Recommended Octane by Compression Ratio
Type the engine compression ratio and get the minimum octane band: below 9 takes 87, up to 10.5 takes 89-91, up to 12 takes 91-95, above that 95-98.
Minimum octane
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Octane rating and compression ratio
Octane tells you how well a fuel resists detonation (knock), the premature combustion under pressure that chews up pistons and rings. As a rough rule, min_RON ≈ 80 + 8 · compression_ratio. The Brazilian numbers: regular gasoline = 95 RON (≈ 87 AKI in the U.S.), additivada = 97 RON, and premium (V-Power, Podium) = 100+ RON. Ethanol = 105+ RON. Take an example: an engine with 11:1 compression → ≈ 95 RON minimum, so regular gasoline does the job. Go above 11:1, or add a turbo, and you'll want premium. The owner's manual sets the minimum. Run a lower octane than that and the engine knocks; modern ECUs respond by retarding ignition timing, which costs you power and efficiency.
Choosing fuel and applications
There are three reasons to reach for this. Fuel selection, so you don't damage the engine. Performance tuning, since more compression or boost asks for higher octane. And the economic decisions: paying for premium only makes sense when the manual actually requires it. Naturally aspirated 1.0/1.6 engines and most Brazilian flex cars are perfectly happy on regular. Turbocharged engines (Audi 2.0 TFSI, BMW B48, GTI) and high-performance imports want at least 95 RON. A knock sensor does protect the engine, but only once knock has already started.
FAQ
Does premium give more power on a common car? In an engine built for regular, you won't see much, if any, measurable gain. That extra 15–25% per liter is money down the drain.
What does knock sound like? A metallic ping when the engine is under load, like climbing a hill or lugging along at low rpm in a high gear. Let it keep happening and it will destroy your pistons.
Why is ethanol "more octane" yet less energy? The two properties have nothing to do with each other. Ethanol shrugs off detonation better (105 RON), yet it carries 30% less calorific value per liter than gasoline.
Can I mix premium and regular? You can. The resulting octane works out to roughly the weighted average. It comes in handy when only premium is on hand and your engine asks for regular, and it does no harm.
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