Q&A

Mailbag: Explaining Gasoline Octane Ratings

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Q: What do the different gasoline octane ratings mean?

A: The octane rating measures the performance of a particular fuel blend.

The higher the number, the more compression it can withstand before self-igniting.

How is it measured?

Gasoline is a mixture of various chemicals. A sample of the fuel blend is run in a test engine in a lab. The octane rating is assigned based on the test results.

There are two common octane tests:

  • Research Octane Number (RON)
    • In this test, the engine is run at 600 rpm with variable compression ratio. The results are compared with mixtures of isooctane and heptane. RON is commonly used in Australia, New Zealand, and Europe.
  • Motor Octane Number (MON)
    • In this test, the fuel is preheated. Then, the engine is run at 900 rpm. Variable ignition timing further stresses the fuel’s knock resistance.

The results of the tests can also be combined. This is known as the Anti-Knock Index (AKI). AKI is the average of RON and MON. It is typically written as (R+M)/2. It is also known as the Posted Octane Number (PON). AKI is commonly used in the United States, Canada, Brazil, and other countries.

How does it affect performance?

Fuel with a higher octane rating is less prone to self-ignition. It can withstand a greater rise in pressure during the compression stroke. This allows the engine to make more power.

Due to higher cylinder pressure, high octane fuel is required in most race engines. It is also required when using power adders. Low octane fuel can cause the engine to knock.

What octane fuel should I use?

Use the octane recommended by the engine manufacturer. Running gas with a lower octane rating will cause a reduction in power and fuel efficiency and possibly engine damage. This is because the ignition timing will need to be retarded to prevent detonation.

On the other hand, a higher octane rating will not improve the engine’s performance. The compression ratio of the engine is set by the cylinder head and piston design.

Therefore, the only way to increase the pressure inside the cylinder is to change the compression ratio or add boost. If you don’t do either of those things, the high octane fuel is just a waste of money.

This is another in a series of weekly Q&A Mailbag sessions with Summit Racings tech department, in which there are hundreds more. Click here to see them all.

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4 Comments

  1. Ronald W Petersen says:

    simply put. there are to grades of gasoline. 87 unleaded and 93 unleaded. you blend the 2 in different ratios to get other grades. if its above 93 its added boosters and chemical to get it there

  2. James Brantley says:

    Well I’m a drag racer and I have always ran higher octane in my daily driver not because it has high compression and needs it but lower octane fuel leaves more carbon deposits and have less detergents than higher octane fuel so I have a cleaner engine period!!!!! I have done my own testing of this principle and it works for me!!

    • Daniel Wilson says:

      James….I’m also a drag racer and I can understand both sides of the octane logic. But the main thing is that if it works for you then by all means just do it !
      One of my daily drivers has a factory supercharged V-6 and all of the literature that came with the vehicle doesn’t just suggest a particular type of fuel, it states in bold print that a high octane “PREMIUM” fuel MUST be used or “SERIOUS ENGINE DAMAGE WILL RESULT”. They certainly don’t have to twist my arm because it just makes good sense to use the highest octane available at the pump for any forced induction application. My blown V-6 is from 2002 and the electronic engine controls seem primitive compared to the technology that’s currently being used.

      A good example is the rental car I recently had for two weeks. A 2020 Mustang Ecoboost with the turbocharged/intercooled , 2.3 liter four cylinder with the 10 speed automatic transmission. Since I build and race a well prepared 351 Cleveland “old school” V-8 in a ‘70 Mustang, I didn’t expect the surprisingly strong performance from the Ecoboost. I would imagine the previous renter filled the tank with cheap 87 octane fuel before returning it so after using about half a tank, I filled it up with 93 octane. The performance increase was quite noticeable using the higher octane fuel. Equipped with the traction lock differential, it would smoke both tires through first, second and into third gear at 22 psi of boost !!!
      Not too shabby for a four cylinder using Fords latest turbo technology.

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