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Octane and High Compression Engines

Q: My car manual says to use 91 octane fuel. On occasion I have used 89 or 90 octane and the car performs just fine. Am I damaging my engine?

A: Some cars have high compression engines, requiring a mid-grade or premium fuel. The higher the octane number, the more the fuel resists “knocking” or “pinging” from premature ignition. If you hear a pinging or knocking sound, increasing the octane level of the fuel may help. You may not have experienced a problem with 89 or 90 octane because the octane rating at the pump is a minimum requirement, meaning you may have actually received fuel with a rating closer to 91 octane. Modern engines use knock sensors, also known as detonation sensors, to identify when slight knocking begins to occur. When a sensor detects knocking (through engine vibrations), a signal is sent to the computer to adjust the ignition timing until the knock is eliminated. You probably are not doing any harm to the engine, but to stay on the safe side I would recommend using the octane rating that the manufacturer suggests – even though that means spending 20-30 cents more a gallon for it.