What do the different fuel octane ratings mean?
Fuel octane ratings are plastered all over your local petrol station – here’s what they actually mean for your car.
91, 95, 98, E10… You’re used to seeing these specific combinations of letters and numbers every time you visit the petrol station, but what do the different fuel octane ratings actually mean?
Often, consumers make their fuel selection based on the dollar figure next to the name, but making the right choice at the bowser isn’t just a matter of price.
Many manufacturers stipulate the minimum octane rating required for their cars, and opting for the wrong fuel has the potential to reduce fuel economy and performance, and even damage your car’s engine.
With petrol prices rising, you might be inclined to focus solely on what’s best for your wallet, but it pays to take a closer look.
Here, we break down the meaning of the different fuel octane ratings.
What is a fuel octane rating?
Put simply, the octane number is a measure of fuel stability – the higher the number, the more ‘stable’ the fuel.
According to Aleks Urosevic, an Expert Technologist at BP’s Innovation and Engineering department, “The octane number of a fuel indicates its tendency to auto-ignite or detonate under high temperature and pressure in the presence of air”.
This premature detonation or ignition is also known as ‘knocking’. The more resistant a fuel is to knocking, the more suited it is for turbocharged or high-compression engines.
What does RON stand for?
There are a number of ways to measure octane, and the two most important are research octane number or RON, and motor octane number or MON.
“RON indicates the tendency to detonate under high load, such as accelerating or climbing a hill. For regular petrol the minimum RON is 91, for Premium it is 95 and for BP Ultimate it is 98,” Mr Urosevic explains.
“MON indicates the tendency of petrol to detonate under low load, such as freeway driving or highway driving. For regular petrol the MON is 81 minimum, for Premium it is 85 minimum, for BP Ultimate it is typically 86 minimum.”
What is a premium fuel?
According to the Commonwealth Department of Environment and Energy guidelines, in Australia a premium fuel must be at least 95 RON or higher.
According to the NRMA, a premium fuel “optimises an engine’s ability to make power through increased fuel density”, which in turn offers “a more controlled burn, reducing engine ping (detonation) and resulting in a smoother delivery of power”.
What are the benefits of a fuel with a higher octane rating?
“Modern engines are more efficient on higher-octane fuel and use less fuel,” Mr Urosevic says.
“Using a higher-octane fuel provides additional protection if the engine is under load, and better fuel economy and power output if the engine has a knock sensor to take advantage of the higher octane.”
Can all petrol cars take all types of unleaded fuel?
No – don’t assume just because your car asks for unleaded fuel that any octane rating will do.
Most cars stipulate a minimum octane rating, which you can typically find inside your fuel filler cap.
For plenty of petrol cars, this minimum rating is 91 RON, but increasingly, modern cars require a minimum of 95-octane fuel, particularly if they feature a turbocharged engine.
What happens if I put the wrong fuel in my car?
The computer systems on a modern vehicle are pre-programmed to expect the minimum-octane fuel stipulated by the manufacturer.
As a result, ignoring this advice could place your car at risk.
“If the vehicle is fine-tuned to use 98 RON, using 91 RON may cause engine damage,” Mr Urosevic explains.
“The driver may notice pinging coming from the engine during high speed, low-load driving, for example, highway driving.
“In addition, the driver may experience poor vehicle performance and fuel economy.”
On the flipside, if you use a fuel with a higher octane rating than required – for example, 98 RON in a car that only requires 91 RON – the worst-case scenario is that you will see no benefit, because your car is an older model that hasn’t been tuned to operate on 95 RON or higher.
Why is fuel with a higher octane rating more expensive?
Put simply: “The cost of components to manufacture a higher RON grade are more valuable, and that is reflective of the end price of the grade,” Mr Urosevic says.
What is E10? Can I use E10 fuel in my car?
E10 is regular unleaded petrol blended with between nine per cent and 10 per cent ethanol.
Ethanol is a colourless alcohol derived from wheat that is considered a renewable fuel or biofuel.
E10 might appeal to consumers because it is often the cheapest option at the bowser – but it might not be the right choice for your car.
“Most cars built after 1986 should be able to use E10,” Mr Urosevic says.
“However, some models have fuel systems that may not be resistant to ethanol; it is, therefore, essential that customers check their car manual to ensure compatibility.”
To check whether your car is compatible with E10 fuel, you can also use the NSW government’s online registry here.
MORE: Is it okay to use 98-octane petrol in my car?
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