When you pull into your local filling station, chances are there will be two types of petrol on offer: the cheaper premium grade and pricier super unleaded. So what’s your poison? Go for the super just because it sounds like a good idea, or be thrifty and stick with the premium?
The most significant difference between the two is the research octane number (RON) of the petrol. The octane rating tells you how resistant the fuel is to detonation, known as knocking or pinking. In a petrol engine, petrol is mixed with air, then it’s compressed and ignited by a spark. When that happens, the mixture burns outwards from the point of ignition like a grassland fire (but faster). The burn should be smooth and controlled, but if the mixture is compressed too much, random pockets of the mixture spontaneously detonate too early. It’s audible and can make a sound like dried lentils being poured into a tin can, or a diesel-like knocking noise.
A key way of increasing a petrol engine’s performance is by raising the compression ratio, or in a turbocharged engine increasing the boost – or both. Either one increases pressure inside the combustion chamber when the fuel ignites. In older engines, the threshold at which detonation became a threat had to be carefully managed by engine designers and tuners, especially when turbos came along. Then back in 1982, Saab’s engine genius, Per Gillibrand (known as ‘Mr Turbo’), dreamed up Automatic Performance Control. APC listened for the onset of knock by using a microphone attached to the cylinder block – a knock sensor – and monitoring boost pressure and engine revs.
Today petrol engines use similar anti-knock systems, but thanks to much faster processors in engine computers they can also use algorithms to predict when knock will occur. Naturally aspirated engines delay the point at which combustion is triggered (retarding the ignition) if knock threatens, all of which brings us back to the question of whether you need to fork out the extra dosh for super unleaded.
The answer is, there’s only one real reason to and that is because your car has a high-performance engine or the handbook explicitly says you should use it. Using fuel of a higher octane than your engine needs or can benefit from won’t hurt it, only your wallet.
The difference between premium and super unleaded at the UK forecourt these days is a maximum of two points (97 octane versus 99) and the chance of a modern engine being damaged by the lower of the two is nil. However, the engineers calibrating higher-performance engines and chasing the best performance numbers are likely to have done so using the highest-octane pump fuel available.
The higher octane allows the engine to use a higher boost pressure and more aggressive settings to pump out a little more power. With the lower octane, it may back off those settings a tad to stay below the knock threshold. Whether you can notice the difference subjectively, though, is down to how attuned you are to your car.
Worst case scenario
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In France, we have the choice
In France, we have the choice between 95 Octane (SP95) and 98 Octane (SP98) fuel.
For all my cars, there is no difference in terms of consumption or performance.
It's the same conclusion between fuel from compagny stations (BP, etc.) and from supermarket stations.
Why not just two?
Having just read most of the posts on this subject from some well informed posters, I want to ask a few questions,why did we ever really have to have two types of Petrol?, if modern technology has eliminated knocking in engines, and it doesn't seem to make that big a difference, why not just have one unleaded?, ok, some of you notice a bit more grunt in the lower gears etc, but, the rest of us would've wanted better consumption, and not just ten or twenty more miles per tank extra, you could say that, if the cars brain alters the knock when you fill up with either fuels is immaterial, and, I've said before, almost all your fuel stations are supplied from one or two refineries in your area, I know this because I've seen them going into a refinery one after the other, I think super unleaded is a revenue maker, if someone can find an article on a strip down of a new engine that's been run exclusively on super unleaded.
A vote for Superunleaded
I was surprised to find that, when we got a 1.6 naturally aspirated Nissan Note Auto, it also saw better torque. How do I know? The car changes up to a higher gear lower down the rev range. This is most noticeable when driven gently. Better mpg was also noted.
Why is this? It was explained to me that manufacturers, not unsurprisingly, want to get the best fuel economy figures when their cars are tested by the authorities. What better way than to set them up to run on superunleaded?
BUT Joe public wouldn't buy a non-performance car that needed superunleaded fuel. No problem. Just tell Joe it just needs 95 RON premium fuel and the knock sensor etc will adjust the engine accordingly.
As most of us are not doing many miles these days I would suggest that the extra cost of superunleaded is well worth the benefits. Have a look at what John Coates (aka Honest John) has to say.