Can regular unleaded gasoline be used in a Audi A8?

Answer 1

Yes but gas economy is worse not recommended

Bdewolff

Lets talk about Octane and what it's doing for you. Octane really matters when you are at wide open throttle. The ecu in your car has two operating methods. Open loop and closed loop. When you are in closed loop that means the ecu is looking at the sensor values and continually changing ignition timing to provide the best gas mileage possible. Closed loop will ignore the O2 sensors and run on a fixed timing map that is fixed almost ignoring all the sensor values. So in closed loop at partial throttle lets say at 50mph your car is making bout 25hp to keep the car moving at that speed. Timing is advanced really far and the relative stress/heat on the engine is very low. During this type of driving your octane value is basically irrelevant. A gasolines octane value is a measure of its resistance to knock or pre-ignition due to high heat or pressure in the cylinders. You will not have a situation of knock or pre-ignition during part throttle because the load on the engine is so low. So while driving on the highway and easily around town you can run almost water in the engine (if it was combustible) and it would be fine. So the BTU value of the fuel is the most important, what will give you the best gas mileage is the fuel with the most energy.

During open loop driving (wide open throttle) the motor is under much more load. Obviously fuel efficiency is thrown out the window here for maximum power and acceleration. The more fuel you can burn the more power you make.. Horsepower comes from burning gasoline. That's why an engine has a turbocharger so you can force more air in it therefor burning more fuel, thus making more power. So octane here is paramount as any knock or pre-ignition can potentially damage the engine (burn holes in pistons, bend rods, melt valves etc.. lots of fun stuff) and cost a lot of money in repairs. So in a racing application where MPG are irrelevant the highest octane fuel is used. That's what "alcohol" funny cars refers to is alcohol or a really high Octane fuel