Octane: what it actually means

Every petrol pump shows a number, such as 95 or 99 in the UK, or 87 or 93 in the US. It’s the fuel’s octane rating, and it’s one of the most misunderstood numbers in motoring. It doesn’t say how much energy is in the fuel, how clean it is or how fast it burns. It says one thing: how well the fuel resists knock.

Knock starts after the spark. As How ignition works shows, the spark lights the mixture next to the plug, and a flame spreads across the cylinder in a few thousandths of a second. As it spreads, the hot gas behind it squeezes and heats the mixture still waiting at the edges, called the end gas. If the end gas gets hot enough, it explodes on its own before the flame reaches it. The pressure jumps all at once and rings round the cylinder like a hammer hitting a bell. You hear it as a metallic rattle under load, called pinking or detonation.

A little knock now and then does no harm, but heavy knock hammers the pistons and overheats them. It can crack a piston crown, break the thin metal between the piston ring grooves and blow a head gasket.

A higher octane fuel can be squeezed and heated more before its end gas goes off on its own. That’s all octane is.

Where the number comes from. A lab runs the fuel in a special single-cylinder test engine whose compression ratio can be changed while it runs, and raises the ratio until it knocks. Then it finds a blend of two test fuels that knocks at the same point: iso-octane, which resists knock very well and is rated 100, and heptane, which knocks very easily and is rated 0. If the fuel knocks like a blend of 95% iso-octane and 5% heptane, it’s 95 octane. Fuels can rate higher than 100: ethanol is about 108.

The demo runs one cylinder of a 2.0-litre turbo engine flat out at 4,000 rpm, at full boost. It’s designed to make its full power on 98 RON, and it has a knock sensor: a small microphone bolted to the block that listens for knock. The slider changes the fuel in the tank, and the graph shows the power the engine makes on each. The demo shows octane the way UK pumps do, in RON, or the way US pumps do if you choose US.

• Start on 95 RON, everyday UK unleaded. Firing the spark at the best moment, 20° before the top, would knock on this fuel. The knock sensor hears it, so the ECU fires the spark later, at 14°. There’s no knock, but the push comes a little late, and the engine makes 140 kW (188 bhp), about 5% less than on 98.
• Press Super 99, or Premium 93 in US mode. The fuel resists knock well enough for the ECU to fire at the best moment, and the engine makes its full 147 kW (197 bhp). Slide down to 98 and nothing changes. Past what the engine needs, more octane does nothing at all.
• Slide down to 91 RON. The ECU fires the spark as late as it’s allowed, 6° before the top, and power falls to 131 kW (176 bhp). Below that is the shaded Knock zone: even the latest spark can’t save it, and the end gas explodes at the edges of the cylinder, shown in Coral.
• Press Knock sensor to turn it off, like an older engine that was set up for one fuel. The spark stays at 20° whatever’s in the tank. On 98 or more that’s fine, but on anything less the end gas explodes on every stroke. Before knock sensors, this is how the wrong fuel wrecked engines.

Why does firing later cost power? The burning gas pushes hardest when the pressure peaks just after the piston starts down. Fire later and the peak comes later, after the piston has started to run away from it, so less of the fuel’s heat becomes push and more goes out with the exhaust.

The model is simplified. A real turbo engine’s ECU also turns the boost down on poor fuel, so the loss can be bigger, and how much any engine gains depends on how it was designed and tuned.

Anything that makes the end gas hotter, squeezes it harder or gives it longer to go off makes knock more likely, and so needs higher octane fuel:

• A high compression ratio. Squeezing heats the mixture, so a higher ratio leaves the end gas hotter. Compression ratio explained shows the knock limit.
• Boost. A turbo or supercharger packs in more air before the piston even starts squeezing, so the pressure and heat at the top are higher. That’s why so many turbo cars ask for super unleaded.
• Firing the spark earlier. The pressure builds sooner and higher, while the end gas is still waiting.
• Hot intake air. A hot day, a hot engine bay or an intercooler that isn’t working all leave the charge hotter before it’s even squeezed.
• Heavy load at low revs. Flooring it in a high gear up a hill means a full cylinder and a slow-moving piston, so the end gas has longer to go off.
• A lean mixture. Air-fuel ratios explains why a slightly lean mixture burns hottest.

Engineers fight back with designs that resist knock. Direct injection sprays the fuel into the cylinder, and as it evaporates it cools the charge. Compact combustion chambers with the plug in the middle mean the flame has less far to travel, so the end gas has less time to go off. And an intercooler cools the air after the turbo. All of these let modern engines run more compression and boost on everyday fuel.

There are two lab tests, and they give the same fuel different numbers:

• RON, the Research Octane Number, runs the test engine slowly, at 600 rpm, on a cool mixture.
• MON, the Motor Octane Number, runs it faster, at 900 rpm, on a heated mixture, more like hard driving. The same fuel scores about 8 to 10 lower.

The UK and Europe print the RON on the pump. The US and Canada print the average of the two, the anti-knock index or AKI, shown as “(R+M)/2”. So US regular at 87 isn’t 8 numbers worse than UK unleaded at 95: it’s about 91 or 92 RON. As a rough guide, take 4 or 5 off a RON figure to get the AKI. The demo takes off 5.

It depends entirely on the engine.

• If your car is designed for regular fuel, the ECU already fires at its best moment on it. There’s no knock to cure, so higher octane makes little or no difference to power or economy. That’s most everyday cars.
• If the handbook says regular is the minimum but recommends super or premium, the engine is held back on regular, like the demo’s engine on 95. Higher octane lets it fire earlier and get back a few percent of its power, most noticeably on hot days and under hard driving. Many turbo and high-performance engines work this way.
• If the handbook says higher octane is required, use it. The engine can protect itself on lower octane for a while, but it isn’t designed to run that way.

Octane isn’t energy. Litre for litre, super unleaded holds about the same energy as regular. Ethanol, though, holds only about two-thirds of the energy of petrol, so a fuel with more ethanol in it, such as E10, holds a few percent less, and economy can drop slightly.

Premium fuels often have more cleaning additives, which help keep injectors and intake valves free of deposits. That can help some engines, especially direct-injection ones, but it’s a different thing from octane.

• Filling up with lower octane than the car needs. A modern car’s knock sensor will cope, firing the spark later, so you lose a little power and economy. Drive gently and avoid towing or flooring it until the tank is topped up with the right fuel. The real danger is the wrong type of fuel: putting petrol in a diesel, or the other way round, is a different problem altogether. Don’t start the engine.
• Pinking you can hear. A light metallic rattle, like ball bearings shaken in a tin, when accelerating hard at low revs or climbing a hill. On a car with a knock sensor it’s rare, and usually means something else is wrong: the wrong fuel, an engine running hot, a lean mixture from an air leak, carbon build-up in the cylinders or the wrong spark plugs. On older cars without a knock sensor, it can mean the timing has been set too far advanced.
• A faulty knock sensor. If the sensor or its wiring fails, the ECU stores a fault code, such as P0325, and lights the engine management light. It can no longer hear knock, so it plays safe and fires every spark later. The car feels sluggish and uses more fuel.
• Carbon build-up. Deposits on the piston crowns and in the chambers take up space, raising the compression slightly, and hold heat. An older engine can slowly need more octane than it did when new.
• Pre-ignition is a different fault. In knock, the spark lights the mixture and the end gas then explodes. In pre-ignition, something hot lights the mixture before the spark: a glowing lump of carbon, a spark plug that runs too hot or a sharp edge in the chamber. Firing the spark later can’t stop it, because the spark didn’t start it, and it can melt a piston quickly. Small turbo direct-injection engines can suffer low-speed pre-ignition at low revs and heavy load, set off by droplets of oil in the cylinder. Engine oils made since 2020 to the latest specifications, such as API SP, are designed to prevent it, so use the oil the maker specifies.

Knock is the wall that every petrol engine tune runs into, so octane matters a lot to tuners.

• Remaps are written for a fuel. Most performance remaps for turbo cars assume 98 or 99 RON, advancing the spark and raising the boost to the edge of knock on that fuel. On 95, the knock control pulls the timing back and the gain shrinks; an aggressive map may knock. Many tuners offer a separate map for 95.
• More compression or more boost needs more octane. Raising the compression ratio or turning up the boost leaves the end gas hotter, so it brings the engine closer to knock.
• E85, a blend of about 85% ethanol, rates about 100 to 105 RON, and as it evaporates it cools the charge strongly. Together these let tuners run much more boost and timing. The catch is that the engine needs 30% to 40% more fuel, as Air-fuel ratios explains, so it means bigger injectors and a bigger pump. Flex-fuel kits add a sensor that measures how much ethanol is in the tank, so the ECU can adjust as it changes.
• Race fuel can be rated over 100 RON, but it’s expensive, and some types contain lead, which ruins catalytic converters and oxygen sensors. It’s not for road cars.
• Octane boosters in bottles from car shops raise the octane only slightly, often by less than one number at the dose on the bottle. Filling up with super unleaded does far more.
• Water-methanol injection sprays a fine mist into the intake. It cools the charge and resists knock, letting a tuned engine run more boost on pump fuel.

However the engine is tuned, the ECU’s knock control is the safety net. On a rolling road, tuners log how much the ECU is pulling the spark back on each cylinder, and back the map off until it stops.