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Summary
  • The air-fuel mixture inside engine cylinders can ignite early because of the high pressure and temperature, which can cause engine knock and engine damage.
  • A fuel’s octane rating is directly proportional to the fuel’s ability to withstand heat and compression. This means fuel with a higher octane rating is more resistant to compression and is less likely to create engine knock.
  • If your vehicle is designed to run on regular gasoline and the vehicle doesn’t require premium gas, then there’s hardly any benefit to putting high-octane premium gasoline.

You’ve probably seen numbers like 87 or 93 on fuel pumps and wondered what they actually meant. Well, these are the fuel’s octane ratings, but what is a gasoline octane rating?

You might think that this number dictates a gasoline’s quality and that fuels with higher octane ratings mean that they’re higher quality. After all, premium gasoline will always have a higher octane rating and some diesel fuels will have a higher cetane rating than other diesel. We’ll discuss what octane ratings mean and how they affect your engine’s performance in this article.

Fuel Octane Ratings

In an engine, fuel should only ignite inside the cylinder with a spark from the spark plug. This ensures that ignition occurs at a very specific time and that most if not all of the fuel is burned.

However, the high pressure and temperature inside the chamber can cause the air-fuel mixture to ignite before the spark plug fires; this is known as detonation or pre-ignition.

This happens when the fuel is too volatile. Believe it or not, when gas begins to degrade as it ages, it can become so that it burns too fast, and this causes spontaneous combustion that results in knocking due to pre-ignition. When gas ages past a certain point, it won’t burn at all.

Fuel Grades and Their Respective Octane Ratings

You might be wondering, what octane is premium gas? What octane is regular gas?

Regular gasoline typically has an 87 octane rating. Meanwhile, midgrade gasoline has an 89 to 90 rating. Lastly, premium gasoline has an octane rating of 91 to 94.

, What Do Gasoline Octane Ratings Mean?

Pro Tips are nuggets of information direct from ASE-certified automobile technicians working with CarParts.com, which may include unique, personal insights based on their years of experience working in the automotive industry. These can help you make more informed decisions about your car.

Pro Tip: Fuel with a low octane number is more susceptible to spontaneous combustion than fuel with a high octane number.

How Fuel Octane Ratings Affect Your Vehicle

Fuel is formulated with additives so that it can have a specific octane rating. This formulation allows the fuel to resist compression and prevent engine knock.

A fuel’s octane rating is directly proportional to its ability to withstand heat and compression. This means fuel with a higher octane rating is more resistant to spontaneous ignition during the compression stroke.

However, does higher octane fuel make your engine run more reliably? Does putting high-octane fuel in your engine give it more power? Higher octane fuel offers more resistance against knocking, but does this mean an engine can have better performance or reliability? Well, it depends on your vehicle.

If your vehicle is designed to run on regular gasoline and the vehicle manufacturer specifically doesn’t require premium gas, then there’s hardly any benefit to getting premium gasoline. However, some performance-oriented vehicles made by manufacturers like BMW or Mercedes Benz specifically say that their engines require premium gasoline. This might be because their engines have a higher compression ratio that needs to resist more compression before being ignited by the spark plug. Performance vehicles also tend to have turbochargers or superchargers inside, which significantly increases the amount of pressure inside the engine and can therefore increase the risk of engine knock. Putting regular gasoline in these vehicles can damage their engines since they’re specifically made to run on high-octane fuel.

In most cases, putting premium gasoline in a standard vehicle designed to run regular gasoline won’t lead to more performance, better mileage, or cleaner operation.

However, some manufacturers like Mazda do say that putting premium gas in their vehicles can lead to a substantial horsepower increase of up to 25 horsepower. If your vehicle manufacturer doesn’t recommend premium gasoline, then using it probably won’t result in increased performance or reliability.

How Octane Ratings are Determined

Since only the petroleum hydrocarbons from pentane to octane are used in gasoline, manufacturers use a process called cracking to break large hydrocarbons into smaller ones when processing raw petroleum. Gasoline consists of these molecules, along with some additives.

Heptane and octane are the two main constituents of gasoline, and each behaves very differently when misted into the engine and compressed. Heptane is very volatile under compression. Even when slightly compressed, heptane spontaneously combusts.

Octane, on the other hand, can be compressed quite a lot without spontaneously igniting. Thus, gasoline’s capacity to resist spontaneous ignition upon compression is compared to the capacity of a standard mixture of heptane and another hydrocarbon element of petroleum called isooctane. This is the basis for the octane rating given to fuel at a gas.

Heptane and octane are the two main constituents of gasoline, and each behaves very differently when misted into the engine and compressed.

– Richard McCuistian, ASE Certified Master Automobile Technician

The octane rating at gasoline pumps is the average of two different values: A Research Octane Number (RON) and a Motor Octane Number (MON). The RON is determined at a low engine speed of 600 revolutions per minute, while the MON number is determined at a high engine speed of 900 revolutions per minute. These two values are averaged together to provide the octane rating of the gasoline available at the pump.

High- performance (high-compression) engines typically require gasoline with a higher octane rating to prevent detonation/pre-ignition. So, if you’re driving a high performance vehicle rated for higher octane fuel, well, that’s the only reason to purchase fuel with a higher octane rating.

A car built to run on gasoline with an 87 octane rating will get the same mileage regardless of which gasoline is being used.

About The Authors
Written By Automotive and Tech Writers

The CarParts.com Research Team is composed of experienced automotive and tech writers working with (ASE)-certified automobile technicians and automotive journalists to bring up-to-date, helpful information to car owners in the US. Guided by CarParts.com's thorough editorial process, our team strives to produce guides and resources DIYers and casual car owners can trust.

Reviewed By Technical Reviewer at CarParts.com

Richard McCuistian has worked for nearly 50 years in the automotive field as a professional technician, an instructor, and a freelance automotive writer for Motor Age, ACtion magazine, Power Stroke Registry, and others. Richard is ASE certified for more than 30 years in 10 categories, including L1 Advanced Engine Performance and Light Vehicle Diesel.

Any information provided on this Website is for informational purposes only and is not intended to replace consultation with a professional mechanic. The accuracy and timeliness of the information may change from the time of publication.

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