Straight-run gasoline has an octane number of about 70. In other words, straight-run gasoline has the same knocking properties as a mixture of 70% isooctane and 30% heptane. Cracking, isomerization, and other processes can be used to increase the octane rating of gasoline to about 90. Anti-knock agents may be added to further increase the octane rating. Tetraethyl lead, Pb(C2H5)4, was one such agent, which was added to gas at the rate of up to 2.4 grams per gallon of gasoline. The switch to unleaded gasoline has required the addition of more expensive compounds, such as aromatics and highly branched alkanes, to maintain high octane numbers.
Gasoline pumps typically post octane numbers as an average of two different values. Often you may see the octane rating quoted as (R+M)/2. One value is the research octane number (RON), which is determined with a test engine running at a low speed of 600 rpm. The other value is the motor octane number (MON), which is determined with a test engine running at a higher speed of 900 rpm. If, for example, a gasoline has an RON of 98 and a MON of 90, then the posted octane number would be the average of the two values or 94.
High octane gasoline does not outperform regular octane gasoline in preventing engine deposits from forming, in removing them, or in cleaning the engine. Consumers should select the lowest octane grade at which the car's engine runs without knocking. Occasional light knocking or pinging won't harm the engine, and doesn't indicate a need for higher octane. On the other hand, a heavy or persistent knock may result in engine damage.
Additional Gasoline and Octane Ratings Reading
- American Petroleum Institute - The API represents the US oil and natural gas industry.
- Automotive Gasoline FAQ - This is Bruce Hamilton's very well-referenced article, converted into HTML by Kyle Hamar.
- Gasoline FAQ - Part 1 of 4 - This is the starting point for Bruce Hamilton's (Industrial Research Limited) comprehensive gasoline FAQs.
- Gasoline FAQs - Detailed information about octane ratings is provided.
- Howstuffworks: How Car Engines Work - If you don't know how it works, then this is the article for you! The graphics are cool, but a printable version of the article is also available.
- Howstuffworks: What does octane mean? - This is Marshall Brain's answer to the question.