Should you use premium unleaded petrol (gasoline) in your car? Auto Expert John Cadogan

Does a high octane fuel make your engine go better, and therefore, is it worth running the good stuff? That’s next.
Engineering for dummies. A really high compression ratio: Totally a good thing. In theory. Expansion over a greater range during the power stroke means more performance and greater efficiency. I love it already. Sign me up.
But there’s a problem. Fuel/air mixtures are not infinitely tolerant of compression. At some point the mixture starts to burn spontaneously, too early. That’s what engine knock or pinging is. You’ve heard of that.
If this occurs, the delightfully synchronous ballet of sucking, squeezing, banging and blowing (just like those numerous free online documentaries...) the ballet of sucking, squeezing, banging and blowing is upset by virtue of early blowing, which can spoil so many otherwise satisfying encounters.
Engine knock at high revs and big throttle inputs can destroy your engine - so the designers try to avoid that, oddly enough. And sometimes they avoid it by using a high octane fuel.
At its core, octane rating is simply a scale that defines a fuel’s tolerance for compression. The higher the rating, the more tolerant of high compression. High octane fuels don’t burn hotter - whatever - that crap is up there with ‘we faked the moon landing’. It’s knock resistance - end of story.
Adding confusion here is of course the three different flavours of octane rating used around the world. RON is the ‘research octane number’ - where that special test engine is run a 600rpm for the test.
Then there’s the Motor Octane Number, or MON. Same basic process, except the test engine runs at 900rpm. MON is always lower than RON, by about 8-12 points. And then there’s the Anti-Knock Index or AKI, which is the average of RON and MON. So: AKI = RON + MON over two.
Choreography - so important. In everything. But especially engines. An engine at 6000rpm is doing the ‘suck-squeeze-bang-blow’ ballet 50 times a second, per cylinder. And the precise timing of the spark - so critical. The spark has to occur at exactly the right time in relation to the movement of the piston.
Basically the spark needs to occur early enough to give the flame front sufficient breathing space so that pressure can build up and generate a deliciously satisfying and effective blow. Just like in those documentaries. A critical thrust just as the piston kicks over top dead centre.
This suck, etc., ballet business happens right down in the millisecond domain. It’s impossible to conceive of the timing without being a mechanical Jedi and using the Force. And the dark side is always calling your engine, inviting it to knock.
This is of course why modern engines have knock sensors - little acoustic microphones that listen for engine knock. It’s all they do. Most boring job ever. A modern engine manages its ignition timing in a feedback loop where the engine keeps advancing the ignition timing until the knock sensor detects incipient knock, and then it backs off a bit. Amazing.
So your engine might be doing a particular job on 91 RON fuel - punting you down the freeway at 100 kays an hour, or something. If you waved Harry Potter’s wand and swapped magically to 98, and keep everything the same, the same throttle position, same road, same everything - you’d look down and see the speedo on 105 or something.
This is because the engine listens for knock and advances the timing. High octane allows slightly more advance, and that delivers slightly more performance. Hence, 105. (Footnote for the tech-savvy: high octane fuel is also a little more dense, and this also contributes slightly to the result.)
Alternatively, you could close the throttle slightly and cruise at 100 again. In other words, there’s a slight increase in fuel economy when you switch to a higher octane fuel. Slight.
I’d suggest there’s a big difference between planning a top-speed run, or racing, in which case high octane fuel makes perfect sense. You’ll get vestigially more peak power.
But for average driving, this ‘more power’ presumption is nuts. A ridiculous way to look at fuel. This is because in average driving, you don’t need an engine capable of delivering more power. If you want more power than you’re making now, just open the throttle a little bit more.
It’s difficult to exploit peak power in ordinary driving.
You will certainly get better fuel economy on premium, but driving will be more expensive per kilometre because of the additional cost of the fuel. So, for most car owners premium gasoline a nice idea that simply doesn’t add up.
My advice for average car owners is: run the fuel the manufacturer specifies. You cannot hurt an engine by running premium when the manufacturer tells you regular will suffice. All you’ll be doing there is wasting a little money.
But running regular in an engine that demands premium will be a very bad day for you, and at the same time a very good day for the service department.