What is Knock? With Harvey from The Boostcreep

Here is a summary of the key points discussed in this episode:

What is Engine Knock? [00:43]
Abnormal Combustion: Normal combustion starts with a spark and results in a smooth flame front propagating at a controlled speed to push the piston down. Knock (or detonation) is abnormal combustion where the air/fuel mixture burns all at once or in unpredicted, localized areas inside the cylinder.

The Danger: Instead of a smooth pressure rise pushing down the piston, knock creates localized pressure spikes and shockwaves that "slap" the piston or cylinder head. These events can severely damage engine internals.

Frequency and Noise: The shockwaves from knock events travel through the metal of the engine and create a specific audible frequency or "ringing" tone based on the engine's mass and bore size.

How Does a Knock Sensor Work? [11:00]
It's a Microphone: A knock sensor is essentially a piezoelectric microphone bolted to the engine block. It doesn't actually sense the combustion event directly; it "listens" for the specific frequency tone that knock generates.

False Knock: Because the sensor just listens for a specific noise frequency, other things can trigger it (false knock). Loose bolts, shifting hard, solid motor mounts, twin-plate clutches, or even hitting bumps/gravel can send vibrations through the block that mimic the frequency of engine knock.

ECU Filtering: Subaru knows that the engine is inherently noisy. The ECU applies logic and filtering to decide if the noise the sensor picked up is a legitimate knock event or just drivetrain/mechanical noise. For example, cylinder 4 tends to log hundreds of "knock sums" simply because the sensor is bolted directly above it, yet the ECU ignores much of it knowing it's not real knock.

Understanding Accessport Knock Parameters [15:20]
When monitoring your engine via the Cobb Accessport, tuners usually focus on three main parameters:

Feedback Knock: This represents a real-time, reactive timing adjustment. When the ECU hears a knock event, it instantly pulls timing (e.g., -1.4 or -2.8 degrees) to protect the engine. Once the knock noise goes away, the ECU quickly tapers the timing back to normal.

Fine Knock Learning (FKL): This is a predictive/learned response. If the ECU sees consistent knock under specific RPM and load conditions, it will "learn" that zone and preemptively pull timing the next time you enter that same load/RPM range, even if it hasn't heard knock yet. FKL will gradually reduce its timing pull over time if it stops detecting knock in that zone.

Dynamic Advance Multiplier (DAM): This is an overall indicator of engine happiness and fuel quality. Tuners layer their maps: there is a base timing table and a dynamic advance table (a "bonus" timing table). The DAM dictates how much of that bonus timing the engine is allowed to use.

On modern ECUs, a DAM of 1.0 means the engine is happy and utilizing 100% of its available timing.

If the ECU detects excessive or consistent knock (or bad fuel), it will drop the DAM (e.g., to 0.5), universally lowering timing across the entire rev range to protect the engine.

How to Properly Interpret the Data [25:25]
Don't Freak Out Over Minor Numbers: It is completely normal for a healthy engine (especially on lower octane like 91 pump gas) to occasionally record -1.4 or -2.8 degrees of Feedback Knock or Fine Knock Learning during daily driving, shifting, or light cruising.

Look at the DAM: If you suffer from "data anxiety," just watch the DAM. If the DAM remains at 1.0 (or 16 on older 16-bit ECUs), the engine is fundamentally happy. If the DAM drops, that is the ECU signaling that the overall fuel quality is poor or a larger mechanical issue exists.

How to Actually Test for Knock: The best way to check your tune is to put the car in 3rd or 4th gear at a lower RPM (e.g., 2,800 RPM), apply smooth, wide-open throttle, and pull to redline. You want the knock values to remain at 0.0 while deep in boost. Random -1.4 readings while casually shifting gears or tapping the throttle in traffic are usually just false knock or drivetrain noise.