MortalGuar
We have a 95 Volvo 850 GLT. The manufacturer recommends using 91 octane gas. But if you are 5000 feet above the level of the sea, you can use 87 octane gas. Why?
Answer
Octane is needed in gasoline to prevent "knock". "Knock" is when the gasoline ignites from compression before the spark fires. Your car runs most smoothly and optimally when the gasoline ignites at the proper moment, not before and not after. Your electronic ignition system chooses the optimal moment and fires the spark plug then. It's less good if the gasoline explodes "accidentally" before the ignition system decides it's the right time.
Gasoline will ignite from compression at a certain pressure that depends on the octane. Higher octane gas will be fine at high pressures where lower octane gas would ignite. An engine compresses the air-gas mixture by compacting it to a smaller volume. That multiplies the atmospheric air pressure by the same as the ratio of the large to the small volume, which is called the compression ratio. If the multiplied pressure is higher than the low octane gas can take, then you get knock. But if you go to higher altitude, the atmospheric pressure starts at a lower pressure, and even when multiplied, it stays below the limit of the low octane gas.
Most "economy cars" have low compression ratios, so that they can use low octane gas even up to sea level atmospheric pressure. They multiply the atmospheric pressure by a smaller amount and it stays under the limit for the low octane gas. But European luxury or sports cars like yours often have high compression ratios.
Octane is needed in gasoline to prevent "knock". "Knock" is when the gasoline ignites from compression before the spark fires. Your car runs most smoothly and optimally when the gasoline ignites at the proper moment, not before and not after. Your electronic ignition system chooses the optimal moment and fires the spark plug then. It's less good if the gasoline explodes "accidentally" before the ignition system decides it's the right time.
Gasoline will ignite from compression at a certain pressure that depends on the octane. Higher octane gas will be fine at high pressures where lower octane gas would ignite. An engine compresses the air-gas mixture by compacting it to a smaller volume. That multiplies the atmospheric air pressure by the same as the ratio of the large to the small volume, which is called the compression ratio. If the multiplied pressure is higher than the low octane gas can take, then you get knock. But if you go to higher altitude, the atmospheric pressure starts at a lower pressure, and even when multiplied, it stays below the limit of the low octane gas.
Most "economy cars" have low compression ratios, so that they can use low octane gas even up to sea level atmospheric pressure. They multiply the atmospheric pressure by a smaller amount and it stays under the limit for the low octane gas. But European luxury or sports cars like yours often have high compression ratios.
Does a 1993 Cadillac Seville require 93 octane?
Mike K
Im thinking about buying a 1993 Cadillac Seville, but, I see it "requires" premium fuel, 93 octane. Do these cars actually require 99 octane, or can it be run off 87 or 89?
Answer
91 octane is required for the first-gen Northstar engines. If you are not prepared to properly maintain a luxury car, then there is Camry with your name on it.
91 octane is required for the first-gen Northstar engines. If you are not prepared to properly maintain a luxury car, then there is Camry with your name on it.
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