I've done a lot of research on this topic because I couldn't find an answer. Obviously we are not going to get any info out of BMW about the stock tune. But we can conclude the following:
1. Based on charts of octane Vs. compression ratio, our engines require 102 RON for 10.2:1 compression. It took me forever to find a chart that specified that it was RON and not AKI octane (I suspected all along). With the average 10 point spread between RON and MON, 102 RON is most likely equivalent to 97 AKI.
The chart mentioned above is most likely based on a carbureted engine with iron heads. With aluminum heads, ECU with knock control and direct injection, you could probably drop that octane requirements by 1-2 points.
2. In Germany: "Big suppliers like Shell or Aral offer 100 RON gasoline (Shell V-Power, Aral Ultimate) at almost every fuel station..." (wikipedia). I've seen some evidence that 102 RON was offered at one time.
So based on the above, I would concluded that 95 AKI would be needed for optimum power on the N54, and I would assume that the BMW engineers set the spark advance based on that since 100 RON is readily available in Germany. The BMW can then retard timing for our crap fuel here in the U.S. especially in the state with the most BMW's and the worst gas (California).
As those of you who live in the P.R.C. (People's Republic of California) know, our premium gas is only 91 AKI and the only alternative at the pump is street legal 100 AKI race gas $7-$10 per gallon depending on where you live.
Our 91 AKI CARB ReFormulated Gasoline (CARB RFG) is only 5.7% ethanol. I believe they reduced the ethanol to reduce vapor pressure for EVAP emissions. Our E9x fuel tanks are 16.1 gallon.
Even at my closest E85 station in Brentwood ($$$) E85 is $3.75/gal. Adding 1 gallon of E85 to your 91 AKI CARB RFG would add just enough ethanol to boost it to E10, raising the AKI by one point while still staying within the 10% ethanol that BMW requires for warranty coverage. It takes 1.7 gal of 100 AKI race gas ($9.89/gal in Brentwood) to accomplish the same thing. This is because ethanol has a very high blending octane value (112.5 AKI) when added in small amounts. In the winter you'll have to add about 1.2 gallon E85 since the ethanol will be reduced to 75%.
A lot of guys on this board are running way more than 10% ethanol and claim no problems. EPA recently approved E15 for all cars made after 2001. So I'm pretty sure E10 is going to be OK for our engines and fuel systems. *That said, try this at your own risk*. If in doubt you can buy an ethanol test kit and see how much %E in in your gas.
BTW, based on my research, adding more Ethanol brings diminishing returns. Sunoco has a good explanation of this on their race gas website. That's why I suggest not boosting the ethanol past 10%. If I need more AKI I can add some of the race gas (which also has ethanol) and/or some Torco Accelerator (MMT/Napathalene based octane booster).
To boost 91 AKI to 95 AKI using only 100 AKI race gas, you'd need 7.1 gal of the Race 100 for our 16.1 gal tanks.
For Sale: 2009 335i 6MT
(E92/N54), Sport / M-Technic / no iDrive / Black Sapphire / Black Dakota Leather, ZCW, 6FL, PDC, 19" VMR VB3, Pilot Super Sport, M3 control arms + tension links (fr) S. California
Last edited by NoTempoLimitN54; 11-22-2012 at 02:04 PM.