Detonation and Knock: What It Is and How to Prevent It in Your HEMI

What Is Detonation?

In a properly functioning gasoline engine, the air/fuel mixture ignites when the spark plug fires. The flame front travels smoothly across the combustion chamber in a controlled burn.

Detonation (also called knock or ping) is uncontrolled combustion — the compressed mixture ignites spontaneously before the spark plug fires, or secondary ignition points cause multiple flame fronts colliding in the chamber. The resulting pressure spike creates a sharp metallic knock sound and, more importantly, enormous mechanical stress on the piston, rings, and rod bearings.

A single detonation event is usually harmless. Sustained or severe detonation can crack piston ring lands, damage piston crowns, and in extreme cases destroy connecting rods — catastrophic engine failure.

What Causes Detonation in the HEMI

Low-octane fuel: The primary cause. Running 87 octane in a 6.4L HEMI tuned for 93 provides insufficient detonation resistance under high cylinder pressure. At wide-open throttle, the mixture can auto-ignite.

Excessive ignition timing advance: If the PCM (or a tuner) advances spark timing too aggressively for the current fuel quality, the mixture ignites too early. Stock PCM is conservative. Aggressive tunes that aren't matched to fuel quality cause knock.

Carbon deposits: Over time, carbon builds up on intake valves and piston crowns (especially common on direct injection engines). Carbon retains heat between combustion cycles, creating hot spots that can trigger pre-ignition.

High intake air temperature: Hot, dense air ignites more readily. A heat-soaked short ram intake or a badly routed cold air intake can raise IAT enough to cause knock at WOT.

Engine overheating: Coolant temperature above 230°F raises combustion chamber temps significantly, reducing knock threshold.

The PCM's Knock Protection

The 2022 Challenger's PCM has knock sensors on both banks of the engine. When knock is detected, the PCM retards ignition timing on the affected cylinders by 2–4 degrees immediately and gradually advances back once knock stops.

This is your safety net, not your plan. Consistently relying on PCM knock retard means you're making less power (retarded timing = less peak cylinder pressure = less power) and still experiencing some detonation.

How to Prevent Detonation

Use the correct octane fuel: 91 minimum for the 6.4L. 93 if your tune is calibrated for it. Never use 87 in a HEMI under load.

Don't push a hot-soaked engine: After sitting in traffic on a hot day, avoid WOT pulls for a minute or two. Let the IATs drop.

Install a quality closed-box cold air intake: Keeps intake air temperature significantly lower than a short ram, reducing knock risk at high loads.

Monitor with gauges or a data logger: HP Tuners, Diablosport's data logging feature, or an OBD-II scanner app with knock retard parameter visible lets you see in real time how much the PCM is pulling timing. If you're seeing 4+ degrees of knock retard consistently under WOT, something needs addressing.

Get a proper tune: An aggressive canned or remote tune on 87 octane fuel is asking for trouble. Always run the fuel your tune was calibrated for.