Higher octane is formulated to be *less* combustible. Here's why: the turbo produces a lot of compression. Lower octane fuels can ignite under increased pressure....before the valves close and the spark ignites. This is pre-ignition (or knock) which can burn valves and cause other collateral damage. To protect your engine, Volvo engineers installed a knock sensor that determines when this condition is present. The ECU then reduces engine performance to correct the situation.

If you want the full performance of your turbo, you need to run at least the specified octane.

Filling stations only have two fuel tanks, regular (87) and super (93). All grades in the middle are achieved by mixing at the pump. Since my car calls for 91 octane, a grade which almost no station in my area carries, I achieve the rated 91 by filling with super (93) then at half a tank I refill with midgrade (89).
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(98 S70 T5SE misc mods, mostly lighting) (92 940GLE)