What is Octane Rating? Differences Between High-Octane and Regular, and its Relationship with Knocking

What is Octane Rating?

At gas stations, you'll see two types of gasoline: "regular" and "high-octane."

So, what's the difference between these two?

One major difference is the "octane rating."

Simply put, octane rating is an indicator of "how resistant gasoline is to abnormal combustion, known as knocking."

The higher the number, the less likely unintended self-ignition will occur even in a compressed, high-temperature, high-pressure environment, meaning the gasoline has a higher resistance to knocking.

What is often misunderstood here is the idea that:

"Higher octane rating = burns better"

"Higher octane rating = more energy"

The octane rating primarily indicates the "resistance of gasoline to knocking." It is not a number that represents simple power output.

What's the Difference Between High-Octane and Regular?

Automotive gasoline in Japan is categorized by octane rating according to JIS standards.

Regular gasoline (Grade 2) is specified to have a Research Octane Number (RON) of 89.0 or higher, while high-octane gasoline (Grade 1) is specified to have a RON of 96.0 or higher.

In broad terms, this means:

Regular is for general gasoline engines.

High-octane is for engines that require higher knock resistance.

However, the octane rating of gasoline actually sold is not always the same as the minimum value specified by JIS. Specifications may vary by brand and product.

Furthermore, some high-octane gasolines contain detergents, but it's important not to assume that "all high-octane gasolines contain the same additives" and instead check the product specifications of each oil company. ENEOS also explains that the difference between its high-octane and regular gasoline is not just the octane rating but also the inclusion of detergents.

What Exactly is Knocking?

In a gasoline engine, air and gasoline are typically mixed in the cylinder, compressed, and then ignited by a spark plug.

From there, the flame spreads, and when combustion occurs at the right time, the engine generates power.

However, if the temperature or pressure inside the combustion chamber becomes too high, the air-fuel mixture, which has not yet been reached by the normal flame, can spontaneously ignite.

As a result, rapid pressure fluctuations occur within the combustion chamber, which can cause vibrations and sounds described as "pinging" or "knocking." This phenomenon is generally known as knocking.

Gasoline with a higher octane rating has greater resistance to such self-ignition, making it crucial for engines used at high compression ratios or under heavy load.

Why Do Some Engines Require High-Octane Fuel?

Engine design influences conditions such as combustion chamber temperature and pressure, ignition timing, and boost pressure.

Especially in engines designed for high performance, high compression ratios and supercharging can create severe conditions in the combustion chamber.

Therefore, engine control may be designed with the assumption of using high-octane gasoline, which has higher knock resistance.

In other words,

Rather than "high-octane for luxury cars,"

it's more accurate to think, "high-octane is necessary for that engine's design and control."

What Happens if You Put Regular Gasoline in a High-Octane Vehicle?

This varies by vehicle model, so always check your vehicle's owner's manual.

Many modern engines have knock sensors that detect signs of knocking and adjust ignition timing and other parameters. Therefore, some vehicle models may allow the use of regular gasoline in emergencies.

However, in engines originally designed for high-octane fuel, using regular gasoline may cause ignition timing to be adjusted, potentially preventing the engine from delivering its full performance in terms of output and fuel efficiency. Toyota also advises that for some high-octane specified vehicles, using regular gasoline may lead to knocking or reduced output.

Therefore,

Instead of "it runs, so regular is fine,"

the fundamental rule is to use the fuel specified by the manufacturer.

Will a Regular Gasoline Vehicle Go Faster if You Put in High-Octane?

Conversely,

Some people might think, "Will a regular gasoline vehicle gain more power if I put in high-octane?"

However, this is not necessarily the case.

If the engine is designed for regular gasoline, simply increasing the octane rating will not necessarily lead to a clear increase in power, unless the control system actively utilizes it to change ignition timing or boost pressure.

ENEOS also states that there is no problem with using their high-octane fuel in regular gasoline vehicles, but that is separate from the claim that "horsepower or fuel economy will definitely improve."

What's important is,

not that higher octane is better,

but rather to use fuel that meets the octane requirements of the engine.

Octane Rating and Gasoline Degradation are Separate Issues

Another point that often causes confusion is:

"Can high-octane fuel be stored for longer periods?"

While one of the main differences between high-octane and regular gasoline is knock resistance, it does not indicate the long-term storage stability of the gasoline itself.

Gasoline changes state during storage due to factors like air, temperature, and moisture. High-octane fuel does not mean it won't degrade. A current Next Science article clearly explains this point.

"For detailed information on long-term stored gasoline, please refer to 'What is Gasoline Degradation? Changes in Old Gasoline and Precautions for Long-Term Storage'."

Do Fuel Additives Increase Octane Rating?

This is another point that requires attention.

There are various types of fuel additives.

Some products aim to improve octane rating, while others are for cleaning injectors or combustion chambers, improving fuel stability or rust prevention, or focusing on combustion conditions.

Therefore,

"Fuel additive = something that increases octane rating"

is not always true.

If a product does not explicitly state that it has an octane-enhancing function, it should not be considered an octane booster.

Next Science also explains that fuel additives have different purposes, and the approach differs between cleaning products using PEA and products focusing on fuel or combustion conditions.

"For information on types of fuel additives and their differences with PEA, please see 'What are Fuel Additives? An Easy-to-Understand Explanation of Their Effects, Types, PEA, How to Use, and How to Choose'."

Beyond Octane Rating: Considering "Combustion Condition"

Octane rating is a very important indicator for understanding gasoline.

However, the actual state of an engine is not solely determined by the octane rating.

Fuel injection.

Intake air.

Ignition.

Combustion chamber condition.

Engine temperature.

Sensors and electronic control.

Spark plugs.

Injectors.

These various conditions combine to determine the final combustion state.

Therefore,

Even if you experience symptoms like:

"The engine feels sluggish recently"

"There's something unusual during acceleration"

"The idling is different than before"

you cannot determine the cause solely by octane rating. For more details, please also refer to "Causes of Poor Engine Acceleration."

Considering Gasoline from the Perspective of "Combustion"

When considering regular and high-octane gasoline, the first important thing is to use the correct fuel specified for your vehicle.

Beyond that, when thinking about engine condition, it's not just about

"What octane rating?"

but also

"How is the fuel currently being used combusting inside the engine?"

The same applies when choosing a fuel additive.

Do you want to change the octane rating?

Do you want to clean the fuel system?

Do you want to consider fuel storage stability?

Do you want to condition the fuel or combustion state?

The product you choose will vary depending on your purpose.

Summary: Octane Rating is an Indicator of "Knock Resistance"

Octane rating is an important indicator of gasoline's knock resistance.

According to Japanese JIS standards, regular gasoline has a Research Octane Number (RON) of 89.0 or higher, and high-octane gasoline has a RON of 96.0 or higher.

However, octane rating is not a direct measure of gasoline's "power."

High-octane for vehicles requiring high-octane.

Manufacturer-specified fuel for vehicles requiring regular.

First and foremost, using the fuel specified by the vehicle manufacturer is fundamental.

And when considering engine condition, it's important to think comprehensively, not just about octane rating, but also about ignition, fuel injection, intake, combustion, friction, electronic control, and more.

Understanding fuel leads to understanding the engine.