If it’s better quality then it’s better for you, right? What about when it comes to the fuel you tip into your car? Is premium fuel better for your car?

If you believe the marketing, premium unleaded is liquid magic, able to clean your engine while you drive, give you more power and better fuel economy. But you shouldn’t always believe the hype.

Sure, fuel is incredibly cheap at the moment, so there’ll be people out there thinking, well, ‘I might as well fill my 20-year-old Corolla with the good stuff. Might turn the thing into a hot hatch, or give it wings’. Only it won’t. Don’t get me wrong, in the right engine that requires premium fuel (98RON), then filling with that fuel is absolutely the best thing.

So, I shouldn’t fill up with premium unleaded?

No, not if your vehicle’s handbook doesn’t recommend it. And that’s because most vehicles 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-powered car, thanks to a fairly common compression ratio of, say, eight to one (8:1), is designed to tolerate lower octane fuels.

That means, according to William Green, a chemist at Massachusetts Institute of Technology, “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”.

Inside the fuel flap it says, 98RON only…

When you’ve got a vehicle running a higher compression ratio, say, a turbocharged vehicle then running higher octane fuel is necessary. If it says 98RON on the filler flap then run that, but if it says to run 95RON then run that. Running 98RON won’t give you a noticeable boost, and those on BookFace who tell you otherwise are imagining things.

Hang on, what does RON actually 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 8 carbon and 18 hydrogen atoms bonded together, and isooctane is particularly resistant to knocking. Isooctane requires high pressure to ignite whereas its equal and opposite n-hepathane, another hydrocarbon, combusts easily. See where we’re going? 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 it’s missing are n-hepathane.

What is knock?

With higher compression engines, like turbocharged vehicles or high-performance sports and supercars, running lower octane fuel than specified can see them ‘knock’. Huh?

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. Make sense?

For most cars premium fuel is a waste of money

See, the normal firing procedure sees the piston dropping in the cylinder, allowing fuel and air to enter 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 uncontrolled explosion, or burn in the cylinder can cause problems in the long run but the occasional knock or ping won’t kill your engine. But car makers are here to help, developing the ‘knock sensor’ which has been a common feature on most cars built since 1996. This little device can detect knocking and adjust the timing of the spark plug to cancel it out.

But premium unleaded is meant to get rid of gunk?

Yep, that’s the marketing claim. Gunk monsters chasing you down the road until you fill your car with premium unleaded and then they’re vanquished.

Now, this is kind of true and kind of misleading. See, as above, running the correct fuel for your car is the best thing you can do. But, yes, premium unleaded is designed to help clean your engine but that’s got nothing to do with the fuel itself and more to do with the detergent tipped into it.

We’ll cover this off in a later article but, in short, all fuel (petrol and diesel) has detergent mixed into it. And while some car makers will say just how much is included in their mix not all will. Just know that, you’ve got the same detergents in a tank of 91RON as you do in a tankful of 98RON.

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