Should I use hi octane premium petrol, and what is octane rating?

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• published: 02 Nov 2015
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Will high-octane fuel make your car go better? Will premium petrol make your car run more economically? More smoothly? Cleaner? Or any combination of those things? No. Premium petrol claims are almost entirely bullshit, and the bits that aren’t bullshit are almost entirely over-blown marketing hyperbole. If you need high-octane gas, you need it. If you don’t, it’s just a waste of money. The brand names of petrol (gasoline) are almost entirely bullshit, too. In Australia, most of the petrol is actually imported from mega-refineries in Singapore in humungous ships, and the branding of that petrol is an entirely artificial exercise that takes place only at the point of sale. Almost everything fuel companies tell you about their brand of gasoline is either grossly overstated, or else outright bullshit. Especially advertisements promoting the purported benefits of premium petrol to most drivers, who really don’t need it. Brand A gasoline is just as good as brand B gasoline because it’s the same gasoline. It came here in the same big tank, on the same big ship. The number is really all that matters - the octane rating. Here in Australia we have 91-, 95- and 98-octane gasoline, and other countries are different … marginally. We also have e10, which is up to 10 per cent ethanol, and because the ethanol’s an octane booster, it’s up to about 94 octane. It’s not always 10 per cent, but it’s never more than that. People think it’s the high octane fuel that delivers the performance, but it’s really the engine’s compression delivering that performance. The high octane fuel just tolerates the compression, allowing the engine to do its thermodynamic voodoo. There’s two main kinds of octane ratings, and they’ve had several children out of wedlock. Of course. After all, why use only one octane rating, when 15 would do? Research octane number, or RON, and motor octane number, MON, are the Adam and Eve of knock-resistance numbers. They’re just different test protocols - different revs, pre-heated fuel in MON, and variable ignition timing, too - to stress the mixture even more. And that means a RON rating is likely to be 10-12 points above a MON rating - for the same knock resistance. Here in Australia, and across the ditch in New Zealand, as well as in most of Europe, they use RON, but in the United States, Canada and Brazil, they use an average of RON and MON, called the ‘anti-knock index’, or AKI. So, AKI = (RON + MON)/2. Sometimes it’s also called the PON, or ‘posted octane number’. The biggest issue for drivers is: Which fuel you should put in your car? And the answer is: The minimum octane rating recommended by the manufacturer. Obviously this is because if you tip an octane rating less than that which the engine is designed for, it’ll knock. Too much squeeze. And then the engine will get damaged, compose a letter to its solicitors, habeus the crap out of your corpus, and then hand you a bill you can’t jump over. That’s bad. Many people fall for the marketing hype here - and fuel companies are very good at pumping up the tyres of their premium gasolines. It’s unprincipled. Most cars don’t benefit from premium. If you drive a car like a BMW M3 that requires 98 high octane fuel, that’s what you need. But if you drive a Toyota Corolla that Toyota says is happy to sip regular 91, all day long, the expensive stuff is not going to help. It really is that simple. Crack open the owner’s manual, or go online, and see what the manufacturer recommends as the minimum octane required for your car. And then just tip that in until the day it dies. And buy the kids better presents at Christmas with the money you save. It’s really easy to save money here, and there’s no tradeoff. This is one of the few cases in life where the minimum required is not merely adequate, but also excellent. Don’t buy premium gas if you don’t need it.