[paul386]

Originally Posted by RSchneider

Less octane will cost you torque and power, plus the engine might not run as smoothly and not as "lively". The higher your altitude, the more you'll feel it.

In Western Europe you fill up with 95/98, so the U.S. 91 is in no way luxurious. And there is no such thing as too high octane in retail fuel. With each ignition the ECU will fine tune the timing, and the better the fuel you feed him the better the engine will run.

On a side note Europe (RON) uses a different method to calculate the anti-knocking properties resulting in a approximately 5 point higher score for the same fuel in Europe as compared with the USA (AKI).

[Steve30dMSport]

Originally Posted by RSchneider

Less octane will cost you torque and power, plus the engine might not run as smoothly and not as "lively". The higher your altitude, the more you'll feel it.

In Western Europe you fill up with 95/98, so the U.S. 91 is in no way luxurious. And there is no such thing as too high octane in retail fuel. With each ignition the ECU will fine tune the timing, and the better the fuel you feed him the better the engine will run.

I don't know if I should be entering this discussion because my knowledge is based on Australian experience. But here is my 2 cents worth.

We have 3 levels of fuel: 91 RON, 95 RON and 98 RON with 91 RON being a basic fuel which smells like rotten egg gas. The 95 and 98 RON fuels have higher degrees of additives such as detergents and lubricants to keep the engine clean and provide longevity, similar to using a premium synthetic oil over a very basic mineral oil. In Australia, European cars are given a minimum of 95 RON as their minimum spec fuel because firstly they are tuned for it and secondly because we mainly use the European formulas in Australia for the 95 and 98 RON fuels. In Australia, BP is the best quality as it is the same premium formula used in Europe with exceptionally low sulphur levels. For a while Shell's 98 RON fuel was blowing up Porsche engines but that has been rectified with a new blend in the last 2 years.

Anyway, here is where the argument becomes interesting. Theoretically, if the car is tuned for 91 RON then the benefit of higher octane fuel is limited unless it comes with improved additives to assist the engine and valve guides in particular. But performance is not dramatically enhanced by running higher octane fuel than what it is rated for. My understanding of cars in America is that they are de-tuned for 91 RON for reasons only known to yourselves. Many Europeans cars have been unavailable to you because of the lower octane minimum fuel requirement there. But this is something to consider if you have fuel pumps stating 89, 90, 91 etc.

Those RON numbers are only a minimum guaranteed octane rating for that fuel. You could be buying 91 RON fuel which is actually 95 RON because the fuel company has guaranteed it is a minimum of 91. The actual octane may well be higher. That's why sometimes you fill up and feel the car responding better or worse even though the octane rating is supposedly the same.

So the lesson here is to know the minimum octane rating of fuel your car is tuned for and always buy that from a reputable fuel company and preferably from a high turn over petrol station so you minimise the risk of getting both stale petrol (yes, fuel goes stale) and petrol with water in it. And be happy in the knowledge that sometimes, even though you fill with 91 RON fuel you will be doing that only as a minimum. You may well be using a higher octane level fuel without paying for it!

And the one golden rule? Never use fuel of a lower rating (it will burn too soon and will overheat the engine as well as increasing fuel consumption) and always use the best oil you can afford. Oil is the lifeblood of the engine. Cheap and incorrect oil is like a high cholestorol diet. Happy travels!