Tech Bulletin - Octane Adjust Ratio

Ford Operational Calibration Strategies

EcoBoost Powertrain Applications - Octane Adjust Ratio

Strategy Background

This vehicle is equipped with several dynamic load, airflow, and torque targeting and limiting strategies. These strategies allow the vehicle to achieve consistent performance with varying atmospheric conditions and fuel quality. There are no set "boost targets" with these strategies, and as such boost pressure can vary depending on driving conditions. 

How It All Works

From the factory, Ford typically designs the base calibration to allow use of 87+ octane fuel and recommends using 91+ octane fuel for optimal power. They can do this by means of a dynamic multiplier that allows for adjustment of each of these targeting and limiting strategies which we call the "OAR" (Octane Adust Ratio). The OAR starts life at a value of 0.0 and is allowed to learn in two directions. When fuel quality and knock sensor feedback are optimal, the OAR will adjust towards -1.0. When these are sub-optimal, the OAR will adjust towards +1.0. I bet you're wondering why -1.0 is better?

Well for one, since this value is a multiplier and not an offset the ECU code can be optimized to use a single table comprised of negative values to handle the spectrum of operating conditions. Take ignition timing corrections for example; with an OAR at -1.0, multiplying against a negative becomes positive which is then added as positive correction to ignition timing. However, when conditions are sub-par a +1.0 OAR will result in negative correction to ignition timing.

How Does It Effect Power?

Additionally, the OAR is used in other strategies such as LSPI (Low Speed Preignition), and part-throttle combustion stability. These are functions designed to dynamically limit load production in the event that fuel quality is not optimal. We want the ECU to limit load as it will remove stress and prevent damage automatically! The part-throttle limiter allows for casual driving at moderate loads while maintaining stoichiometric operation. This allows for increases in fuel mileage but at the expense of heat. Heat is generated much more quickly with lower octane fuels and can cause detonation. This is why it is necessary for the OAR to step in and allow for adjustment. In the case of LSPI, the OAR is used to create a blending ratio between three separate load limits to mitigate the possibility of preignition. 

How Does COBB Use OAR? 

Our calibrations attempt to utilize as many of the OEM strategies as possible to offer the most advanced, powerful, and safest driving experience to you. Most of our calibrations utilize the OAR just as the factory does. However, our calibrations are designed for a specific octane rating and have a no nonsense policy for knock. If the ECU senses knock, we will aggressively adjust the OAR towards positive. We have designed the OAR to be utilized so that maximum performance is achieved for the specified octane. This is why it is important to run the appropriate calibration for your available fuel type!

The OAR is also a KAM (keep alive memory) stored value and will be reset upon initial installation of the Accessport. It will take time to learn, so don't be alarmed after a few days of driving if it's not fully settled. Once learned the value will persist through each start-up (even through "change map"). The value may change from time to time based on knock sensor feedback, which is completely normal and expected. If you find the OAR value does not stay near -1.0 when using top tier fuels, please use the Accessport to change to a lower octane calibration until better fuel can be found.