Updated: Feb 26, 2021
For many motors, the limiting factor in terms of power production is fuel quality.
Under a certain amount of heat and pressure, a fuel will break down and become unstable. When this point of instability is reached inside of a motor's cylinders, pressure spikes, putting the motor under unnecessary and unsustainable stress.
A fuel's octane rating is its resistance to becoming unstable under pressure. Higher octane fuels can support higher cylinder pressures, where-as lower octane fuels have a higher propensity to become unstable under pressure.
Given that cylinder pressure is directly related to power output, it follows that a limit to cylinder pressure is also a limit to power production. When a fuel is unstable, cylinder pressure must be reduced. Because cylinder pressure will be reduced, the motor will make less horsepower.
Detecting Fuel Instability
Pressure spikes from unstable fuel will smack into the crowns of the motor's pistons, making a distinctive pinging sound. This pinging or knocking is one of the first signs that a motor is making too much pressure for the fuel being used.
Manufacturers have developed sophisticated knock detection systems to identify fuel instability, and correct for it, before heat buildup leads to a mechanical failure. "Knock sensors" are specially tuned microphones, that attached to the sides or valley of modern motors. They are used to listen to the engine block for the resonant frequencies produced by knock.
Reacting to Fuel Instability
ECUs measure the output of knock sensors every combustion event. Should the sensors be noisier than they are supposed to be at a given engine speed, the ECU considers the motor knocking, and issues a command for "knock retard" to reduce cylinder pressure.
Knock Retard is able to reduce cylinder pressure by delaying (retarding) when the chemical reaction between fuel and air begins. It should be noted that this process is reactive in nature, and the motor must knock before any corrective measures can occur.
For optimal efficiency and power output, combustion is started by spark just before the piston reaches top dead center on the motor's compression stroke. This lead time allows the burning mixture to build enough potential energy to apply a substantial amount of force on the piston during the motor's power stroke. This lead time is called "Spark Advance".
When a fuel is unstable and pinging before the power stroke is underway, pressure must be reduced. This is accomplished by reducing spark advance so that the chemical reaction's temperature is lower during the time period where the piston is near top dead center. This delayed combustion will remain at a lower relative pressure, and have less mechanical advantage against the piston during the power stroke.
Along with Knock Sensors, modern ECUs have strategies to compensate for poor fuel quality. With these strategies, along with working knock sensors, vehicles will rarely audibly knock. Lack of power will be the only external evidence that a motor is octane limited.
It should be noted that knock control strategies are intended to keep a motor relatively safe when running on low quality fuel, and are not intended to adapt to lower octane fuels. Factory knock control strategies are reactive in nature, and are usually over-aggressive in terms of removing ignition timing.
Next, I'd like to provide a practical example of long-term knock correction that is used in most GM vehicles from 1997 through 2019.
GM's ECUs address long-term knock suppression by blending between two ignition timing tables. The default high octane timing table is usually intended for 91 octane fuel, where-as the low-octane timing table is designed for fuel of lower quality than 87 octane.
As the ECU hears knock, it learns that ignition timing needs to be blended closer and closer towards the low octane table. This adjustment is often overly aggressive in terms of how much timing needs to be removed in the event of measured knock.
Below, are the high octane (top) and low octane (bottom) timing tables from a 5.3L truck. For convenience, I've highlighted some cells that are often used during a full throttle pull.
As you can see, at this amount of load, the low octane table is removing 10* of ignition timing in the presence of long-term knock.
Most of the 5.3 trucks running 87 octane that we see are running fully on the low-octane table. These trucks often tolerate closer to 20* of timing when custom tuned for 87 octane, and will benefit from custom tuning.
It should also be worth noting that an unmodified truck with a factory calibration on 91 octane will out-perform an unmodified truck with a custom 87 octane tune, as fuel octane is the upper limit in terms of power production.
Choosing Your Octane
When having a custom calibration created for your vehicle, it's important to fill your gas tank with the fuel you will primarily use. We will be using data gathered from burning that fuel to calibrate your ECU. Any other fuel will make the car run differently, and less efficiently.
While it is safe to run gasoline with a higher octane rating than your vehicle has been calibrated for, it is not good to run a lower octane. If your use case requires you to run lower octane fuels due to availability or travel related issues, your vehicle will need to be tuned on the lowest octane fuel that you will use.
If you need to run your vehicle on lower octane fuels, it may make sense to purchase the hardware required to run multiple tunes that are calibrated for different octanes. Professional grade electronics to re-write your vehicles calibrations are typically in the $400 range, and we strongly encourage our customers to learn and take advantage of these technologies.
Keep Ethanol Content Consistent
Related to the topic of fuel octane, it will also be important to discuss Ethanol Content. For each 10% increase in ethanol content, your motor will need to inject 4% more fuel. Without an ethanol content sensor, your motor does not know when it is running on E10. Since all 87 octane in the Midwest is required to contain 10% ethanol, this presents challenges when switching between ethanol free 91 octane and 87 octane E10.
As there is mechanically no reason to avoid ethanol content, we suggest vehicles that switch between 87 octane and higher grades use fuels that all contain 10% ethanol content. With custom calibration, this prevents the factory ECU from having to reactively adapt by 4% as ethanol content will remain consistent in the fuel used.
Run good fuel! If you're coming to us for a custom calibration, we'll want to see your vehicle on the lowest octane fuel you will frequently use. We want our customers get the most out of their investments, and running quality fuel is critical to accomplishing that goal!