OK, here we go..........

the higher the octane the faster the evaporation of the gas from a liquid state to a gaseous state inside the cylinder. U NEED to use 91 octane because if u use a lesser octane the spark plug will spark before all of the gas has evaporated within the cylinder. Liquid gas burns slowly, gaseous gas explodes. If not all of the gas has evaporated after the spark u will have an explosion (pistion fireing which is good) and leftover liquid gas burning slowly (bad).

a4's may retard (slow) the timing to help deal with the improper usage of gas but i'm not positive if a4's are programed to do so.

If u use less than premium fuel (lower octane) for long periods of time you very likely may cause premature wear to your internal engine parts (pistons, valves, seals and rings). Also the gas that has not burnt off may seep into your exhaust system exiting via the valves and can burn out a cars catalyic converters. Also since the turbo runs off exhaust u may screw ur turbo.

AKA, no dont use anything less than premium fuel. Audi said premium only for a reason and its not because they have stock in oil companies....

 

also, the only purpose to running higher octane fuels than is recommended is if u have increased ur cars timing.

 

I always like this question we get it at least once a week, use only 91 or 93.

 

heh heh - I no longer waste my time responding this question - its all in the archives ready to be searched.