Using 87 octane gasoline instead of 85 octane is a common question for drivers, especially those in high-altitude regions. This dilemma exists because 85 octane is often the “regular” fuel grade in elevated areas, while 87 octane is the standard regular grade found at sea level. The difference between these grades relates to the fuel’s chemical stability and the unique atmospheric conditions at higher elevations.
Defining Octane Ratings and Engine Knock
The number displayed on a fuel pump, such as 85 or 87, represents the fuel’s Anti-Knock Index (AKI), which is a measure of its resistance to premature ignition. Gasoline is meant to ignite only when the spark plug fires, but under high pressure and heat within the engine cylinder, the fuel-air mixture can spontaneously combust before the spark, creating a powerful shockwave. This uncontrolled explosion is known as engine knock or detonation.
The Anti-Knock Index (AKI) is calculated by averaging two laboratory measurements: the Research Octane Number (RON) and the Motor Octane Number (MON). The resulting AKI value, often written as (R+M)/2, indicates the fuel’s resistance to pre-ignition. A higher octane number signifies greater stability, which is necessary for high-compression or turbocharged engines that generate more heat and pressure. Using a fuel with too low an octane rating can lead to inefficient operation, reduced performance, and potential engine damage from repeated detonation.
Why 85 Octane Exists
The presence of 85 octane as a regular fuel grade is a direct result of the physics of high-altitude operation, typically in regions 4,000 feet or more above sea level. At higher elevations, the atmospheric pressure naturally decreases, making the air less dense. This reduction in air density means that when the engine draws in air, a lower mass of oxygen molecules enters the cylinder compared to the same engine operating at sea level.
Because less air enters the cylinder, the effective pressure generated during the compression stroke is reduced. This lower cylinder pressure decreases the likelihood of the air-fuel mixture igniting spontaneously before the spark plug fires. Historically, 85 octane was introduced in these regions as a less expensive blend that worked adequately in older, carbureted engines. While modern, fuel-injected engines automatically compensate for the thin air, 85 octane remains viable in many high-altitude areas due to this fundamental reduction in effective compression pressure.
Practical Effects of Using 87 Octane
When a vehicle designed to run on 85 octane at high altitude is filled with 87 octane fuel, the immediate effect is generally neutral. For an engine that is already operating efficiently on 85 octane, switching to a higher-octane fuel will not provide any noticeable increase in power, acceleration, or fuel economy. The engine’s computer, or Electronic Control Unit (ECU), is programmed to optimize performance based on the minimum octane required to prevent knock.
Since 87 octane fuel is more resistant to pre-ignition than the required 85 octane, the ECU will simply operate the engine as designed, without needing to adjust the ignition timing. The only practical difference is the increased cost, as 87 octane is typically priced higher than 85 octane at the pump. Using a higher-octane fuel than necessary is primarily a waste of money, as the engine does not have the high compression ratio or aggressive timing needed to fully utilize the fuel’s increased knock resistance.