What is Engine Knock

What is Engine Knock

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Engine knock is also known as pre-ignition or detonation. There’s several common reasons what cause knock in your engine. When your engine is running, there’s a certain mixture that your engine computer tries to achieve. This mixture is known as the stoichiometric ratio, or in easy to understand terms 14 to 1 air / fuel ratio. While driving your late model car around town, this is also known as closed loop operation.

Any gear head or automotive enthusiast wants his engine to make more power. The basics of an engine are easy enough to understand. Your engine works on a four cycle combustion cycle, and uses this kinetic energy to produce torque and horsepower. To increase horsepower you want to increase the rate in which your engine consumes and expels air.

You’ve heard the rudimentary comparison of a combustion engine to an air pump. In it’s very base terminology this is an accurate metaphor for a four cycle combustion engine. What you don’t hear people talk about the most is of course ignition timing. Ignition timing is very different than your cylinder head or valvetrain timing, although it’s mixed up all the time.

Setting your base ignition timing is something that everyone has had done. While this is an important part of tuning your engine, it’s just one part of the timing equation.

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Ignition timing is the the process of igniting the air fuel mixture in your combustion chamber. The timing aspect of your ignition controls when your spark is delivered.

From a mechanical standpoint ignition is delivered close to the end of the compression stroke. Because combustion time and space to be efficient, ideally you want your spark to be delivered before this compression point.

This process is called advancing your ignition timing, and it can be done a variety of ways. Also known as setting your spark Before Top Dead Center (BTDC) this is done with the sole purpose of increasing power. Spark that’s delivered After Top Dead Center (ATDC) can cause engine knock and severe damage in your engine.

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In the simplest terms possible, any performance engine makes the most horsepower at the most efficient rate possible when your ignition spark is advanced as far as safely possible.

To safely tune your vehicle, you want to adjust your fuel mixture in your engine as close to stoichiometric mixture as possible, and advancing the spark to the point just before detonation would occur.

This is because advancing your spark too far leads to engine damage. When the air fuel mixture is ignited at the wrong time, horsepower is lost and fuel is wasted. This occurs because the detonation in your combustion chamber causes most of the kinetic energy to escape through the exhaust or cooling system. This is the complete opposite of what you want happening when you are trying to make more horsepower.

Even worse the spike in cylinder pressure at the wrong time can cause catastrophic damage to your engine. Concussive explosions of engine misfire can destroy ring lands, blow away bits of piston rings and even punch holes through your piston crowns. Prolonged engine misfire at high engine speed and you run the risk of melting your pistons before you even have a chance to figure out what is going on.

engine knock 4Before the age of modern engine computers and knock sensors, cars used distributors to set their base timing. Because of the limitations of this technology, ignition distributors had to be set very conservatively.

This was due to the fact that ignition distributors advanced spark using a combination of mechanical and vacuum advance controls. Needless to say this is a very crude method of controlling ignition and because these were wear items, it meant that spark advanced moved as the engine become older. This is why many people who still own ignition distributor controlled or carburetor equipped vehicles often install new points and wonder why there is so much engine knock.

With the advent of engine computers and subsequent piggyback tuning computers, this has changed the way cars operate quite a bit. This technology allows automotive engineers to come closer to the danger zone to increase power and torque. Nowadays people can install a piggyback and wire it inline of their Powertrain Control Module and tune it to gain even more horsepower.

knock sensor
How To Replace your 350Z knock sensor

Even with ignition events perfectly timed however, engine knock can still be a huge problem. Variations in fuel quality, atmospheric changes, driving habits, and engine condition still have big roles to play. To combat this automotive engineers needed a device that could detect the onset of detonation, otherwise known as today’s knock sensor.

While the common knock sensor in your vehicle may work in many different ways, the most common is known as the piezoelectric sensor. Using a special ceramic casing this sensor produces voltage when they are put under pressure or strained. This kind of ceramic is also common in microphones and other audio devices.

In an automotive sense the knock sensor uses a self contained slug of piezoelectric ceramic. One end of this ceramic sensor is attached to the engine housing and the other is connected to a smaller weight. When this sensor is shaken by the vibration or the sound of engine detonation, it produces an electric signal.

When your engine computer receives this signal, it scales back your ignition timing. Other kinds of knock control go a step further, by shutting down cylinders or fuel injectors. Either way the knock sensor is there to help you and your engine. When your knock sensor is bad or engine knock is becoming a problem, check here for our collection of detonation tech tips and guides.

Have any questions about our FAQ sheet on engine knock? Want to learn more about tuning your vehicle or changing your knock sensor? Subscribe to My Pro Street for all the latest tech tips and How To guides.

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