Ford Tuning - What is the Octane Adjustment Ratio?
We have all seen the people talk about premium fuel as if its worthless additional cost that has no benefits, while others are conned into thinking adding premium fuel to a economy engine is going to result in a cleaner injectors and more power. The truth is that premium fuel definately can contribute to increased power, both with aftermarket and factory tunes. Ford has chosen to deal with this with something called the octane adjustment ratio.
What is the octane adjustment ratio on a Ford PCM?
The octane adjustment ratio (OAR) is a strategy that the PCM incorporates to adjust for octane level of the fuel used. This is a parameter that can be pulled up with the usage of a scan tool. If you don't have access to a fancy scan tool, give this a try FORScan - The cheapest and best scan tool for Ford DIY Mechanics or use something like HpTuners. Sometimes it can be found under a few different PID names such as Infered Octane, Octane_Ratio, or OAR. The values can also be displayed as a simple digit like 87 or 91, or it will be displayed as its true value -1.0 to 1.0.
The simple digit explanation is easy to understand. Your scan tool is interpreting the actual value as either 87 or 91.
The actual value that has been calculated by the PCM is a little more confusing to read. An OAR of -1.0 is actually when the engine is at maximum octane adjustment and generally is only realized on Premium 93 Octane gasoline. An OAR of 1.0 is when the PCM recognizes the fuel is of minimum octane recommended by Ford which is Regular 87 gasoline.
Why is the octane adjustment ratio backwards?
The OAR seems backwards because you must understand how the PCM is making the calculations. The factory tune is comprised of several different spark tables. For example, maximum brake torque (MBT) spark table is your maximum spark allowed, the borderline knock spark table is maximum spark achievable before spark knock occurs in perfect conditions.
There are then so many modifier tables such as intake temp spark modifier, variable cam position spark modifier, and of course the octane adjustment ratio that are futher used for calculating spark. These modifiers all work together for the PCM to dynamically come up with a spark value.
The octane adjustment reference table is a negative number that is ADDED to the total spark output. So the OAR is a negative number when higher octane is used so that when multiplied the two negative numbers when become a positive number which is ADDED to total spark output.
How does the car know if I filled up on premium 93 gasoline?
In short, it does not know for certain. The strategy is constantly adjusting to signals from the knock sensor. The OAR is slowly subtracted from to reach a normalized base of -1.0. When the knock sensors detect spark knock, it reverses and starts adding to the OAR. The amount of knock determines the speed at which the ratio is added to.
When the knock sensor detects spark knock, the PCM immediately enacts a spark retard table based on the strength of the knock. It quickly removes advance from the spark until the knock no longer happens. The PCM will always push the boundries of the fuel to keep the spark advanced pegged at the furthest forward advance that leads to no knock, and adjust it when knock is detected.
Premium 93 Fuel Advantages
The 93 octane rating of the premium fuel allows higher resistance to ignition at higher cylinder temperatures. Higher cylinder temperatures occur when higher compression ratios are used or other power adders like turbochargers or superchargers. This resistance to ignition is avoiding uncontrolled pre-ignition of the fuel called spark knock.
On a 3.5L Ecoboost in a Ford F150, using premium fuel can result in an estimated 10% increase in fuel miles per gallon and a similar increase in horsepower. The extra efficency gained is almost exclusively from reduction of spark retard. The increased efficency doesn't make up the cost difference but every horsepower counts. You can reduce the cost of premium fuel by signing up for Upside - Fuel Cashback App and using this promo code (BRAD27932) to get an additional $5 bonus.
I challenge you to drive an F150 with a 3.5L Ecoboost and watch / graph the knock retard PID while the vehicle is on regular unleaded fuel. You will practically see spark retard on every tip-in on the throttle.