Hey! Why even bother with pricey 87 octane?
Just piss into the tank; you'll do just fine.
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When the ECU senses detonation it takes more or less drastic measures to prevent the engine from blowing up. The three biggest measures are richening up the fuel/air mixture, turning down the turbo boost, and retarding the ignition timing. These actions work to (a) decrease your mileage and (b) decrease your performance. When it's all said and done, the money you saved on fuel prices will be eaten up in reduced gas mileage. I would call that "breaking even" except -- remember -- you gave up a ton of performance.