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Understanding Detonation

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To understand what detonation is, you first have to know what is going on inside the combustion chamber. I should hope that if you are interested enough to read this article that you would already know such things; however, I will go into it a little bit. Detonation occurs during the power stroke.  This means the piston has returned to the top, compressing the gas and fuel mixture, the spark plug has ignited this mixture and the piston is now on its way down. This is what makes the engine run, hence, ?power stroke?. During a normal power stoke, the spark plug ignites the fuel, which burns creating pressure in the cylinder which forces the piston back down again. The ideal way for this to take place is not in an actual explosive form, but more like a whoosh. Think of the effect you would have on a piston by smacking it with a hammer.  You could give it a good hard whack but chances are it wouldn?t travel very far, and also receive some damage in the process. Now, think of your hand pushing the piston down with even force the whole way to the bottom.  You wouldn?t have as much energy released in a single moment like you would with the hammer, or explosion, but you would sure get a lot more power over the entire stroke.

Ok, so, now you know what happens during the power stroke of the engine, and the ideal conditions of that power transfer. Detonation, sometimes referred to as a knock, occurs when the air fuel mixture in the combustion chamber burns abnormally. By abnormally I mean in an explosive manner. If you could somehow slow down the burning of gases in the combustion chamber, you would first see the spark plug fire igniting the gasses. These gases would begin to burn outward filling the combustion chamber and forcing the piston down. Late in the cycle, but before the flame has reached the cylinder walls you would see small, bright flashes of light in front of the flame front. This is the last bits of the air fuel mixture burning by themselves in an explosive form. In fact, the tiny explosions that make up detonation can burn at the local speed of sound. This creates a shockwave that hits parts and makes them ring audibly, and can even in the worse case, blast heat-softened aluminum out of pistons or the head.

The physical cause of detonation in the combustion chamber is the prolonged or excessive heating of the fuel and air, which breaks down the molecules into a form that can ignite by themselves, and then burn with tremendous speed. Higher compression ratios, resulting from turbo charging or supercharging an engine, or introducing extra oxygen into the combustion chamber by running Nitrous, are usually to blame for detonation. However, even naturally aspirated engines running high compression ratios are not immune to its effects. Normally, compression in these engines is raised to just below the point of detonation or to the point in which it is simply not efficient to tighten the combustion chamber any further. This is where higher-octane fuel is used. The higher the octane rating in the fuel, the slower it burns, and thus is less receptive to being changed into a auto-ignition state.

Now, you probably want to know how to check for detonation. Besides using your ears, you can also read the spark plugs. You may want a magnifier and the brightest light you can find, perhaps the sun is a good choice. What you are going to be looking at is the center wire of the spark plug. You want to do this test with a pretty fresh set of plugs that have been run hard for a short time. First check for deposits on it.  If they are there and have not been blown off, you?re in pretty good shape. If you run the engine hard and the wire looks completely bare but still resembles a new plug in good shape you're getting close to detonation. When the edges at the end of the center wire become rounded, like a broken glass rod you are at risk for some serious engine damage. Remember, this is to be done with new plugs; of course, a plug with 50k miles on it will start to round off from normal use. Do not mistake this for reading the plugs insulator. That is for judging the fuel mixture. You can also see the affects of detonation inside the combustion chamber itself. This comes in the form of sand blasting usually on the edges of the piston, a matte finish look instead of the normal light tan or cream-colored deposits. Another symptom of detonation is the rise in temperature of engine coolant. This is caused because as detonation occurs it blasts away the tiny film of stagnant gas that sticks to the combustion chamber surfaces. This film, sometimes called the boundary layer helps insulate the chamber surfaces from heat. Without it, more heat transfer takes place between the cylinders and coolant.

A lot of people confuse pre-ignition and detonation.  Really though, they are forms of one another with slight differences. Pre-ignition is generally referred to as the air fuel mixture being ignited prematurely by a hot spot in the combustion chamber. This could be a glowing build-up of carbon, too hot of a plug, or a hot spot on the piston. This is what you hear 'most of the time' as a pinging sound. The fuel is starting to ignite as it is being drawn into the combustion chamber or soon after the upstroke begins, but before the spark. On the other hand, detonation occurs 'after' the spark has happened, and is usually considered a 'knocking' sound. A slight ping, although not good for the engine, is not as harmful as detonation. Sometimes detonation can occur, but be so small that it doesn?t catch up with the piston on its downward stroke. You might not even be able to hear this! It can however, still cause significant damage.

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