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      04-11-2013, 06:28 PM   #25
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Drives: 2011 E90 M3, Jerez Black
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Originally Posted by meyergru View Post
Two more differences that I know of:

1. seats differ in the lower area (US model is like standard E9x).
2. among the different codings, US model is adapted to lesser fuel and has more restrictive M dynamic mode (only if you have the M button, which it seems is not the case).

And yes, US models start in S3 while EU start in D2 (numbers mean setting, not gear). That is not a coding thing, car has to be reprogrammed to have that changed. As far as I know, there is no coding possibility to change this (I have a EU-spec E92 and would like D3 at start).
Is it really true that the US version is "adapted to lesser fuel"? The difference between European Octane and US Octane is the method used to calculate octane. The average octane available in both countries is pretty similar if I understand correctly.

If there is a tuning difference, it may have more to do with emissions or other governmental regulation?

Research Octane Number (RON)

The most common type of octane rating worldwide is the Research Octane Number (RON). RON is determined by running the fuel in a test engine with a variable compression ratio under controlled conditions, and comparing the results with those for mixtures of iso-octane and n-heptane.

Motor Octane Number (MON)

There is another type of octane rating, called Motor Octane Number (MON), or the aviation lean octane rating, which is a better measure of how the fuel behaves when under load, as it is determined at 900 rpm engine speed, instead of the 600 rpm for RON. MON testing uses a similar test engine to that used in RON testing, but with a preheated fuel mixture, higher engine speed, and variable ignition timing to further stress the fuel's knock resistance. Depending on the composition of the fuel, the MON of a modern gasoline will be about 8 to 10 points lower than the RON, however there is no direct link between RON and MON. Normally, fuel specifications require both a minimum RON and a minimum MON.

Anti-Knock Index (AKI)

In most countries, including Australia and all of those in Europe, the "headline" octane rating shown on the pump is the RON, but in Canada, the United States, Brazil, and some other countries, the headline number is the average of the RON and the MON, called the Anti-Knock Index (AKI, and often written on pumps as (R+M)/2). It may also sometimes be called the Pump Octane Number (PON).

Difference between RON and AKI

Because of the 8 to 10 point difference noted above, the octane rating shown in Canada and the United States is 4 to 5 points lower than the rating shown elsewhere in the world for the same fuel.