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

[madmaxdvl]

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!) ;-)

[ThoreauHD]

At or above 91 octane. And the reasoning is that your engine will stutter and no workie when you have it running at native spec's, like say with a jim conforti or EATS chip, at a lower octane.

[madmaxdvl]

At or above 91 octane. And the reasoning is that your engine will stutter and no workie when you have it running at native spec's, like say with a jim conforti or EATS chip, at a lower octane.

I realize that I'd need 91+ "octane" (US spec AKI) IF I was using an EAT chip or similar, but not all e34 owners want to chip their engines (I know I don't). The question really revolves around the true meaning of "Premium Unleaded Fuel" as defined by BMW, and not the "octane" requirements for those owners who chose to chip their cars (who should, of course, follow the chip programmer's recommendations).

Hasn't anyone else thought of this?

[liquidtiger720]

it dosnt say on your fuel cap?

Mine says 89 on the fuel cap, which is mid-grade.

[JonE]

Here in Minnesota "regular" is 87AKI, "premium" is 89 or 90, and "super premium" is 92 or 93. If I were you I'd probably opt for the above "premium", if not chipped. In other words, use 89 or 90AKI and see how it runs. If you start knocking, you can always spend more and fill with the 92 or 93 AKI. FYI, I use 90 in my 535i, which is chipped, no knock sensors and it runs fine.

[rob101]

doesn't the M50 without vanos have knock sensors?........ thats news to me if it doesn't , haha 91 ron is premium, thats a joke! a euro 525i runs on "premium" which is 95 RON i run mine on 98 RON. but i am pretty sure its not that high i haven't heard anything but 91 or 92 octance recommended for US m50's in the articles i have read.

[BigKriss]

Australia uses a different rating of fuel than the USA. So when USA chips specify a minimum of 91 octane, thats 91 PON (pump octane number) which is roughly equivilant to 95 RON (Reseach Octane Number).

[rob101]

that would explain alot. i did read your post on that before....... but i didn't think the difference was that large.

[madmaxdvl]

LiquidTiger:
Nope, my gas cap is only metal and doesn't say a thing. But since you have the M50TU, you've got the VANOS and a knock sensor that can compensate, if needed.

JonE:
That's another problem with the "octane" and "premium" wording... it's not even consistent within the US! I remember 91-92 "octane" gas being called "super premium" in some places, too. Then again, I remember "regular" being 85 octane (AKI) in Montana and parts of Idaho, too!

Rob101:
You and BigKriss have hit upon the very crux of my original confusion

So is there anyone here that runs a non-VANOS M50 without a chip on 89 octane (US spec, R+M/2 or PON). Next time I fill up I'll probably mix half a tank or so of mid-grade with the high-octane I've got in there right now... but I know, too, that engines can "knock" (predetonate) without us being able to necessarily detect it by sound or feeling--which is something I'd like to avoid, too, especially since the head is aluminum (I'm not so picky on my wife's '79 Jeep CJ--that motor is nearly indestructable (and ran great on 85 octane in Montana, in fact).

I'm a patient man, though... just verbose. I hope to be lurking and posting here regularly in the future, so just fair warning that I may tend to be a chatterbox in my replies.

[rob101]

so just fair warning that I may tend to be a chatterbox in my replies.

i give you fair warning i may tend to be a chatter box in my thread hijacking.