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Ask a Mechanic: Fuel Octane  

Simply put, an octane rating is an index of at what temperature a fuel (which includes additives) will ignite.
Nida Zafar
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Every week, we take your questions about what is going on under the hood of your vehicle and pose them to a knowledgeable mechanic in the Greater Toronto Area. With a number of readers commenting on our March 6 column, where it was erroneously reported using the wrong octane rating could clog your filter (it actually causes something called “knock.” More on that in a minute), we decided to take a closer look at what an octane rating is and how it works.

Simply put, an octane rating is an index of at what temperature a fuel (which includes additives) will ignite. When you go to the gas station, there are typically three octane options to choose from, generally referred to as grades. Each is represented by a number. This number is based on the calculated average of the results of two specific laboratory tests (Research Octane Number and Motor Octane Number) that measure a fuel’s resistance to causing knock. You may have seen “(R+M)/2” on the pumps alongside the rating number.

A vehicle’s engine has cylinders that house pistons. “As the piston in the cylinder moves down, it draws in the fuel and air. As it moves up, it compresses that air-fuel mixture and creates heat and pressure in that cylinder,” said Robert Lirette, a professor of automotive technology at Centennial College in Toronto.

If there’s too much heat, the fuel will pre-ignite. It can also occur as uncontrolled spontaneous ignition when higher cylinder pressures and temperatures occur during the relatively slower process of controlled combustion. These will lead the engine to knock and can cause damage to the vehicle. Knock is very literal: It sounds like a knock on a door.

Wilson Almeida, the director of training at Vast-Auto Distribution, said that when this occurs it can lead to a number of mechanical problems and could cause your engine to fail. “This forced explosion was not supposed to [happen] and it can damage the internal combustion engine,” he said.

The octane rating recommended for use in a vehicle depends on the compression ratio of the engine, Lirette added. Every engine has a specific ratio comprised of the volume of the cylinder when the piston is at the bottom versus the volume of the cylinder when the piston is at the top.

Most vehicles on the road have a smaller compression ratio compared to premium vehicles, like some sports and luxury models, tend to tend to have engines with higher relative power outputs and a greater likelihood of higher-octane rating requirements.

This brings us back to our reader question from March 6: Could they save some money by using a lower-octane fuel in their luxury vehicle? “If you’re driving a high-end car or a car that’s turbocharged, and you’re putting low octane gas in it, you could potentially have a knock condition. In some cases, it would void your warranty if you caused damage,” Lirette said.

If you drive a luxury vehicle and are tight on money, fill your tank a quarter way with fuel one octane grade lower and listen to how your car reacts, Lirette said. He also warned that if an issue does occur, and you go the dealer to get it fixed, your warranty may not cover the damages if it is discovered you used a lower octane. If you’re confused by what gas to use, Lirette recommended either looking at your owner’s manual or your gas cap or door. It will list the ideal fuel to use in your vehicle.

Correction – Mar. 29, 2021: This article has been edited to correct some of the information about a fuel’s octane rating. A fuel’s octane rating is an index of at what temperature a fuel will ignite according to heat, not pressure. The article also misidentified a piston as a valve. A further description of how a fuel’s octane is calculated has also been included.

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