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JCL · #2 12-23-2009, 12:12 PM

I would assume that in Australia you are using the Research Octane Number (RON) to refer to octane rating, similar to Europe. In North America, we use an average of RON and the Motor Octane Number, two different test protocols, and call it the Anti Knock Index (AKI)

In North America, BMW recommends 91 or 92 AKI, I can't remember which. Vehicles require a minimum of 87 AKI, with a possible loss of mileage and performance, and many run fine on 89 AKI with no loss of power or performance. Other vehicles don't run well on anything less than 92, primarily because fuel varies widely in quality across North America.

There is no precise conversion between RON and AKI, but it is about 5-6 points usually. So, 98 RON is the same as the 92 AKI recommendation, and many vehicles will run fine on 95 RON, which is similar to 89 AKI in North America.

If you are using the RON scale, it is not surprising that your vehicle would run better on 98 than 91 (which is similar to 87 AKI). I would use 95 RON in any case, and see if you can tell a difference. If you can't then carry on. If you can, then use 98, unless BMW Australia has a different recommendation.

If you are using the AKI scale, and it runs fine on 91, there is no advantage to providing fuel with more AKI than the engine is designed for, apart from the fact that the tank of fuel you purchased may be fresher, or may have higher concentrations of cleaning agents, both of which would explain improved engine response.

Higher octane fuel than that which the engine is designed for does not run hotter, does not contain more energy, and does not benefit an engine, but it does lighten a wallet. The exception to the above is if you have aftermarket modifications that increase the compression ratio, such as different pistons or forced air induction (turbocharging or supercharging).

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