Octane around the world

[LVR]

I have been using BP Ultimate for near on a year now...... I was under the impression that it was 98 octane and @ $85 for a tank of approx 55 litres, figured I was giving the 4.8 the best around, yet it knocks like a bitch. - would hate to see what would happen if I used 91 RON...........

[fast4d]

in the US we use R+M divided by 2

87,89 and 91 octane using this method.

[aeroresh]

Octane isnt measured the same around the world. Dont ask me how but US 91 is similar to uk 95, etc

[JCL]

Paomien: Pretty good summary.

In short, the research method will always yield higher numbers than the motor method. (R>M, but it is not a linear or fixed relationship in any way

What the US and Canada use is called Pump Octane, or Anti-Knock Index, and as was noted above, it is (R+M)/2.

From a 98 RON, you can expect 5 to 6 points different to get to the pump octane, so that is like 92 - 93 pump octane. Again, it isn't a linear, fixed conversion, because a fuel that reacts one way in a test can react differently in the other test. Still, it is a good rule of thumb at the octane ratings under discussion.

89 pump octane is, similarly, very similar to 95 RON in Europe and Australia.

The article made another very good point, which is that different fuel blends will perform differently in various real-world engines, despite having the same octane rating. That is why the "89 is crap" and "I can use 87 just fine" discussions have to be qualified with "from my supplier, in my geographic region" to be valid. For example, 89 works fine for me, no loss of power or mileage, but that doesn't mean it is true for anyone else. (but it might be true if they live near to Vancouver, have a 3.0, and drive similarly to me)

BMW recommends 91 octane, which is a good way of covering all the variables in local refining and distribution systems. Sometimes it won't be sufficient. Sometimes it is more fuel ($$, and anti-knock index) than you require. Safest approach is to just use 91 in North America, and 98 elsewhere. Or, experiment and see what happens. My experience is not necessarily what yours will be.

Now, has anyone else ever run one of those single-cylinder Waukesha engines and calculated a test lab octane rating? It was a university course in mechanical engineering, years ago. Good fun.

Jeff