What octane rating is right for the REx: 89 or 91?

In the i3 manual, BMW makes the curious statement that they "recommend" using 91 octane gas, but the "minimum" octane rating is actually 89. What does that even mean? No one sells 91 octane gas where I live. There's 87 Regular, 89 Plus, and 93 Super. I don't use the REx very much, so the price difference isn't THAT much of a factor, but it seems silly to put 93 octane gas into my car when 89 meets the minimum requirement.

I assume that the REx runs at some higher-than-normal compression ratio that requires higher-than-87 octane gas. I'm guessing that BMW determined that 89 octane is the minimum required to prevent pre-ignition. But why in the world would BMW recommend any more than that? Why would it make any difference?

Good question. The "Technical training. Product information. W20 Engine" (©2013 BMW AG, Munich) document has this to say about that:

Compression ratio: 10.6
Fuel rating [RON]: 95
Fuel [RON]: 87- 98

Click to expand...

I think that means that the 'nominal' octane rating is 95 RON and it should work on anything from 87 to 98 RON.

https://en.wikipedia.org/wiki/Octane_rating

US and Canadian pumps show the "AKI" (Anti-Knock Index) which is the average of RON (Research Octane Number) and MON (Motor Octane Number). Often it is labeled as "(R+M)/2".

In Germany, 95 RON gas ("Aral Super 95") has a MON of 85 and therefore an AKI of 90. Same for "EuroSuper" or "EuroPremium" or "Regular unleaded" in Europe, "SP95" in France, and "Super 95" in Belgium. It looks like an AKI of 90 would be a close approximation of the 95 RON gas used in Germany and the rest of Europe. This is in the range that the US calls "Midgrade":

US 'Regular' (87 AKI) is 91-92 RON
US 'Midgrade' (88 to 90 AKI) is 94-95 RON.
US 'Premium' (91 to 93 AKI) is 96-98 RON.

Likely nothing to do with octane rating. My understanding is that there are far more additives in premium gas compared to regular. Top tier gas also has more additives. Mid grade is simply a mix of the two grades. Fuel injectors keep cleaner with more additives. This is important for injectors that are used infrequently.

TOEd said:

Likely nothing to do with octane rating. My understanding is that there are far more additives in premium gas compared to regular. Top tier gas also has more additives. Mid grade is simply a mix of the two grades. Fuel injectors keep cleaner with more additives. This is important for injectors that are used infrequently.

Click to expand...

Yes, this makes the most sense to me.

Doug - out

Many modern engines use a knock sensor to adjust the timing so that they can get the most performance out of the fuel used. While many cars will work at the lower allowed octane, their output can improve when supplied with the top recommended. OTOH, using more than they suggest is usually just a waste of money, since the adjustments the engine can make only go so far. If your driving is really stressing the REx, I'd say you want the maximum output possible, but for most situations, that is not needed. The amounts of detergents is a factor...the suppliers do use more on their premium versions.

This was a while ago, but I picked up a flyer in a BMW dealer from BMWUSA that said if you did use a lower grade gas, as especially if you didn't use gas from one of the TopTier alliance, you should periodically add some injector cleaner. That could pretty much erode any savings you might get by using the middle grade, and, you'd not have the maximum output available should you need it occasionally.

FWIW, detuning the engine also (at least usually) decreases the efficiency of the engine, so running the lower grade gas may not get you as far. IF you never stress things, you may not notice a performance difference, but you may notice a range difference. It won't be much given the size of the tank, but every little bit helps.