Dealer recommended mid grade fuel. Why?
Senior Member
Joined: Nov 2008
Posts: 488
Likes: 0
From: Michigan
W203-4M W163
http://en.wikipedia.org/wiki/Octane_rating
Regional variations
The selection of octane ratings available at the pump can vary greatly from region to region.
In the Rocky Mountain (high altitude) states, 85 AKI is the minimum octane, and 91 AKI is the maximum octane available in fuel. The reason for this is that in higher-altitude areas, a typical naturally-aspirated engine draws in less air mass per cycle due to the reduced density of the atmosphere. This directly translates to less fuel and reduced absolute compression in the cylinder, therefore deterring knock. It is safe to fill up a carbureted car that normally takes 87 AKI fuel at sea level with 85 AKI fuel in the mountains, but at sea level the fuel may cause damage to the engine. A disadvantage to this strategy is that most turbocharged vehicles are unable to produce full power, even when using the "premium" 91 AKI fuel. In some east coast states, up to 94 AKI is available [1]. In parts of the Midwest (primarily Minnesota, Iowa, Illinois and Missouri) ethanol based E-85 fuel with 105 AKI is available [2]. Often, filling stations near US racing tracks will offer higher octane levels such as 100 AKI.
