Premium unleaded. It’s the holy grail of fuels. Able to clean your engine, give it more power and make it sprout wings and fly off into the sunset; it’s a ‘fitness programme for your engine’ claims one retailer of 98RON fuels (both petrol and diesel). Only that’s largely nonsense.

Sure, premium unleaded (98RON) is undoubtedly a better fuel in the right engine and it does offer more power and burns cleaner in engines tuned for this type of fuel (more on this at the end of the article). But that means pumping premium unleaded into your 1984-model Toyota Corolla is a complete waste of money.

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• This article first appeared in Practical Motoring

And treating your car to the occasional tankful of premium unleaded – if it’s not specified to run on premium – won’t give it more power, allow you to travel further on a tank, or clean your engine. It’s all marketing hype.

Why? It’s simple. Most modern cars run a specific compression ratio which is a measure of how much room there is available for fuel when the piston is at the bottom and top of the cylinder. Indeed, your average petrol and diesel powered car, thanks to a fairly common compression ratio of, say, eight to one, is designed to tolerate lower octane fuels.

William Green, a chemist at Massachusetts Institute of Technology, says that “the regular fuel will burn properly and the premium fuel will burn properly and therefore there is no reason you should pay the extra money”.

Where premium unleaded does become necessary, is when you have a higher compression ratio, say, in a turbocharged vehicle. Most car makers will specify on the fuel flap what type of fuel the vehicle is happy to run on, and if it says 95RON, then there’s absolutely no reason to run 98RON in it. If, however, the maker stipulates premium unleaded then you absolutely have to shell out the extra coin for the heady brew.

But why? Well, with higher compression engines, like turbocharged vehicles or high-performance sports and supercars, there is greater propensity to ‘knock’ when run on lower octane fuels because of the design of the cylinders to produce more pressure and thus extract more power from the fuel. You might remember many, many years ago the NSW Police found itself with detonating engines in its Subaru WRXs after ignoring the manufactures instructions to fill them only with premium unleaded.

So, what’s ‘knock’? Knock is, in its most basic form, an uncontrolled explosion, or burn in the engine’s combustion chamber, which is designed to handle controlled explosions (or burning) only. Basically, knock occurs when the compression of fuel and air alone ignites in the combustion chamber (without the spark plug) usually in a far corner of the cylinder, while elsewhere in the cylinder the spark plug ignites the mixture as normal – the knock is the two explosions, or burns colliding.

See, the normal firing procedure sees the piston dropping in the cylinder, allowing fuel and air to rush in and, when the piston returns to the top of the cylinder the spark plug ignites the mixture, forcing the piston back down again. And the whole process repeats itself.

The knock, or burn in the cylinder can cause problems in the long run, but the occasional knock won’t kill your engine. Beyond that, most cars built since 1996 carry a knock sensor which is a clever little device that can detect knocking and tweak the timing of the spark plug to cancel it out.

So what does RON really mean? It stands for Research Octane Number, and relates more to the fuel’s ability to withstand combustion than it does, necessarily to octane. See, petrol is made up of around 200 different hydrocarbons but its isooctane that’s the key to all of this; it’s made up of eight carbon and 18 hydrogen atoms bonded together, and iso-octane is particularly resistant to knocking. Iso-octane requires high pressure to ignite whereas its qual and opposite is n-hepathane, another hydrocarbon, and it combusts very easily indeed.

Basically, 91RON, 95RON and 98RON relate to the fuel’s resistance to knocking. Meaning that 98RON fuel is only two off pure octane in terms of the compression it can handle before firing, and the two its missing are n-hepathane. This means that unless your car is specifically meant to run on premium unleaded then you’re simply wasting money pumping it into your car if the maker has said normal unleaded (95RON) is fine. Your car’s engine won’t knock because it’s been designed to run on that fuel.

What about marketing claims of premium fuel providing more power? Well, that’s only true, again, if your car’s specified to run on premium unleaded. It means the maker has designed the cylinders to produce more pressure and thus extract more power from the fuel. Sadly, if your car is tuned to run on regular unleaded, then pumping premium into it won’t extract more power.

Sorry.

This article first appeared in Practical Motoring