Octane rating is determined by comparing a fuel’s resistance to knocking against two reference chemicals in a standardized test engine. The scale runs from 0 to 100, where 0 represents n-heptane (a fuel that knocks very easily) and 100 represents iso-octane (a fuel highly resistant to knocking). A gasoline with an octane rating of 87, for example, resists knock as well as a mixture of 87% iso-octane and 13% n-heptane.
The Reference Fuels Behind the Scale
The entire octane scale is built on two specific hydrocarbons. Iso-octane (technically called 2,2,4-trimethylpentane) is a very stable molecule that resists spontaneous ignition under pressure, earning it the reference value of 100. Normal heptane sits at the opposite end: it ignites easily under compression, so it’s assigned a value of 0. By blending these two in precise ratios, engineers create what are called Primary Reference Fuels with known octane values. If they need a reference fuel rated at 92, they mix 92% iso-octane with 8% n-heptane. Other well-known compounds like toluene are sometimes added to create reference blends for specific testing needs.
What Engine Knock Actually Is
To understand why octane matters, you need to know what it prevents. In a normal engine cycle, the spark plug ignites the fuel-air mixture, and a flame front spreads smoothly across the cylinder. Engine knock happens when the unburned fuel ahead of that flame front gets so hot and compressed that it ignites on its own before the flame reaches it. This creates a sudden pressure spike, producing the metallic pinging or rattling sound drivers sometimes hear.
Knock isn’t just annoying. It weakens power output, raises engine temperature, increases fuel consumption, and can cause real mechanical damage over time. Higher-octane fuel resists this premature ignition, which is why high-compression and turbocharged engines require it.
The CFR Engine Test
The standard tool for measuring octane is a Cooperative Fuel Research (CFR) engine, a single-cylinder test engine designed in the 1920s and still used today under internationally recognized ASTM standards. The engine has an adjustable compression ratio, which is the key to the whole process.
Here’s how it works: the test fuel is run in the CFR engine, and the compression ratio is increased until a specific level of knock is detected by sensors in the cylinder. Then the test fuel is removed and replaced with blends of iso-octane and n-heptane. The tester adjusts the reference blend ratio until it knocks at the same compression ratio as the test fuel. Whatever percentage of iso-octane is in that matching blend becomes the fuel’s octane number.
RON, MON, and the Number on the Pump
There are actually two standard octane tests, and they produce different numbers for the same fuel. The Research Octane Number (RON) test, governed by ASTM D2699, simulates moderate driving conditions with a lower engine speed and cooler intake air. The Motor Octane Number (MON) test, governed by ASTM D2700, uses a higher engine speed, hotter intake air, and variable ignition timing to simulate more demanding conditions like highway driving, heavy loads, or hill climbing. MON values are typically 8 to 12 points lower than RON for the same fuel because the harsher test conditions make knocking more likely.
In the United States and Canada, the number you see on the gas pump is neither RON nor MON. It’s the Anti-Knock Index (AKI), calculated as the simple average of both:
AKI = (RON + MON) / 2
So when you see “87” at a U.S. pump, that fuel might have a RON of roughly 91 and a MON of roughly 83. Most of Europe, Australia, and other regions post the RON value instead, which is why gasoline labeled “95” overseas is roughly equivalent to American “91” octane. The fuels aren’t dramatically different; the labeling systems are.
Ratings Above 100
Since iso-octane defines the top of the original scale at 100, you might wonder how racing fuels and aviation gasoline carry ratings of 104, 110, or higher. These values can’t be measured by simply mixing iso-octane and n-heptane, so they’re determined through extrapolation.
ASTM International defines octane numbers above 100 using a formula based on adding small, measured amounts of tetraethyllead (TEL) to pure iso-octane. The more TEL required to match the test fuel’s knock resistance, the higher the rating. This isn’t a simple extension of the 0-to-100 curve. The octane “units” above 100 grow at a different rate, which is why the scale tops out in practical terms around 120 or so, where the unit size would become infinitely large if the original curve were simply stretched.
In practical terms, an octane number above 100 reflects how much additional knock resistance the fuel provides beyond pure iso-octane, expressed through a performance number conversion that keeps the scale roughly linear.
Modern Alternatives to Engine Testing
Running every batch of gasoline through a CFR engine is expensive and time-consuming, so refineries and fuel labs increasingly use faster screening methods. Near-infrared (NIR) spectroscopy has become one of the most promising alternatives. It works by shining infrared light through a fuel sample and measuring how different molecular groups absorb that light. The absorption pattern reveals the concentration of specific chemical structures, like the types of carbon-hydrogen bonds present, which correlate strongly with octane rating.
Researchers have paired NIR spectroscopy with machine learning models to predict RON values with impressive accuracy. In one study using 200 gasoline samples with RON values ranging from 90 to 100, an artificial neural network trained on spectral data predicted octane numbers so accurately that about 94% of its predictions fell within the normal error range of the traditional engine test. These methods don’t replace CFR testing as the official standard, but they allow refineries to monitor octane in near real-time without shutting down production for hours of engine testing.