Detonation is the uncontrolled combustion of the end-gasses in the cylinder and, by definition, always happens after the spark-ignition event (as opposed to before spark-ignition, as is the case with pre-ignition). It occurs when the fuel lacks sufficient octane to resist uncontrolled combustion for the properties of the engine it is used in, but it can also be caused by an excessively lean fuel mixture.
If the shape of the combustion chamber of the engine doesn’t lend itself well to extremely quick flame propagation and a lower-than-appropriate octane-rate fuel is used, the build-up of heat and pressure waves in the cylinder as the fuel mixture burns can ignite the end gasses in the cylinder (detonating) and thus cause additional heat and pressure waves that can potentially destroy the engine. Detonation can be tolerated by an engine if it is not severe relative to the engine design, but it can be extremely destructive if it is severe enough. As far as the engine itself is concerned, one of the key differences between detonation and pre-ignition is that an engine can be designed and built to tolerate light-to-moderate detonation, but not necessarily so with pre-ignition. Neither, however, is desirable as neither aids in combustion stability and control. To oversimplify, if you plan on running your engine hard, run the proper heat-range spark plug for your engine, don’t run an excessively rich or lean fuel mixture, inspect and change your plugs often, and run the best quality fuel you can afford with an appropriate octane rating for your engine (we'll have more on fuel characteristics and octane rating in an upcoming article).
If you've tuned your engine and have corrected the airflow modelling characteristics by properly adjusting the MAF curve and/or mapping the VE table to achieve the desired fuel mixture, set the power enrichment values at WOT to the value that will result in the greatest power output (MBT) without compromising the engine longevity you wish to achieve. For example, most production engines (assuming the use of premium unleaded pump gas) such as GM's LS engines, Ford modular and Coyote engines and Chrysler/Dodge Hemi engines, an air/fuel ratio of 12.8:1 works really well as a compromise between great power output and long-term longevity. If power output is prioritized, you can run as lean as 13.2:1 (if dyno tests show a power gain) without killing the engine, however, you will stress the engine significantly more than with an AFR of 12.8:1. Heavier vehicles (those over 3,800 lbs) may require a richer mixture, such as a 12.5:1 AFR, to keep detonation to a minimum at WOT. With lower rear axle ratios (3.55:1 and numerically higher) you can try leaning again and may find that no detonation occurs. Running leaner than 13.2:1 may result in more engine power (and in fact, some engines may run best at these leaner fuel mixtures), but beware that running too lean can cause excessive heat, wear and increase the likelihood of pre-ignition. Whenever detonation occurs, you have several options (assuming you're sticking with the fuel in question): Option 1 is to enrich the fuel mixture and test to verify that detonation is avoided with no loss in power. Option 2 is to command less spark advance timing in the main ignition map. Option 3 is to tolerate the detonation if it's slight. Option 3 may work if the knock sensor is only picking up slight detonation and is causing the ECU to retard timing by no more than 2-4 degrees. If you're drag racing, this might work for you. If you're road racing, avoid Option 3 and stick with Option 1, and if no success, go to Option 2. If the engine runs at WOT for extended periods of time (think long straights for endurance racing, like Daytona, Sebring or Road America), you may want to consider creating a separate engine calibration specifically for these tracks to run a little bit richer of a fuel mixture at WOT and, if needed, a couple of degrees less of spark advance to be certain the engine survives happily at WOT during these heavy-throttle endurance races. Making these changes only results in a loss of a few horsepower compared to the more radical tuning route, so unless you're racing for some serious sum of money, it might be the prudent course of action. After making these changes to your ECU calibration, remember to re-check the spark plugs to be certain you're using the appropriate heat range for the new set-up.
Knock & Pinging
Engine knock and pinging are similar and are thus named due to the sound that is heard when the fuel mixture pre-ignites or detonates. Engine knock is more commonly associated with detonation as it is most often heard due to large-amplitude pressure waves that bounce off the engine block walls and cylinder head. This is heard as a sharp sound, most commonly described as a knock or ping, hence the name. Knock/pinging, however, can be produced by any combustion event that creates a pressure rise in the cylinder that exceeds a certain pressure-rise threshold for the individual design of the engine. For example, if an engine operates with a pressure rise of 50 psi/degree crank rotation and remains relatively “quiet”, that same engine may exhibit a drastic knock with a pressure rise of 65 psi/degree crank rotation. This is one of the reasons you hear old diesel engines clattering on while new diesel engines are rather quiet: Smooth and careful control of combustion and cylinder pressure rise.
Modern knock sensors work quite well in terms of sensing engine knock, however it must be remembered that like any sensor, the accuracy and sensitivity of the knock sensor cannot be completely relied upon to keep the engine safe should detonation occur. It's also worth remembering that any significant valvetrain noise may cause false knock readings from the sensor. Engines that are equipped with large, aggressive cam lobes can create enough mechanical noise to trigger false knock readings by the knock sensor and therefore cause the ECU to retard engine timing.
To recap, if you plan on running your engine hard on the track or for a long duration of time, run the proper heat-range spark plug for your engine and driving conditions, don’t run an excessively rich or lean fuel mixture, inspect and change your plugs often, and run the best quality fuel you can afford with an appropriate octane rating for your engine. For those of you running large cylinder bores, try to use a fuel with the quickest burn rate possible to keep the possibility of detonation to a minimum. If you follow these simple rules and keep your eyes and ears open (check your data logs!), you'll be able to race and have the most fun possible while spending the least amount of cash. MET
Pre-Ignition, Detonation, Knock and Pinging: What's the difference? We get asked this question a lot, and rightfully so. If you don't understand exactly what each of these events are and how they occur, you won't know how to prevent them from happening, or possibly even recognizing when it happens. While it's not as catastrophic as we illustrate in the photo (chuckle, chuckle), it's not pretty when it happens in your engine and the results could be permanent engine damage. So, what is the difference? Let's take a
caught before any damage to the engine occured. When racing, you can run a colder spark plug to help protect against pre-ignition as cold-start characteristics, plug fouling and exhaust emissions are not necessarily a priority. If your engine is running too rich, however, the accumulation of carbon deposits in the cylinder can cause pre-ignition, so keep an eye on your spark plugs and exhaust-gas temperatures to help determine whether or not pre-ignition is occurring. When it comes to choosing the correct spark plug heat range, a little bit of testing is necessary. In terms of numbers, the optimal temperature at the firing end of the spark plug is between approximately 930°F to 1470°F (500°C - 800°C). Running colder than 900°F will cause carbon fouling while running much hotter than 1470°F will cause spark plug overheating. By choosing a sensible heat range to begin with (usually 3 steps colder than you would use on the street does the trick) run your engine for at least 30 seconds under race conditions and then check the plugs. Look for signs of over- or under-fueling (rich or lean fuel mixture) as well as carbon deposits and/or overheating. Moving up or down one heat range generally results in a plug temperature change of 160°F - 210°F (70°C - 100°C ), so this should give you an idea of how far to move in one direction or the other.
When testing your engine, removing and inspecting the spark plugs will allow you to discover any signs of an over-heated electrode before you see any major damage to the piston (this is not always the case, as severe pre-ignition can kill an engine quickly, but such a severe case is not typical). It's good practice to check the spark plugs often to monitor engine performance. This is exactly why many drag racers check the spark plugs after each and every run: It helps you find trouble as would be indicated by the spark plug pictured on the right. This damage occured due to a lean condition that then caused the overheating of the electrode which in turn could cause pre-ignition. Fortunately this was
look at each of the events in some more detail, starting with Pre-ignition, which can also be called self-ignition.
Pre-ignition (self-ignition) occurs when the fuel mixture in the cylinder burns before the spark-ignition event at the spark plug. Pre-ignition may or may not cause permanent engine damage, but it does lead to engine inefficiencies and may cause engine damage if it is a severe or continuous event. Pre-ignition causes a significant drop in power output and efficiency as it robs the crankshaft of power that must now be used to drive the piston up through the forces of early fuel mixture combustion and expansion. The most common causes of pre-ignition are the overheating of a spark plug electrode (using a spark plug that has a heat rating that is too high for your application and intended use) and excessive build-up of carbon in the cylinder.
Engines designed for street use are usually equipped with “hot” spark plugs to ensure better cold-start characteristics and reduced fouling. The compromise with using a hotter spark plug is that the spark plug electrode can become the “burning ember” that pre-ignites the fuel mixture under excessive load and WOT operation. This is one of the reasons the OE manufacturers run excessively rich mixtures at WOT if the engine is held at WOT for long periods of time; to keep the spark plugs cool and prevent them from causing pre-ignition events that are severe enough or continuous enough to destroy the engine. The downside to this is the potential for carbon build-up in the cylinder. If an excessively rich fuel mixture only occurs during WOT, it's unlikely that a carbon build-up will occur within the cylinder, but it can happen. If the fuel mixture is excessively rich most of the time while operating the engine, including during light loads and lower engine speeds, such as occurs during short and infrequent engine start-up and stop phases or by simply having a poorly tuned engine, the possibility of carbon fouling increases dramatically and so does the likelihood of pre-ignition. On the other hand, if the engine runs too lean while at WOT, cylinder temperatures go way up and can again cause overheating of the spark plug electrode regardless of heat range, thereby again increasing the likelihood of pre-ignition. If you notice what feels like a significant power output drop from the engine while racing, the prudent thing to do is stop the engine when it's safe to do so and check for signs of pre-ignition.
When severe or prolonged pre-ignition occurs, it generally leads to the failure of the piston(s) due to heat and pressure load. The cylinder heads and engine block have greater material thickness and thermal inertia than the pistons, so their tolerance to heat/pressure is greater than that of the pistons and rings which usually take the fall when pre-ignition occurs. The first sign of pre-ignition can most often be found by inspecting the spark-plugs.