Saw someone posted that they use 87 octane in their A4. I've noticed on the inside of the fuel door it states that the minimum is 91. Is this true? If so, what gas station sells 91 octane? Mobil, Sunoco, etc. only have 92.

Electronic control unit and knock sensors will probably retard your timing enough that you can use 87 octane without ping, but you'll lose power. Also, Road and Track reported in some column or article a while back that there are more de-carboning cleaners in higher octane gas, even though the lower octane stuff is supposed to have "sufficient" amounts.

This is a confusing topic since the gasoline sold in the US is rated on the research/motor octane average. The number posted in the fuel door cover are based on a EC standard which is based on motor octane. Motor octane gives more advantage to octane boosters while research numbers are based solely on composition

Different R+M/2 values are regionally available with lower octane numbers in the mountain states. This is due to the altitude requirements (lower atmospheric pressure lowers the compression ratio as compared to sea level pressure)

Stick to 91 minimum as a standard. If this is not available, go with the highest available. Here in Houston, the three grades available name brand wise are 87, 89 and 93. I say "name brand" to differentiate between major oil company marketed products and those sold by independents. Only two grades of gasoline specified by ASTM are marketed for automotive use. 87 is the standard pipeline product with a detergent package containing no octane boost. 93 is the 92 pipeline product with detergent and octane package added. The intermediate grade is mixed at the station, usually 1/3 premium and 2/3 standard.
Pipeline grades are 87 and 92 with 89 "created" at the point of service by mixing the two. These usually are straight from the pipeline and have no additives. Avoid their use as the fuel system will suffer with deposits.

Economics show that the intermediate grade is the highest margin product. Mix your own and save!

Kieth,

You wrote "The number posted in the fuel door cover are based on a EC standard which is based on motor octane." As I stated above, the fuel door on my new U.S. Audi 2.8 uses the American system to define the octane requirement of 91.

However, following some French phrase is "RON 95", so if the pumps in your area show the Research Octane Number, then 95 is what to look for.

Bill Houston
99.5 2.8 Avant QMS

Both the Audi brochure and the owner's manual say that you may use regular unleaded (87 octane) gasoline. The caveat is that you must use 91 octane for "maximum performance" and with 87 octane "this may lead to a slight loss of engine power."

If you do not do any extreme driving, my opinion is that the use of other than regular unleaded gasoline is a waste of money. There is a law that says that cars sold in the US must be able to use regular unleaded gasoline.

If you do not have high performance requirements for your car, if your engine is properly tuned, your engine should not knock using regular unleaded gasoline, except for transient situations (hard acceleration). If you notice undesirable knocking, and you are sure that your engine is tuned properly, then the use of a higher octane gasoline is required. In other words, use whatever does not give the car undesirable knocking.

Manufacturers of aftermarket chips require use of 91 octane or greater with their chips.

I use regular unleaded gasoline and have not experienced any knocking under any conditions.

I'm sure it has something to due with the lighter oxygen content up here. I've been using 91, but am going to try 87 on the next trip to the pump. Any other high altitude residents try 85 or 87 yet?