Does a high octane fuel make your car go better? Is it actually worth running the 'good' stuff? Or are you really just pumping cash out the exhaust pipe?
This report is inspired by this question from the audience:
"What about cars that recommend premium but do not require it? The ford mustang ecoboost is like that. Ford says to get all that 310 hp you’ll need to run premium. Mid-grade will give you around 300. Not sure how true that is." - TVinmyEye
The short answer is: Yep. It's true. Three per cent sounds about right. But while you could measure it, you’ll probably never feel it, and you’ll never notice it.
Engine power and fuels is one of the most misunderstood automotive topics. So, let's crack this code:
THE FINE PRINT ABOUT OCTANE RATING
Engineering for dummies: A really high compression ratio is 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. Knock is when combustion literally goes off half cocked.
If this occurs, the delightfully synchronous ballet of sucking, squeezing, banging and blowing in the engine (just like those numerous free online documentaries you may have seen...) is upset by virtue of early banging, 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.
Wikipedia: Octane rating >>
WHAT OCTANE RATING IS
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.
Octane rating is an applied chemistry thing. There’s a very particular kind of octane - the chemical - called iso-octane. Sometimes it uses the catchy name 2,2,4 tri-methyl pentane. You run it in a particular test engine with variable compression ratio. It’s - kinda - the chemical standard for 100 octane.
98 RON fuel has the same knock resistance as a mixture of 98 percent iso-octane and two per cent heptane. Just how this works. Maybe this is too much detail - I don’t know. It’s a percentage.
Wikipedia: Gasoline >>
THREE FLAVOURS OF OCTANE RATING
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.
In most countries - including ‘Straya’ - RON is used routinely. But in the US, Canada and Brazil they use AKI. And the upshot is: AKI is always about four to six points lower than RON.
I point this out because it’s a very bad idea for anyone in a RON country (like Australia) to download the US specs and take them as gospel without knowing it’s a different rating system. You might draw the entirely erroneous conclusion that standard gasoline is OK, when in fact premium is required. Because an AKI of 90 is actually a RON of 95. Jeep Grand Cherokee SRT is like that.
LIMITS ON FUEL USE
You should never use a lower octane rating than the manufacturer recommends - but you can use a higher octane rating. There is absolutely no technical problem with doing that.
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.
(If they taught applied science in schools with this level of innuendo, the entire population would emerge technically cognizant. Horny, perhaps, but tech savvy horny - and that’s the best kind. I’m sure the social justice warriors would be appalled. If I were king, I’d just take them all on a cruise and not bring them back.)
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. It never stops.
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.
It does this continuously: advance - little knock - back off - repeat. Many times a second. The point about high octane fuel is: it will also tolerate a bit more advance before pinging, and that will deliver a little bit more torque at the crank - meaning a little bit more performance.
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Keyword here: ‘a little bit’ - certainly not the same as changing the compression ratio. The difference here is quite minor. A few per cent.
People who tell you - ‘mate, she just runs better on 98’ (or whatever - smoother, cooler, etc., - or goes heaps better) they are bullshitting you. It’s at best a minor difference.
Here’s how it plays in practice: People like TVinmyEye, earlier - and you should never watch TV - TV will rot your brain and suck out your soul, leaving you a withered husk. You should watch YouTube instead. And in particular, this channel. Intellectually stimulating, spiritually fulfilling and I think you’ll agree, a whole lot classier…
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.
WILL YOU BENEFIT?
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.
The engine rapidly makes more power if you do this. Peak power is irrelevant for mundane driving because it requires only (I dunno) 25 or 30 kilowatts to drive down the road at 100. It’s difficult to exploit peak power in ordinary driving.
Even overtaking - you might be at peak power for a few seconds, and then you change gear. Peak power is mostly a kind of interesting comparative specification for engines. But it’s not all that practical.
ECONOMICS OF PREMIUM UNLEADED: WILL YOU SAVE?
Here’s the thing: High octane fuel delivers more power or better economy (they’re flipsides of the same efficiency equation). But in most markets around the world, the small improvement in efficiency is smashed into irrelevance and economic irrationality by the substantial difference in the cost of premium petrol.
What really matters to most people is the cost per kilometre of driving - and for whatever reason, the manufacturers of premium fuel do not offer it at an economically rational price. And if you listen to the bullshit marketing you’d draw the conclusion that what it generally does is clean your engine better than regular.
In other words, 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.
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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. I hope this atrocious level of innuendo helped you understand more about what goes on 'down there' (in your engine).