, reducing ignition timing accordingly.
• FLKC values persist in the ECU’s memory but are not active knock events. If an FLKC value of -2.8 appears at 3,000 RPM, it doesn’t mean the engine is knocking continuously—it means the ECU has learned that knock occurred previously and is applying that correction until it can verify conditions are stable again.
Knock correction values gradually phase out over time once the ECU determines that knock is no longer occurring. This process may appear as repeated knock entries in a log, but in reality, it’s just the ECU progressively restoring timing adjustments back to baseline.
When Should You Worry About Knock?
• Mild knock adjustments at low loads are normal. A -1.41 to -2.8 FLKC reading during cruising is common, especially in cooler months. During hotter weather, -3 to -4 readings may occur more frequently, sometimes alongside minor DAM fluctuations.
• A single knock event may appear in logs multiple times due to the way the ECU “unlearns” stored FLKC values over time. A single -1.41 knock event might take 15-20 logging cycles to completely clear out.
• If your DAM drops to 0.312 or lower, it’s an indication that knock is persistent and severe enough for the ECU to make a significant global timing adjustment. In extreme cases, logs might show -7° FBKC or more, meaning the engine is pulling timing aggressively to prevent damage.
However, occasional FBKC or FLKC readings alone aren’t necessarily a sign of trouble. Unless the car exhibits poor performance, misfires, or unusual noises, these adjustments are simply the ECU doing its job.
Resetting the ECU Too Often Can Be Counterproductive
If the ECU has learned knock through FLKC and you frequently reset it, the learning process starts over—meaning the car will have to experience knock again for the ECU to relearn how to compensate. Unless you’ve made mechanical changes or resolved a clear fueling/timing issue, frequent ECU resets can create unnecessary fluctuations in engine behavior.
Modifications That Can Reduce Knock (only For non-US wrx owners)
Two of the most effective upgrades for minimizing cruise knock and tip-in knock are:
• TGV (Tumble Generator Valve) Deletes
• EGR (Exhaust Gas Recirculation) Deletes
These mods improve intake airflow and significantly lower charge air temperatures substantially.
Under low-load cruising conditions (2,000-5,000 RPM), the EGR system injects hot exhaust gases into the intake manifold, raising air temps and making the engine more prone to knock. If conditions become too unfavorable, the ECU might pull -8 to -10° FBKC due to excessive intake heat. By eliminating the EGR system, the intake manifold remains cooler, reducing the likelihood of knock under light load conditions.
Ethanol fuel/blends are also a great option to help mitigate knock for any setup if you have it available in your region.
Example of Knock on a Stock 2015+ WRX Tune
Logs from a stock WRX running 91-octane fuel show that factory tunes often exhibit frequent FBKC and FLKC events. In one example, the DAM dropped to 0.312 from 1.0, meaning the ECU was running with only 30% of the normal ignition advance due to excessive knock. The same log also recorded -7° FBKC and mild FLKC adjustments, showing how aggressive the stock ECU strategy can be when it detects persistent knock.
Final Thoughts: Don’t Panic Over Normal Knock Events
Unlike in movies where cars immediately explode from “engine knock,” real-world knock control systems are designed to handle occasional knock without causing damage. If knock has already appeared in your logs, it means the ECU has already taken action to mitigate it.
The real concern arises when:
• DAM drops significantly and stays low
• FBKC consistently shows high negative values (-7 or worse)
• The car exhibits poor performance, hesitation, or audible knock
Otherwise, mild knock correction is just the ECU doing its job to maintain safe and efficient performance.
Additional Resources
• Innovative Tuning: Critical Knock Control Differences in Subaru EJ vs. DIT Engines