Everything You Needed to Know About Octane

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Everything You Needed to Know About Octane. When you pull up to the pump, you’re often presented with a variety of fuel options that are graded and priced differently. The number next to each variety indicates its octane rating. The grade of fuel you choose is important for both your pocketbook and the proper running of your car.

Octane is a hydrocarbon produced during the conversion of petroleum into gasoline. The octane rating is a standardized measure of a fuel’s stability, or resistance to spontaneous combustion. So, there is octane in your fuel, which is assessed using precise tests.

Fun fact: Octan is the Danish term for octane. It’s also the fake gas brand included in numerous Lego play sets, as well as the firm led by Lord Business in “The Lego Movie.”

Everything You Needed to Know About Octane

How Octane Ratings Are Determined

It may appear that fuel that is difficult to ignite would be a problem for an engine, yet this is really a desirable attribute in gasoline. This stability allows for more controlled combustion and eliminates the likelihood of the fuel igniting at the incorrect point in the piston’s stroke due to high temperature and pressure in the engine’s cylinders, rather than when the spark plug fires. When fuel ignites before the spark plug ignites, it causes pre-ignition, also known as knocking or pinging.

In the United States, octane ratings—the anti-knock index—are an average of two engine measurements: the motor octane number (MON) and the research octane number. Other countries just utilize the RON rating at fuel pumps, which is slightly higher than the anti-knock index.

The difference between the MON and RON tests is the engine’s operating conditions. MON refers to a test that simulates an engine at idle, whereas RON tests raise temperature and engine rpm to model an engine under load.

These figures are derived from a calibrated piece of lab equipment that compares fuel samples to iso-octane and heptane mixes. Stable iso-octane has a reference value of 100, but unstable heptane has a value of 0.

Operators test the gasoline until a predetermined knock threshold is met. They then compare the sample fuel’s performance to that of reference fuels with known octane levels. So, a fuel with a knock resistance equal to 92% iso-octane and 8% heptane would have an octane rating of 92.


To compensate for knocking caused by lower octane fuel, modern cars have sensors that detect it and can delay spark timing via the engine computer. When you put the recommended octane fuel in later, the computer will recognize it and make the necessary modifications. While a tank of lower-than-recommended octane fuel is unlikely to cause permanent damage, neglecting the manufacturer’s octane standard over time may. Knock, on the other hand, can cause engine damage in older vehicles that lack electronic fuel and ignition systems.

The best thing to do is consult the vehicle’s owner’s manual. It should state the minimum octane rating required for your engine, as well as a range of permissible ratings, with higher octane producing more power.

Using fuel with a lower-than-specified octane level might raise emissions while decreasing fuel economy. In short, it’s not worth saving a few dollars on a tank of gas.

There’s no reason to put premium or even mid-grade fuel in your automobile if it doesn’t require it. Stick to the less expensive choice.


Anyone who travels through or lives in mountainous areas may notice that the octane numbers on petrol stations begin at 85 or 86 instead of the 87 your automobile can handle. This is a relic from the carbureted engine era. Because the air is less dense at altitude, the engine’s compression ratio is essentially lower, allowing a carbureted engine to use lower-octane fuel while resisting knock.

However, today’s vehicles use fuel injection rather than carburetors and have computers to adapt for air pressure. They don’t expect fuel with an octane rating lower than the recommended level. Consult your owner’s handbook for recommendations.

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Ethanol is a renewable fuel produced mostly from corn in the United States that, when burned, absorbs carbon dioxide from the atmosphere. Gasoline firms are forced to purchase substantial volumes of ethanol and mix it into much of the gasoline they sell. Ethanol has a higher octane rating than ordinary gas, increasing the actual rating of the gas blend, but it produces one-third less energy than gasoline.

Ethanol is also hygroscopic, which means it absorbs moisture from the air. This is troublesome in engines without sophisticated sealed emissions systems, particularly those driven infrequently and thus do not go through a tank of fuel very often. Fuel stabilizer may assist extend the life of a gas tank in those cars by preventing corrosion, which could otherwise result in water entering the system.

Ethanol-containing gas is classified into two types: petroleum-derived gasoline with some ethanol added and ethanol-based fuels. The former, known as E10 or E15, typically includes 5% or 10% ethanol, and it is becoming increasingly difficult to avoid. E85 can include anything from 51% to 83% ethanol, and it is solely intended for use in flex-fuel vehicles.

According to the US Department of Energy, E10 is the limit for vehicles produced prior to 2002, as well as motorbikes, watercraft, and lawnmowers.