Otto Explains — Octane Rating
What Is Octane Rating, and Does Higher Octane Mean More Power?
Short Answer
No, not by itself. An octane rating measures how well petrol resists knock, the early, uncontrolled ignition that makes a pinging or rattling sound; it is not a measure of energy or power. In most cases, using a higher octane than your owner's manual recommends gives no benefit in performance, speed or fuel economy. Engines designed for higher-octane petrol are the exception the other way: on a lower grade they can lose some performance and may knock.
What does an octane rating measure?
It measures resistance to knock. In a petrol engine, the spark should ignite the compressed fuel-air mixture and a flame should spread smoothly through it. Knock happens when part of the mixture ignites on its own before the flame reaches it, and the colliding flame fronts make a pinging or rattling sound. A higher octane number means the petrol is more resistant to that early ignition. It does not mean the petrol contains more energy.
What do RON and the numbers at the pump mean?
The Research Octane Number (RON) is measured in a standardised single-cylinder laboratory engine by comparing the petrol with reference fuels. A related test gives the Motor Octane Number (MON). Oman's petrol standard covers two grades, 91 RON and 95 RON, which are sold as Mogas 91 and Mogas 95. Pumps in the United States show a different figure that combines RON and MON, so the same petrol carries a lower number there: Mazda, for example, equates US 91 with 96 RON.
Does higher octane petrol give more power?
Not on its own. According to the US Federal Trade Commission, using a higher octane than the owner's manual recommends usually gives no benefit: the car won't perform better, go faster, use less fuel or run cleaner. What matters is matching the fuel to the engine. Some engines, often high-compression or high-performance ones, are designed for higher-octane petrol. On those, a lower grade than recommended can slightly reduce performance and cause knocking, and petrol below the minimum the manufacturer states can cause serious engine damage. Fuel use has many other causes, covered in why a car uses more fuel.
Which octane should you use?
The one in your owner's manual. If it specifies 95 RON, Mogas 95 meets it; if 91 RON is enough, a higher grade won't add anything. Check whether a figure is RON before comparing it: a manual written for the US market may quote the US pump number, which is lower than RON for the same petrol. As a rule, higher-octane petrol also doesn't clean an engine better than the grade specified. Branded premium fuels are a separate subject, with their own additives and claims.
What if your engine knocks?
Petrol with a lower octane than the engine needs is a common cause of knocking, and heavy knock can damage parts of the combustion chamber. A knocking sound can also come from mechanical faults, so it can't be diagnosed by ear. If an engine still knocks on the recommended fuel, or a new, loud or persistent knock appears, take it seriously: stop safely, switch the engine off and have the engine checked before driving on. AutoTronix Garage handles engine diagnosis and repair, and AutoTronix Lab carries out diagnostic testing. A noticeable loss of performance has its own causes, covered in why a car loses power.
Common Questions
- Is US 91 petrol the same as Mogas 91 in Oman? No. US pumps show an index that combines RON and MON, while Oman's grades are rated in RON. Mazda's manual equates US 91 with 96 RON, so a US-market manual that asks for 91 is asking for a premium grade.
- Will a higher octane improve my fuel economy? Usually not. If your engine is designed for the lower grade, the FTC says a higher octane won't improve mileage. Using the grade your manual recommends is what keeps the engine running as designed.
- Can a lower octane than recommended damage my engine? It can. Manufacturers that specify higher-octane petrol warn that a lower grade can cause knocking and reduce performance, and that petrol below their stated minimum could cause serious engine damage. Heavy or persistent knocking can damage the engine.
- Does premium petrol clean the engine? As a rule, higher-octane petrol does not clean an engine better than the grade your manual specifies. A fuel's octane number and its additive package are different things.