Why higher octane gas better

The octane ratings that you see on the gas pump indicate a fuel blend's ability to resist knock. Knocking is a pinging sound that is caused when fuel in your engine ignites prematurely, which can throw off the rhythm of your engine's combustion.

Gas with a high octane rating requires more pressure to ignite, and certain engines take advantage of this quality. Low-performance engines generally do not pressurize the fuel-air mix inside engine cylinders as much as high-performance engines. This allows them to use lower-octane blends, because the designers don't need to worry about the gas lighting early due to pressure. In very powerful engines, however, the pressure in each cylinder is much higher, and gas that has too low an octane will cause the off-time ignition that leads to pinging and a loss of power.

So is high octane always better? Many people who own cars designed for low-octane fuel choose to fill their cars with high-octane options. They believe that doing so will improve their car's performance by ensuring that the engine runs perfectly, and that this will also improve durability. While it's nice to think that a simple change of gas can act like an auto insurance policy and safeguard you from potential issues, save your money. Retail gasoline stations in the United States sell three main grades of gasoline based on the octane level:.

Some companies have different names for these grades of gasoline, such as unleaded, super, or super premium, but they all refer to the octane rating. The large number on the yellow gasoline pump octane label is the minimum octane rating. Of the 18 isomers of normal octane C8H18 , octane gets its name from the 2,2,4-Trimethylpentane compound, which is highly resistant to auto-ignition.

This iso-octane has been assigned the reference value of for testing purposes. The extremely unstable normal heptane C7H16 molecule is the 0 octane reference fuel. Engines are designed to burn fuel in a controlled combustion. A flame starts at the spark plug and burns throughout the cylinder until all of the fuel in the cylinder is burned. In comparison, spontaneous combustion , also called auto-ignition , detonation , or knock , happens when rising temperature and pressure from the primary combustion causes unburned fuel to ignite.

This uncontrolled secondary combustion causes pressure in the cylinder to spike and causes the knock to occur. The competition between the intended controlled and unintended spontaneous combustion causes the energy from the burning fuel to disperse unevenly, which can cause damage and place high pressure on the engine's piston before it enters the power stroke the part of the cycle when the piston's motion is generating power.

Before electric computerized ignition was widely used, this knocking commonly occurred and could cause significant engine damage. Most modern engines have sensors to detect knocking. When detected, the computer delays the initial spark, which causes the controlled combustion to take place at a point when compression is not at its highest point.

Although this eliminates the knock, it can cause the engine to run less efficiently. A similar undesirable condition is called pre-ignition, when the fuel ignites on its own before the spark ignites it.

Modern engine computers minimize this condition by controlling the timing of valves and fuel injection; however, this control mechanism can also come with a fuel-efficiency or emissions penalty.

The standard means of testing octane is with an octane testing engine. If you buy fuel with an octane rating above the manufacturer's requirement, you're likely to feel it in your wallet more than the seat of your pants. How your engine constantly invites and silences engine knock to estimate a fuel's octane rating. Your car doesn't know the octane rating of the fuel in its tank.

Instead, the engine controller calculates an inferred octane with closed-loop logic that continuously advances the ignition timing until it detects knock, which occurs when a portion of the fuel-air mixture ignites before the spark-plug-initiated flame front reaches it. The further the computer can advance the timing without provoking knock, the higher the octane rating. During knock, the flame front travels through the combustion chamber up to 10 times quicker than the normal spark-initiated flame front.

Left unchecked, these pressure waves can damage the head gasket, pistons, or cylinder head. But the occasional brief knock is a useful tool for checking that the engine is operating efficiently.

It's detected with one or more knock sensors bolted to the block to sense the oscillations created by the pressure waves with a typical frequency between 7 and 16 kilohertz. Stephen Russ, senior technical leader for gas engines at Ford, says this normal knock is usually detected and addressed within one or two combustion cycles and poses no threat to the engine.

New Cars. Buyer's Guide. Type keyword s to search. Today's Top Stories. Marc Urbano Car and Driver. From the July issue of Car and Driver. Related Stories. This content is created and maintained by a third party, and imported onto this page to help users provide their email addresses.

You may be able to find more information about this and similar content at piano. Advertisement - Continue Reading Below. More From Features. Octane allows an engine to run at higher compression ratios and use more of the energy in gasoline.

More energy equals the ability to do more work, and that means a properly engineered vehicle will go farther or faster on the same amount of gasoline. The U. In addition to adding higher-octane fuels, the lowest current octane levels will probably be phased out over time.