Fuel Octane/AKI/RON... so what is "Premium" according to BMW?

[madmaxdvl]

I consider myself warned.



I'm always up for a "lively" conversation.

[Bill R.]

stock m50 in your year.

Hey all.

Been lurking around the boards all morning looking at recommended octane ratings, etc. for a 1992 525i with the M50B25 (non-VANOS). I know that:
a) I have no knock sensors and, hence, the engine will not "adapt" to the wrong gasoline grade and
b) my dashboard and fuel filler say "Premium Unleaded Only"

The question, though, is with all the different fuel ratings, what does BMW mean by "Premium?" I found the following passage on WikiPedia which would seem to suggest that, in the US, "Premium" would be 89/90 octane (AKI) (in other words, our mid-grade)... since this is referenced as "European Premium":

<<<Different countries have some variation in what RON (Research Octane Number) is standard for gasoline, or petrol. In the UK, ordinary regular unleaded petrol is 91 RON (not commonly available), premium unleaded petrol is always 95 RON, and super unleaded is usually 97-98 RON. In the US, octane ratings in fuels can vary between 86-87 AKI (91-92 RON) for regular, through 89-90 (94-95) for mid-grade (European Premium), up to 90-94 (RON 95-99) for premium unleaded or E10 (Super in Europe)>>>

This would make "Premium" 89/90 AKI (or "octane" for us Americans) or 94/95 RON (for our folks in the UK, Oz and elsewhere). Many of the previous posts regarding "recommended octane" or "fuel grade" are asked in one country and answered in the terms of another... making this answer ellusive. I know that using too high an octane on a stock motor can be detrimental. I'm actually not all that concerned with spending the extra $.10 a gallon to fill up, but am rather wanting to use the "correct" fuel grade to avoid carbon buildup and other issues over the life of my car. This is my daily driver and is only driven in a "spirited" manner on the roads--not raced.

Anyone have the definitive answer on this? My car did not come with an owner's manual so I have no "official" reference from which to pull. However, I did find what appears to be a 1992 e34 owner's manual online (http://luuk.xs4all.nl/bmwboeken/bmwe34manual/) that references "Premium" fuel as "95 RM" (see page 4 of above)... which makes me think that BMW may actually mean 89/90 AKI (Octane), or mid-grade fuel for the US market, but just didn't translate that requirement when they exported their vehicles.

FYI, my Bimmer (henceforth known as Ludwig, or "Vic") is:
1992 (build date 3/92)
bone-stock 525i
M50B25 engine
5-speed manual tranny
Islandgrun Metallic ('cause we all know the color makes a difference in how fast you can go!) ;-)

[madmaxdvl]

stock m50 in your year.

Bill R.

Thanks! That's precisely the kind of info I was looking for.

Since the US typically sees AKI or 89 for mid-grade and 91-92 AKI for high-grade, there seems to be a "fudge factor" in the gasoline selection. I'd be more apt to say, then, that if I was doing low-load, part-throttle driving most of the time, mid-grade would work (and would probably work for the majority of "normal" driving conditions, since engineers tend to be a bit conservative). However, for mountainous driving, very "spiritied" driving, or driving in hot weather (i.e. Phoenix/Tucson in summer), the 92 AKI "super premium" fuel would probably be best. Or, heck, half a tank of each and arrive approximately at the 90 AKI spec... ;-)

Thanks for your help, everyone.

-Chad