Science, Tech, Math › Science Octane Number Definition and Example What Octane Rating Means Share Flipboard Email Print Octane number reflects resistance for engine knock. PM Images/Getty Images Science Chemistry Chemical Laws Basics Molecules Periodic Table Projects & Experiments Scientific Method Biochemistry Physical Chemistry Medical Chemistry Chemistry In Everyday Life Famous Chemists Activities for Kids Abbreviations & Acronyms Biology Physics Geology Astronomy Weather & Climate By Anne Marie Helmenstine, Ph.D. Chemistry Expert Ph.D., Biomedical Sciences, University of Tennessee at Knoxville B.A., Physics and Mathematics, Hastings College Dr. Helmenstine holds a Ph.D. in biomedical sciences and is a science writer, educator, and consultant. She has taught science courses at the high school, college, and graduate levels. our editorial process Facebook Facebook Twitter Twitter Anne Marie Helmenstine, Ph.D. Updated September 09, 2019 The octane number seen on pumps at gasoline stations is a value used to indicate the resistance of a motor fuel to knock—that is, to make pinging or ticking sounds in a car's engine when you step on the gas pedal. Octane number is also known as octane rating. Octane numbers are based on a scale on which isooctane is 100 (minimal knock) and heptane is 0 (bad knock). The higher the octane number, the more compression required for fuel ignition. Fuels with high octane numbers are used in high performance gasoline engines. Fuels with low octane number (or high cetane numbers) are used in diesel engines, where fuel is not compressed. Octane Number Example A gasoline with an octane number of 92 has the same knock as a mixture of 92% isooctane and 8% heptane. Why the Octane Number Matters In a spark-ignition engine, using a fuel with too low an octane rating can lead to pre-ignition and engine knock, which can cause engine damage. Basically, compressing the air-fuel mixture may cause fuel to detonate before the flame front from the spark plug reaches it. The detonation produces higher pressure than the engine may be able to withstand.