Top 10 facts about octane rating every car owner should know | Auto Expert John Cadogan

Coming up - the top 10 things you need to know about octane rating. Number one with a bullet: 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. Two: Octane rating has nothing to do with more energy, intrinsically. Ethanol blended fuels pump up the octane, but they actually have less energy than low-octane gasoline. The two things are unrelated. Three: 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. Four: 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. Five: 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. Six: 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 not as much as there would be if they increased the compression ratio. Seven: 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. Eight: And that’s 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. 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. Nine: 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 in most of Europe. But in 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. 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: 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. 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 the fuel’s sensitivity. It’s certainly possible to have octane ratings greater than 100. 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?