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Octane

Car culture term

Octane
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What is Octane?

A measure of fuel resistance to engine knock. Higher octane fuel resists knock better, allowing more aggressive ignition timing.

Octane is a measure of a fuel resistance to engine knock (detonation). Higher octane fuel can withstand higher cylinder pressures and temperatures before spontaneously combusting. Different countries use different octane rating systems. The United States uses (R+M)/2 (the average of Research Octane Number and Motor Octane Number). Europe uses RON (Research Octane Number) only. The same fuel rated 87 (R+M)/2 in the US is approximately 91 RON in Europe.

Standard fuel grades in the United States are 87 (regular), 89 (mid-grade), 91 (premium), and 93 (super premium). Some markets sell 94 octane (Costco brand). Higher octane is typically required for high-compression or turbocharged engines. Using lower octane than the engine requires can cause knock and engine damage; using higher octane than required typically provides minimal benefit on a stock tune.

Race fuels are available in higher octane ratings. Sunoco GT260 is 110 octane. Methanol is approximately 100-105 octane (varies by formulation). Ethanol-blended E85 (85% ethanol, 15% gasoline) is approximately 105-115 octane. The high octane of race fuels and ethanol allows more aggressive tuning (more timing advance, more boost) and significantly more power than equivalent gasoline.

Related terms

KnockAFRECU tune

Frequently asked questions

Should I use higher octane than my car requires?
Generally no benefit on a stock tune. Modern engines are tuned for the recommended octane; using higher octane provides no power increase or fuel economy gain. The exception is on tuned cars with high-octane-specific tunes, where using lower octane than the tune requires can cause knock.
What is the difference between R+M/2 and RON?
Two different methods of measuring octane. R+M/2 (used in the US) averages Research Octane Number and Motor Octane Number. RON (used in Europe) measures only Research Octane Number. RON is typically 4-5 points higher than the same fuel rated in R+M/2. So 91 R+M/2 is approximately 95-96 RON.
Is E85 higher octane than premium?
Yes. E85 typically tests at 105-115 RON octane equivalent, significantly higher than 93 R+M/2 premium gasoline (about 97 RON). The high effective octane allows more aggressive tuning. E85 is widely used in racing and high-performance street tuning for this reason. The lower energy density of ethanol means more fuel volume must be injected.
Can I mix octane levels in my tank?
Yes. The result is a fuel with octane rating between the two grades, weighted by the proportion. Mixing 5 gallons of 91 with 5 gallons of 93 results in 92 octane fuel. Mixing is sometimes used to achieve specific octane targets when only standard pump grades are available.
What octane do most performance cars require?
Most modern naturally aspirated performance cars (Honda Type R, Mazda Miata, Subaru BRZ) recommend 91-93 octane (premium). Most turbocharged sport cars (Audi RS3, BMW M cars, Porsche turbo) require 91-93 octane. Many tuned cars require higher octane (94+) or E85 specific fuels for the tune to operate safely.
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