What Is the Octane Rating in Gasoline?

The octane rating of gasoline measures its resistance to early, uncontrolled combustion — known as engine knocking. Knocking occurs when the air-fuel mixture ignites prematurely during the compression stroke inside the engine’s cylinders. This reduces engine performance, increases noise, and can cause long-term engine damage.

Gasoline with a higher octane rating is more resistant to knocking and burns in a more controlled manner. This allows the engine to run more quietly and efficiently while reducing emissions.

The octane rating is an indicator of how quickly fuel burns and how resistant it is to detonation. It plays a crucial role in selecting the type of gasoline that ensures the cleanest and most efficient combustion in engines.

Determining Octane Rating: The octane rating is measured through a test defined by the ASTM D910 standard. In this test, the gasoline sample and reference fuels (iso-octane and n-heptane) are burned under controlled conditions in a test engine. The combustion speed is measured by observing the formation of pressure waves. Iso-octane burns slowly and in a well-controlled manner, and therefore has an octane rating of 100. n-Heptane, on the other hand, burns rapidly and uncontrollably, and is assigned an octane rating of 0. The closer the combustion behavior of gasoline is to iso-octane, the higher its octane rating.

Octane ratings generally range between 90 and 110. Gasoline rated at 90 octane or higher is classified as high-octane, while those below 90 octane are considered low-octane fuels.

The required octane level depends on the engine’s design and operating conditions. High-performance engines are more sensitive to knocking and therefore require higher octane fuel. As a general guideline, the following octane ratings are recommended for gasoline engines:

95 octane gasoline: for standard gasoline engines 97 octane gasoline: for high-performance gasoline engines 98 octane gasoline: for turbocharged engines and racing cars.

• Increased engine knocking and noise
• Reduced engine performance
• Decreased fuel efficiency
• Potential damage to the catalytic converter
• Shortened engine lifespan

• Less engine knocking and reduced noise
• Improved engine performance
• Slight improvement in fuel efficiency
• Better protection for the catalytic converter
• Extended engine lifespan