[markjenn]

There are a ton of misconceptions on this thread:

1. Euro fuels are not higher octane, but just have a higher octane number than US fuels because they use the Research Octane Number (RON) method of octane measurement. US fuels use "pump octane" which is an average of the RON and the MON (motor octane number). Since MON is typically 10 pts lower than RON for the same fuel, a 95 RON fuel will typically be 85 MON or a 90 pump octane. That's why BMW specs 95 RON in Europe and 90 or 91 pump octane in the states - both spec essentially the same fuel.

2. Octane appetite in an engine is dependent on many factors, not just CR. Air-cooling and big bores raise the required octane as much as a high CR, that's the reason an air-cooled big-twin Harley with a 9.0:1 CR will ping like crazy on a hot day running 91 fuel, while a water-cooled R6 with a 13:0:1 CR doesn't ping at all running 86 fuel.

3. Energy densities of different grades of fuel at the pump do not typically vary much by octane. (This assumes they're all gas or at least all have the same percentage of ethanol if they have ethanol. Ethanol does have lower energy density by about a third, but I've never heard that the percentage of ethanol varies depending on what grade of fuel you buy, and in any event, it is only about 10% of the mix at worst. And please, please, there are other threads on this and other forums if you want to rant about ethanol.)

4. The idea that you will get more/less power out of a stock engine by running different octane (either higher or lower) generally doesn't hold water. Two exceptions are: 1) If the engine is octane-adapative, continually adjusting engine parameters on the fly to stay right on the edge of knock (which no motorcycle engine currently is); and 2) If you're under such demanding conditions (load, temps) that the engine knocks on the octane you're running. Under scenario #2, if the engine has knock sensors (like the R1200), then engine timing will be retarded to control the knock and that will reduce power. If the engine doesn't have knock sensors, then the knock itself will reduce power. In either case, running higher octane should help restore some of the lost power. But if the engine is not knocking, then you're not losing any power and running higher octane will do diddly-squat.

5. Octane doesn't materially affect the rate the fuel is burned in the engine. The reason you retard the ignition to control knock is to have the spark plug ignite the mixture before it is preignited by compression alone. That's the whole idea of a higher octane fuel - to allow more ignition advance before the fuel ignites on its own.

It's been said a thousand times before by a thousand engineers: USE WHATEVER OCTANE IS RECOMMENDED OR THAT KEEPS THE ENGINE FROM PINGING. Beyond this there is little or no benefit to going higher or lower. Octane is just the resistance to engine knocking - nothing more and nothing less.

- Mark