It goes by several names — knock, pinging, detonation, etc., and many of the terms can make the event seem rather innocuous. The truth is, under moderate to high loads consistent knock counts can cause catastrophic engine failure, usually in the form of crushed rod bearings, cracked ringlands, or a hole in your piston.
Just about every moderate to high power spark-ignition engine will experience random knock events throughout its service life. It’s one of those things that can never be completely avoided, but can still be easily controlled and kept within a safe limit by way of sensors and responsible tuning. To start, it helps to understand what is happening inside the combustion chamber to cause that destructive pinging sound.
What It Is
Depending on engine load and tuning, the spark plug fires anywhere between 45 degrees down to 5 degrees before top-dead-center (BTDC) of the compression stroke and ignites the air/fuel mixture.
During a normal combustion cycle, the flame front expands from the point of ignition out towards the cylinder walls and piston crown, this burn process can take up to 90 degrees of crankshaft rotation to completely burn. Detonation is identified as any spontaneous combustion occurring after the burn process has already begun, and is independent of the initial flame front. This uncontrolled event can originate from anywhere within the chamber and is usually caused by high cylinder temperatures and or pressure.
What It Does
Now that you have a basic understanding of knock and its two main causes (heat and pressure), we can talk about the damaging effects associated with it. Damage is not caused by the energy released by detonation, but rather the rate the energy is released (the energy potential is the same as a normal combustion cycle). Detonation is often considered to be the equivalent of hitting the top of your piston with an explosive hammer.
How To Detect It
When a knock event occurs, an audible pinging sound can be heard. On your average EFI engine, knock detection is relied on by the use of one or more knock sensors installed in specific locations on the motor. These sensors are basically a type of microphone that is calibrated to pick up the specific range of frequencies known to be associated with detonation. When the sensor detects a high enough knock count, the ECU will begin to retard ignition timing and or add more fuel, depending on the ECU used.
These knock frequencies will vary by engine design and will also need to be recalibrated by your tuner after heavy engine modifications.
The Australian aftermarket engine management company Haltech created a great video explaining this calibration process:
Professional tuners often rely on the use of detonation cans (det cans) to detect knock events on highly modified engines. These det cans can be either electronic or mechanical, the former using a microphone to transmit the audio through a pair of headphones, and the latter just using a copper mount and tubing to transmit the sound picked up by the copper to the headphones. Det cans can also help to expedite the knock sensor recalibration process.
How To Control It
When tuning an engine there are two primary sources of heat and pressure, fueling and ignition timing.
Ignition Timing To Control Pressure
When tuning ignition timing, you have to be mindful of how much you advance the timing — more timing doesn’t always equal more power. The idea is to time the spark at just the right moment BTDC to allow enough burn time for peak cylinder pressure to occur at the optimum point ATDC.
Over advanced ignition timing will cause the spark to occur too early, forcing the pressure within the cylinder to build faster than the flame front can spread. This will create two sources of cylinder pressure working at the same time (piston travel and combustion), causing the cylinder pressure and temperature to surpass the autoignition point of the unburned fuel still left in the cylinder, burning it off instantly. This spontaneous combustion is the detonation event and is one of the most common causes of piston, rod, and bearing failures.
Fueling To Control Temperature
When tuning an engine, fuel is used as a form of temperature control. Adding more fuel creates a richer mixture and cools the chamber, removing fuel leans the mixture out and adds more heat.
Haltech provides a great analogy to help you understand this process. “Think of it as baking a cake. When you’ve finished baking, you open the oven and pull the cake out to cool. The air inside the oven is 180 degrees C, so the cake and the steel cake tin are both 180 degrees, yet putting your hands in the 180 air doesn’t burn you. The metal cake tin however, certainly does burn your hands as does the cake itself after a couple of seconds.”
What you want to take away from this is that air is a terrible thermal conductor, because the 180 degree air in the oven will not burn you like the cake tin would at the same temperature. Fuel is a much better conductor of heat — so in general terms, the more fuel you add, the more heat that will be pulled away from the cylinder walls, pistons, valves, etc.
It is possible to add too much fuel though, especially in low load areas like idle or cruise, and you can cause knock or even melt a piston if you are too lean at higher loads. It’s the tuners job to calibrate multiple different engine components (MAF, VE, VVT, Boost, Fueling, Ignition Timing, etc.) to achieve the most efficient fueling and ignition timing targets for the engine and its specific modifications.
Detonation can cause catastrophic engine failure if left unchecked. That’s why most modern engines incorporate a failsafe in the factory tune to retard ignition timing or add fuel when a knock sensor detects too high of a knock count. To prevent knock on modified engines, a tune is required to adjust the factory calibrations and bring your engine into equilibrium with your new mods.