The octane rating is a measure of a fuel’s ability to resist engine knock. Knock occurs when the fuel is prematurely ignited in the engine’s cylinder, leading to reduced efficiency and potential engine damage. This phenomenon is rare in modern engines, primarily because fuels are blended with oxygenates that add oxygen and help prevent knock. These oxygenates are commonly referred to as octane.
Most retail gasoline stations offer three octane grades:
- 87 (Regular)
- 89 (Mid-grade)
- 91–93 (Premium)
The higher the octane number, the more resistant the fuel is to knocking. Higher octane fuels also allow for:
- Higher compression ratios
- Turbocharging
- Engine downsizing/down speeding
These features support greater engine efficiency and higher performance. Currently marketed as ‘premium’, high-octane fuel is gaining attention from automotive manufacturers, who are advocating for raising the minimum octane levels in the U.S. Doing so would enable the development of smaller, more efficient engines, improving vehicle efficiency and reducing greenhouse gas emissions through decreased petroleum use.
Gasoline Octane
The primary difference between regular and premium gasoline is octane rating—a fuel’s ability to prevent engine detonation or “knock.” Uncontrolled knocking can cause severe engine damage and result in costly repairs. Premium gasoline usually has an octane rating four to six points higher than regular (e.g., 93 vs. 87).
Contrary to popular belief, premium-grade fuel does not contain more energy than regular fuel. In fact, its energy content is slightly lower due to the inclusion of additional anti-knock additives.
The idea that high-octane fuel delivers more power stems from the fact that high-performance engines often require premium gasoline. However, it is not the fuel itself that produces more power, but rather the engine’s state of tune—a combination of mechanical design factors optimized for performance. These optimized engines are more prone to knock, hence the requirement for higher octane to avoid damage and expensive repairs.
Gasoline Quality
All regular grade and premium grade gasolines in a given area are made from the same base fuel. What distinguishes one fuel brand from another is the blend of proprietary additives—the brand’s “secret sauce.” These additives serve multiple purposes, with a key function being the prevention of carbon buildup inside the engine. Carbon deposits can reduce performance, lower fuel economy, and cause cold-start problems and other driveability issues.
Since 1995, the Environmental Protection Agency (EPA) has mandated a minimum level of deposit control additives in gasoline. However, in 2004, eight major automakers introduced the TOP TIER™ gasoline standard, which exceeds EPA requirements by mandating higher quality and quantity of additives. TOP TIER certification also requires that these enhanced additives be used in all fuel grades and that they be sold at all retail locations operated by participating companies across the U.S. and Canada.
