Higher octane fuel is designed to ignite more slowly than regular octane fuel. This prevents premature ignition in high-compression engines, allowing the piston to fully compress the turbocharged fuel mix before firing.

With regular fuel, you'll get pre-ignition (knock); the ECU will detect this and restrict the performance parameters of the engine to prevent the damage this condition leads to when left unchecked: burned valves and cracked heads. The engineers who designed these systems are aware of the temptation to save a few cents per gallon and have given their engines this built-in defense mechanism.

So if you're running regular, you are not achieiving the full potential of your engine, as the knock sensor is telling the ECU to hold back.

The engineers who design these cars are not trying to make the fuel companies a few extra bucks. The fuel specification is there for a reason.

HERE"S A TRICK if you want to take a few extra minutes at the pump and....a few pennies: put in half midgrade (87) and half super (93) to get the specified average (91). Octane blends are made at the pump from just two tanks (regular and super)....this way you make your own custom blend.
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(98 S70 T5SE misc mods, mostly lighting) (92 940GLE)