91, 95 or 98 Octane fuel - some are specified for your car - some are promoted as being “better” (in various ways) for your car. But are the claims true? Is it worth spending the extra on filling up? Do you really get some additional benefit, such as increased distance driven for less fuel, or a “cleaner” engine, or more power when you need it? Does anyone know - or has it ever been tested or verified?
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Good question, I’ve always wondered the same thing.
If your vehicle manufacturer specifies a minimum RON, eg. 95 RON, then best advice is not to go below that.
Any extra distance gained from higher octane has to be weighed against the additional cost of the higher RON fuels. I doubt it would be cost effective, but bear in mind the first point.
All fuels contain detergents/additives to help prevent injector deposits. The higher RON fuels might contain better or slightly more, but do you want more detergent or more fuel in your tank ? 
There is also a possible issue with older engines. Mine has done 201,000 km and is happier running on 95, although it should be able to run on 91. It will ping ever so slightly if I fill completly with 91. Even a half 91/half 95 (93 RON) mix is good.
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Great question @dcary! I was having car trouble where my old car would struggle along, bunny hopping like their was air in the fuel line. I took it to my mechanic who did a clean of the fuel line and then advised me to just Premium 98 because my car is an older model and that would help it run more smoothly. It’s been running quite well since (touch wood!) but I’ll be honest - I have no idea about the difference between the fuels.
I’d be curious to find out if there’s been tests done, too!
I saw that @pdtbaum, @peel01, @timthornley and @barry.brooks were chatting about something similar in another thread and seem to know a lot more about fuel than me! Guys, do you have any insights into how 91, 95 or 98 Octane fuel behaves in different cars?
Its funny the “stories” that are told (like yours @rachelbee). I overheard a chainsaw sales man telling a new customer that for their 2 stroke mix that was going into a chainsaw, they …“should only use Premium 95 as that 91 Octane fuel is just rubbish and doesn’t last …” (what ever that may mean?).
Anyway - it appears that there is a lot of smoke and mirror chat about these various fuels, and add to that the fluffery in the advertising from the fuel companies (my understanding is that all the fuels start out from the same tank - irrespective of who gets to finally brand it) this seems an ideal Choice subject to investigate. Hope they do!
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I have a similar response from my 150,000 km Honda Civic. Meant to run on 91 - but much happier on 95.
My question is really about getting Choice involved with a good hard “technical” look at what is good for what AND if you don’t absolutely need it (for the car specs) is it REALLY worth paying the extra for the few extra octane (and additives)?
I have had quite a few cars in my life from “crude iron” to exotic turbos. My experience is that some ran better on higher octane, and with some it made no difference. Some anecdotally seemed to prefer a particular brand! While all the fuel comes from very few sources a “package” of additives and detergents are added, and the minute differences can affect a particular engine in various ways. My current cars include a 2009 1.6L turbo, a 1997 2.0L, and a 2013 2.5L.
The turbo is speced for and runs fine on 95 but gets better economy with 98 and the cost per km difference is a wash so I usually buy 98, having bought into the marketing that 98 has (may have?) more detergents. Maybe and maybe not, but when all other things are equal it doesn’t hurt.
The 2.0L owners manual says 91-95-98. It is not happy with 91, runs very well on 95, and delivers a minor economy gain with 98, but not nearly enough to compensate for the increased cost. It gets 95.
The 2.5L manual also says 91-95-98 and we always use 91; there is zero discernible difference with 95 so have not tried 98.
E10 fuels get mandated by governments in some jurisdictions because they have a renewable component but importantly for pollies politically supports agriculture by legislating a captive customer base. The “E” is for ethanol (derived from corn) and the number is the percentage in the petrol. E10 has high octane but less energy than straight petrol and delivers roughly 5% worse fuel economy on most engines. I believe all cars built since the later 90’s are E10 friendly and some are speced for E30. E10 should not be used in an older car not designed for it.
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Octane rating is the measure of a fuels resistance to detonation. Contrary to popular belief petrol doesn’t explode inside a engine cylinder it burns at a controlled rate. If the petrol tends to explode rather than burn in any given engine it is said to detonate. If that detonation happens at low power levels it can often be heard as the pinging sound sort of like marbles rattling inside the exhaust system. If it happens at high power levels it can be quite destructive and cause breaking of piston rings/lands and even burning holes in pistons in extreme situations.
Manufacturers are aware of the phenomena and when designing and testing they establish the type of fuel the engine should run on. Modern engines with management systems have what is called a knock sensor that if it detects detonation it makes changes to the engines controls like timing to reduce or preferably eliminate the detonation but these changes effect power and economy and emissions. This is why some manufacturers can allow the use of all octane fuels in their engines but they recommend certain grades for better economy and performance and emissions. There are many factors that cause engines to detonate and different designs have different effects, this explains why some engines do detonate and others do not.
Most modern engines are high compression for better power, fuel economy and emissions reasons and high compression is a significant contributor to detonation risk therefore a reason that higher octane fuels are often recommended. Also ambient intake temperature is a factor that is why some engines happily run on lower octane fuel in cooler months but run better on high octane in summer months.
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@pdtbaum
please note pure gasoline does not absorb very much water as it almost immediately separates, gasoline ethanol mix will absorb more water because the alcohol. Diesel fuels are far more problematic as they will absorb a relatively high amounts of water.
All fuels are complex cocktails of hydrocarbons which the blending is controlled in refining and the ‘cocktail’ can vary throughout the year depending on ambient temps and supplies.
All fuels in storage tend to slowly go off, this is due to loss of aromatics and depositing of heavier elements.
Lastly the discussion about detergents makes the uninitiated think that the oil companies add soap, this is not true they do use specific chemicals that have a stronger solvent effect or a scrubbing action but it is not like soap or dishwashing liquid it is usually specific hydrocarbons.
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rachelbee to find the answer to your question may I suggest you need to know a little about how the system works. It aint brain surgery. 3 bits of information are needed and its pretty simple really.No1 Compression Ratio No 2Preignition No3 Knock sensor. Compression ratio is the volume above the piston when its on the bottem of the stroke divided by the volume above the piston when the piston is at the top of the stroke. Old cares 7-8 new cares 9-10 and more.Higher compresson more heat. Think of pumping up a bike tyre. Soon the end of the pump gets hot.because the air is compressed. No2 when the fuel vaperises it extracts heat from its surrandangs . High octane fuels extract more heat than lower octanes so the compresson ratio. If the heat generated by compresson is so high that the cooling effect by the fuel vaperising the new fuel coming in will ignite with out the spark plug fireing. Preignition loss of power damaged bearings.etc. Higher octain fuel used no preignition moter runs well.No3 On the sde of the moter there is a microphone that senses when this knock is starting and tells the computer the computer tells the spark to come in early before the compresson chamber is too hot so the cylinder fires before this high compresson nigh heat can take place. Car maker says any fuel but best performance with octane value at or above stage of preignition .
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My understanding of the facts are similar to @tndkemp and @peel01. I won’t repeat them.
I recall a car I owned many years ago with an after market turbocharger. It was a lot of fun but suffered from “pinging” and needed higher octane fuel that was not readily available at the time. Any re-turning was done manually because it had no computer. Altitude made a considerable difference. At lower altitudes pre-ignition was a bigger issue meaning it needed a higher octane fuel. Just another of many factors that can change the way your engine performs.
My short answer is, I use the fuel recommended by the manufacturer as indicated in the owners manual. Unless you keep detailed and complete records of just how your car performs in terms of fuel economy or measure it’s performance on a dyno it is easy to convince yourself of anything. The difference is unlikely to be more then a couple of % and even with a built in computer it will be hard to be sure you have measuring a difference in the fuel and not just your latest driving habit. I believe a lot of people talk up the more expensive fuels in the belief that if you pay more you get more but in this case that only applies if your engine is build to need the better fuel.
I have found that when measured over a period of around 10,000km my cars both use more fuel on E10 and 91. I believe that this has nothing to do with the octane rating and everything to do with ethanol having a lower energy value meaning E10 simply has less energy per liter. The difference is small but I choose 91 over E10 unless the E10 is 3 or more cents cheaper. In the end this is all little money and more a matter of curiosity then anything else for me. If I was sure of the environmental benefit of E10 I would use it but am unclear of the total impact of producing E10 from farm to pump.
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Higher Octane fuel are chemicals that change the characteristic of the fuels tendency to detonate, the best one was (TEL) tetraethyl lead but for good community health reasons it was eliminated in the 1980’s.
Most modern octane boosters are specialist hydrocarbons chemicals that when added during refining make the fuel more resistive to detonation.
Although to improve fuel economy was not the original design intention of petrochemical engineers when designing fuels to replace old leaded fuels with non-lead fuels of similar octane ratings, but the chemicals used as octane boosters have a slightly greater energy density so improved fuel economy is usually a happy byproduct.
High octane fuel is not recommended by manufacturers for fuel economy consideration it is recommended because of resistance to detonation (potential damage) in a given engine design and allows the engine that is designed to require it to achieve its desired HP/Torque output and emissions requirements.
In essence don’t buy 95 or 98 if you are looking at the fuel economy only, use it if the cars maker recommends it for your vehicle, if the manufacturer says your vehicle’s engine does not need the higher rating then it does nothing and is just a waste of money.
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We have tested our Liberty on long-distance driving and when we filled up with 91 the total expected kms showed 850, when we filled it up with 98 it showed 980. We have only ever filedl up on 98 if and when we drive long distances, so we cannot tell you what the daily useage difference would be for town/city driving. Have you contacted the RACQ - they might have better information, if any - we haven’t, so you have now given me an idea as well, to do that and check whether, as you queried, it has been tested and/or verified.
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The manufacturers of both of our current cars (5 and 9 years old, low km for their age) recommend 95 minimum. Years ago, when it was still fairly new, we filled the older one with 91 octane once (because the tank was empty and the petrol station we stopped at only had 91) and it ran like crap - rough, not responsive, not nice to drive at all - until the 91 was out of its system (it took months of filling up with 95 to flush it all out completely). Even at 9 years old, that car is still a delight to drive (on 95+ octane), but with the 91 it was horrible! We have had some of our more money-conscious friends/relatives question our choice to always use 95, but it’s a bit of a no-brainer to us - when we paid a lot of money for a nice car it doesn’t make sense to not have it operating at its best.
We used to have a 1997 Camry and 1998 Kia, both of which ran fine on 91 octane, so that’s what we used for them, but 91 just doesn’t cut it for our current cars.
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Some hi performance BMW cars have water injection on them. Not added to the fuel directly though, so your statement about steam is incorrect.
tndkemp has 2 much better posts than mine about old fuels etc, and I seem to have fallen into an old folks tale that may (or may not) have been accurate decades ago. I am thus deleting my original post.
To All Concern
Premium petrol is preferable for 2 & 4 - stroke engines. Usually because their use is irregular and the petrol is more stable. Aged low octane fuel deteriorates quicker then premium. Alternatively, buy small (5L) quantitie for these types of machines.
We live in a rural setting so buy fuel in large quantities for use in ride-on mowers, chainsaws, generators, as well as our cars, etc…
I also add Morleys Upper Cylinder stabilizer to all petrol whether for small engines or motor vehicles.
As for premium fuel for cars, I’ve yet to be convinced that the extra cost is justified just in the belief that it gives better economy. My argument - to get better economy is to drive according to the conditions, e.g. if the speed zone is 100 kph, then drive at 90 - 95 kph. Drive like you want to see tomorrow, not like there is no tomorrow!
I have also recently had nitrogen mix inputted into our cars tyres to give a longer life to the tyres as nitrogen allows the tyres to run cooler. Extra milage is also supposed to be a bonus. Only time will show if the cost ($70 for 4 tyres) is justified.
Hope this helps.
David K.
11-8-16
If you are in the NT - the consensus seems to be always go 95 or 98 if you can get it. While the fuel claim low aromatic 91 is fine for engines, there seem to be too many horror stories to ignore.
There’s a lot of text above and I don’t have a lot of time to read everyone’s thoughtful responses but I also wondered the same recently after a friend insisted that 91 unleaded was the devil’s fuel and that I should be filling my car with 95 and that it will last me longer. So, in classic CHOICE style, I decided I needed to test the different fuel types in my car.
I did the maths and filled up my car with both types of fuel and recorded the kms. I drive to and from work every day and around and about on the weekends, I didn’t do any highway driving. I filled up each tank as soon as the light went on at the nearest petrol station. I have a Honda Jazz, so a reasonably fuel efficient car.
And the results are in! From the 95 fuel I was able to drive 12.5km pL (I realise I didn’t calculate the Km per 100L here, apologies!), and from the 91 I was able to drive 10.9km pL. By my calculation this makes the 95 14.6% better performing. At the pump it was only 10% more expensive than the 91.
Moral of the story is that the better the fuel, the more efficient it is and you’ll fill up less often - saving you $$ at the pump.
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FYI @npatch @rachelbee @dcary I’m also planning to test 98 next time I fill up with the same kind of driving. I’m headed overseas in two weeks though so results may be a while! I’ll come back here and let you all know whether 98 is worth it
