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Old 03-06-2011, 09:58 PM   #5
3rd Gear Member
Join Date: Sep 2010
Location: Guelph, Ont.
Posts: 321
First things first, the term 'premium' is a marketing ploy. It is not better gas than regular, it's different gas. If you want good gas, go to Petro or Shell since they don't use ethanol (yet afaik), and avoid esso, pioneer, sunoco, and essentially all others. There are no more impurities in 87 than there are in 91 so your fuel filter WILL NOT get damaged. Furthermore, the engine isn't tuned to the point where damage will occur if you run anything less than 91, even over an extended period of time. If it did get damaged under these circumstances then there wouldn't be any possibility of tuning the engine any further, or running boost on this engine. Think critically, if you needed to run 91 stock, then how could anyone push 6-8psi on the same gas without damage....answer is they couldn't. Furthermore, the octane rating is actually a ratio of septane and octane (two different types of gas). 87 has as you would imagine 87% octane and 13% septane. Does this really matter, well since septane has more btu's than octane, yes it does because you get WORSE mileage out of an engine if you run higher than needed octane...yes, you did read that correctly. Most people look at a compression ratio and automatically assume that alone defines what type of gas is required, that's an incorrect mentality. Compression ratios are important, but so is the material the block is made of, the material the head is made of, the altitude the car is driven in, the humidity the car is driven in, the air temperature, the rpm range (based on idividual drivers), the load (if the car is towing or full of fat people), the temp of the spark plug, the type of fuel delivery, the list can go on for quite some time I've just listed some for illustration purposes. The reason a company like BMW will put a 91 octane required sticker on the gas cap is to imply exclusiveness, and protect themselves. If they didn't put that sticker there you can be damn sure that some jacka$$ would drive the car through a desert, while towing a camper with the A/C on and farting along at 1200rpm...essentially destroying the engine while the car was under warranty.

From my personal experience, when I'm driving on winter tires, I use 87, when on summer tires, 89. And from what I've read compared to other e46 owners I get much better mileage than average. This is owning 2 non-m e46's ('01 325Ci, and '04 330Ci).
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