Out of complete curiosity I filled my car with 99 octane petrol at Tesco the other day.
See
My car (5 year old Volvo) has never run better.
I've yet to see if there's an economy benefit (I'm sure there will be).
I'm nearly convinced. Is there any negative side to running higher octane fuel in your engine?
David
Poverty ;-)
Expense.
Don't be so sure. It may run smoother and could, with tuning, increase power, but it does not guarantee better fuel economy. Increasing the octane actually reduces the energy content of the fuel. It is likely that the formula has been tweaked to restore this, or very slightly increase it over standard fuel, but you shouldn't expect a large increase in economy.
Higher octane fuel is generally much worse for the environment, due to the need for larger quantities of nasty aromatics, like benzene.
Christian.
The message from "vortex2" contains these words:
It costs more! Not all engines are capable of making the best of higher octane fuel - in fact, very few are.
Try asking in uk.rec.cars.maintenance instead.
I thought it just made it less susceptible to detonation.
That's obviously the main feature and why you would want higher octane fuel. However, it has the side effect of reducing the energy content of the fuel.
Take LPG, for example. This has a much higher octane rating and so can be used in petrol engines very successfully without detonation. However, it has a much lower energy content, which is why it gets much lower mpg than 95RON.
There are methods to restore the energy content of the fuel whilst maintaining a high octane, but it won't happen by default.
Christian.
What then is the actual reduction in the "energy content" between petrols with different octane ratings?
That would depend very much on the individual fuel. Obviously, higher octane fuels from reputable manufacturers will have been formulated to restore the loss of energy content, so I would be surprised if there is any on general sale (LPG excepted) that has a lower energy content in practice.
Christian.
My Dad got more MPG from 99 Octane in his old Sierra.
I wouldn't have thought there would be a significant difference in energy content. High octane fuel is less susceptible to premature detonation but a consequence of this is a slightly slower burning fuel. The reason it sounds "better2 could be a result of that. It's likely that you'll get slightly worse mpg for a engine tuned for 95RON.
If the ignition timing can be altered to take advantage of the fuel, you'll get more power and fuel economy than before.
So increasing the octane rating does not reduce the energy content?
Depends on the engine management software. My old Saab 9-3 LT gives 10% improved consumption using "big name" brands like Esso or Shell compared to the bottom of barrel stuff from Tesco, Asda and cut price independents. So for paying 1-2% more there's a 5-fold return. But on a car with a less tuned engine you probably wouldn't get much difference.
john2
For those who have po "The engine can be driven witht he octane ratings 91, 95 and 98 RON." "98 RON is recommended for maximum effect (sic) and minimal fuel consumption."
If I see 4% less fuel usage then I have a neutral cost for using superunleaded.
David
All the Tescos round here sell branded Esso.
No, increasing the octane rating does reduce the energy content. This then can be compensated for by reformulating the fuel to increase the energy content to mask the effect.
Christian.
I often fill up with 97 RON fuel (not 99, I've never seen that) and although I don't monitor fuel consumption carefully, it's my impression that it does give more miles per gallon than the usual 95. I'm also pretty sure that the effect isn't great enough to make up for the increased cost (often a bigger difference than the 4p you quote).
My engine was designed for 97 RON although it will run happily on 95. The manufacturer's (Audi's) consumption figures refer to 97, which leads me to believe that it's better than the figure for 95, but it might be that they have to select same the fuel for performance and economy figures, and choose best performance.
I travel to the North of Scotland quite often and usually do one way on 95 octane, and the other using Super 97-99 or whatever. I use the cruise control where possible set at the same speed. And I can't honestly say I've found a verifiable difference in either performance or economy. My car is sort of the same as yours saying 95 octane on the filler but suggesting in the handbook higher octane may be better. So I'd guess
*most* EU cars are optimised for 95 octane.My old Rover, however, with no ECU control over ignition, objects loudly to 95 but runs well on 97. Of course it originally ran on 4 Star leaded.
Yes, you could be wrecking your engine if the compression ratio can't take the faster burning fuel. In simple terms, it goes bang faster than lower octane ratings, so the engine must be flexible enough to take it. A look in the data section of the handbook should clarify it.
That is simply wrong. High octane pertol burns *more slowly* than low. You won't damage any petrol engine by using a road fuel with a higher octane rating than it actually requires. The only harm is to your wallet.
Perhaps you'd give a direct quote from one?
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