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Understand Octane Rating for Better Do-It-Yourself Auto Maintenance

What is Octane Rating?

Image of 93 octane rating formulaWe are all familiar with the bright yellow and black octane rating stickers on the gasoline pumps. But what exactly does the octane rating mean, and does it really matter?

The octane rating displayed on a gas pump is the Antiknock Index, a number that is the average of the Research Octane Number (RON) and the Motor Octane Number (MON). The Antiknock Index is derived by using the formula (R+M)/2; in other words, the RON and MON are added together, then the sum of those numbers is divided by two.


How Octane Rating May Impact Engine Performance

Image of Mustang GT500 in a dyno test

The RON generally affects low- to medium-speed engine knock, while the MON generally affects high-speed and part-throttle engine knock. A fuel with a RON that is too low may cause low-speed engine knock or engine “run on” after the ignition is switched off. A fuel with a MON that is too low may cause engine knock during acceleration, while passing, or while climbing hills.

Generally, running gasoline equivalent to the manufacturer’s Octane Number Requirement (ONR) will be adequate. However, if your need to boost the octane rating in your current gasoline, consider using 104+® Octane Boost.

Engines are specified by the manufacturer to have a certain Octane Number Requirement (ONR). Running fuel with a lower octane rating than the ONR can result in performance problems with the engine. These symptoms might have the do-it-yourself automotive maintenance mechanic considering a tune-up or other maintenance, when all that is really needed is improving the gasoline.

There are, however, circumstances that will impact the octane requirements of your engine. Advancing the ignition timing or modifying the air/fuel ratio are two examples of engine adjustments that will change your engine’s octane requirements. Similarly, changes in temperature, humidity, barometric pressure, or altitude also impact octane requirements.

Still interested in learning more about gasoline production and performance issues? Gold Eagle has a whitepaper on petroleum production and distribution that goes into considerable detail on the technical issues.


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