Top 10 facts about octane rating every car owner should know


e10, 91, 95 or 98: Which ‘flavour’ of petrol is right for your car - and how does the underlying tech affect your engine? (Pro tip: You can destroy your engine if you get this wrong.)


Number one: Use the right fuel

Never use a fuel with an octane rating lower than the carmaker recommends. That’s a great way to damage your engine. Going higher than the minimum octane the manufacturer recommends is quite OK. But it will cost you more money.

Number two: Octane rating and energy content

Octane rating has nothing to do with a particular fuel having more or less energy, intrinsically. Ethanol blended fuels pump up the octane rating, but they actually have less energy than low-octane gasoline. The two properties are (mostly) unrelated.

Number three: Other common misconceptions

Octane has nothing to do with the speed of combustion, or the heat of combustion. These are two things that scientifically illiterate halfwits claim all the time. Simply not true.

Number four: Knock, knock..

Octane rating is all about knock resistance. It’s about burning in a controlled way under pressure, while hot.

High octane fuels simply resist autoignition better than low octane fuels. Autoignition - which is the fuel burning thanks to the heat and compression in the chamber - before the spark plug fires - causes knock. Which destroys engines at high rpm and big throttle inputs. That’s bad. (Too much ignition advance also causes knock.)

Number five: Octane and compression

If an engine is optimised for high octane fuel the designers can increase compression and add ignition advance, because the fuel is more resistant to autoignition. And it’s these two things that lead to a peak power increasefor engines optimised for high octane fuel.

Number six: Using premium when you don’t have to

If you use high octane fuel in an engine designed for low octane fuel, the engine will adapt up, slightly. The knock sensor will allow a small increase in ignition advance and there will be a slight increase in power. Slight.

Certainly this adaptation will not produce as much additional economy/power as there would be if they increased the compression ratio and optimised for premium.

Number seven: Economic rationalism

Here in Shitsville, it’s almost never economically rational to use premium fuel in a car designed for regular. The extra cost of the premium fuel is, in practice, never offset by the slight increase in economy. You’re just blowing money out the exhaust pipe unnecessarily.

Number eight: Marketing premium to the masses

Point number seven is of course why fuel manufacturers talk up the alleged ancillary benefits of premium - such as the spurious claim that premium will also keep your engine clean. And if you believe that, I’ll sell you the Shitsville Harbour Bridge. (DM me on that...)

It’s such bullshit. They’re not promoting premium because it’s a benefit to you - they’re promoting it because it’s a benefit to them.

Number nine: Overseas octane ratings are different

If you’re reading owner’s manuals from overseas, bear in mind that octane ratings are not constant around the world. Here in Arse-trailer, we use ‘research octane number’ or RON. Same standard as most of Europe.

But in Retardistan and Canada (ie North America) they use the Anti-knock index, which is the numeric average of the RON and another octane measurement standard called the Motor Octane Number (MON).

Essentially, for any given fuel, RON is about four points higher than the Anti Knock Index. So 91 here - our entry-level cat’s piss petrol - is about the same as 87 gasoline in the USA and Canada.

And if you’re wondering why so many Euro cars demand 95 here in Shitsville, it’s because 95 is the default, entry-level cat’s piss in Europe. They don’t do 91.

Finally, number 10: The full techo explanation

Time to go 100 per cent propeller-head: Octane rating is an index of the knock resistance of a particular fuel compared to a laboratory standard kind of fuel called iso-octane. Which is actually 2,2,4 tri-methyl pentane - for those of you who remained awake for carbon chemistry in high school.

Iso-octane has an octane rating of 100, and another chemical - n-heptane has a rating of zero. There’s your measurement scale.

So, essentially, 91 RON unleaded has 91 per cent of the knock resistance of iso-octane when you run the test in a special experimentally controlled engine with a variable compression ratio, against a standard set of test protocols that is basically a miracle cure for insomnia.

(What I’m saying is if you make up a litre of fuel from 910 millilitres of iso-octane and 90 millilitres of n-heptane, it’ll perform the same as 91 RON petrol from the pump, etc.)

The engine runs at 600rpm for the RON test and 900rpm for the MON test and the difference between the two values is an index of what petrochemical propeller-heads call the fuel’s sensitivity.

It’s certainly possible to have octane ratings greater than 100, too. E85 is about 102, straight ethanol or methanol - both about 109, propane and butane (think: LPG) both about 112. Methane - that’s natural gas - is about 120. Toluene - a fairly evil octane boosting additive - is about 121. And hydrogen gas is more than 130.

Who knew?

Learn more on this via Wikipedia’s ‘octane rating’ entry >>


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