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Got this from a post Nazman did on Mighty




As you probably have already figured out, detonation (AKA: "knock") is a big issue in the world of automotive performance. You probably know that detonation is a bad thing, and you must take additional measures to avoid detonation, especially if your engine has other modifications. Normally the simple solution to stop detonation is to run higher octane fuel... but before we get ahead of ourselves, let's start from the beginning.

What is Detonation (Knock)?

Under normal conditions, the combusting air and fuel mixture inside the combustion chamber ignites in a controlled manner. The mixture is ignited by the spark, normally in the center of the cylinder, and a flame front moves from the spark towards the outside of the cylinder in a controlled burn. Detonation occurs when air and fuel that is ahead of the flame front ignites before the flame front arrives because it becomes overheated. Under these conditions, the combustion becomes uncontrolled and sporadic and often produces a pinging noise or a "knock" noise when the conditions become worse.

Why Detonation is bad?

Detonation is definitely not cool. Detonation causes sudden pressure changes in the cylinder, and extreme temperature spikes that can be very damaging on engine pistons, rings, rods, gaskets, bearings, and even the cylinder heads. Even the best engine components cannot withstand severe detonation for more than a few seconds at a time. More severe detonation obviously leads to more severe forms of engine damage. If there is enough heat and pressure in the combustion chamber, detonation can begin to occur before the spark plug even fires, which would normally initiate the combustion. Under these circumstances, known as "pre-ignition", the piston may be traveling up towards a wave of compressed, exploding gas. These are the worst kinds of detonation conditions, and can bend con-rods and destroy pistons.

What Causes Detonation?

Detonation occurs when several conditions / factors inside the combustion chamber exist at the same time. Increased compression, high temperatures, lean fuel/air mixture, advanced ignition timing, and lower octane fuels are all factors that PROMOTE detonation conditions. The good news is that, because there are so many factors in play, you can always find a way to eliminate detonation if it exists.

Let’s Use A Supercharger For Example

A supercharger increases the amount of air inside the combustion chamber, which in turn increases the compression inside the combustion chamber. Along with increased compression comes higher temperatures and higher pressures, which as we know, tend to increase the chances that some form of detonation will occur. In order to compensate for the increased compression and heat, we must change one or more of the other factors / conditions to move us away from our detonation threshold. Tuning the supercharger system to the engine in this way for maximum performance without detonation is something that supercharger manufactures do so, chances are, you won't have to worry about it unless you do other modifications to your engine that place you closer to your detonation threshold.

How Can I Avoid Detonation?

The two most common tricks used by engine tuners looking to obtain maximum performance without detonation are:

1. The use of higher octane fuel.
2. Retard the ignition timing.

Higher octane fuel burns more controllably and is not as likely to combust before the flame front. This is why racing engines use 100+ octane gasolines. The ONLY benefit of racing gasoline is that it moves you away from the detonation threshold, which allows you to be more aggressive with power producing factors like raised compression, advance timing, etc. This is why you'll be disappointed if you put racing gasoline in your mom's bone-stock 1983 Ford Fiesta 1.1L thinking you'll turn it into a race car. If you don't have detonation, the increased octane will do you no good. For cars designed for daily street driving, you obviously won't want to fill up with 100+ octane fuel every week at the tune of 5 bucks a gallon.
Retarding the ignition timing will delay the timing of the spark, which also moves you away from your detonation threshold. Most popular "power programmers" or "chips" increase engine power by advancing the ignition timing, and requiring you to run a higher octane fuel to avoid detonation. These work great, except the advanced ignition timing is NOT compatible when using a supercharger or, unless you're happy to run 100 octane fuel. In fact, many supercharger and Nitrous systems include an "ignition boost retard" or FMUs / controller that retards the ignition timing when it senses boost or WOT. This allows you to maintain stock performance while not under boost or Nitrous, yet still remain safe while the supercharger or Nitrous is making power.

Another way to avoid detonation is to cool the incoming air charge to lower the temperature inside the combustion chamber. On a supercharged application, this task can be handled by an intercooler or by a water injection system (less common). The intercooler takes the incoming air charge and passes it over a series of air-cooled or water-cooled fins and ducts, thus cooling the air in the same way that a radiator cools your engine's coolant. Intercoolers are thus very popular in higher output supercharger systems, where detonation becomes more of a problem. Often times, the intercooler allows you to run more boost and also allows you to eliminate the ignition boost retard, meaning you'll notice increased performance, and still experience no detonation. Another way to lower the temperature of the combusting air and fuel is to run cooler heat range spark plugs. Many supercharger and Nitrous manufacturers will recommend cooler plugs for you engine.

Because lean condition (fuel starvation) also contributes to detonation, it is important to make sure that the fuel system (pump, injectors, etc.) are capable of delivering the increased fuel requirements of your modified engine. Often times, an otherwise perfectly tuned engine will experience detonation just because the fuel pump can't deliver enough fuel to the engine. Upgrading certain fuel components is almost always necessary when supercharging or injecting Nitrous into an engine. Most supercharger systems normally include the upgraded fuel components if they are necessary. Nitrous systems will draw additional fuel form different areas. If you are installing a supercharger on an engine with other modifications, make sure you consider the additional fuel requirements and compensate with larger injectors and / or a bigger fuel pump.

Some modern vehicles come with "knock sensors" that listen for detonation, and automatically retard the ignition timing to eliminate detonation. Although these devices are effective in preventing engine damage, they are not tuned for performance, so you should not rely on the knock sensors and expect your engine to run its best.

In Conclusion:

Although detonation can be potentially damaging to an engine, a simple understanding of what it is, and what causes it, will help you stay away from your detonation threshold. Pay attention to "knock" and pinging noises that come from your engine because they could indicate detonation inside the combustion chamber and should be dealt with immediately. If you ever do notice detonation, perhaps from bad (low octane) gasoline or extremely high air temperatures, just drive with a light foot until you are able to resolve the cause of the problem.
If you have any comments, additions or just more “know how” knowledge about detonation feel free to post them!
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Also, lower heat range spark plugs. to avoid detonation(nitrous/sc).