Octane: is more better for your engine?

Learn what octane does and why it’s important to use the recommended octane rating

Octane is a measure of a fuel’s ability to resist “knocking” or “pinging” during combustion caused by the air-fuel mixture detonating prematurely in the engine. Higher octane ratings denote greater resistance to knocking, which is essential for maintaining engine efficiency and longevity. It’s not a measure of energy, the level of detergent, or a way to gauge fuel mileage.

What is octane?

Octane is a hydrocarbon with the chemical formula C8H18. It exists in several isomeric forms, each with different properties. The most common reference standard is iso-octane, which is used to define the octane rating scale. Pure iso-octane is assigned an octane rating of 100, while n-heptane, another hydrocarbon, is assigned a rating of 0. Fuels are rated against these standards to determine their octane level.

How is octane rated?

There are two primary octane ratings: the Research Octane Number (RON) and the Motor Octane Number (MON). RON is measured under controlled, low-speed conditions, while MON is determined under more stressful, high-speed conditions. The Anti-Knock Index (AKI), commonly displayed at gas stations, is the average of RON and MON, providing a practical measure of a fuel’s performance.

Why using the recommended Octane matters

High-performance and turbocharged engines typically require high-octane fuel to prevent knocking. Knocking can cause significant engine damage over time, leading to decreased performance and costly repairs. Modern engines are designed with advanced technologies like direct injection and turbocharging, which increase the compression ratio and, subsequently, the likelihood of knocking if low-octane fuel is used.

What is detonation?

Let’s review the combustion process in an engine. Air and fuel are combined during the intake stroke. The air/fuel mixture is compressed during the compression stroke. That creates heat, but not enough heat to start the mixture on fire. As the piston reaches the top of the compression stroke, the air/fuel mixture is ignited by the spark plug. As the air/fuel mixture burns, a flame front expands away from the ignition point near the spark plug, and the pressure in the combustion chamber increases dramatically.

Because the pressure in the cylinder was already high due to the compression stroke, the air/fuel rapid pressure rise caused by combustion raises the temperature of the uncombusted air/fuel mixture to the point where it self-ignites—causing multiple flame fronts in the combustion chamber. When those multiple flame fronts collide, they create audible shock waves known as detonation or “knock”.

Detonation robs the engine of power and, if allowed to continue, can destroy the engine.

Octane prevents the high-pressure, unburned air/fuel mixture from self-igniting. So it prevents detonation and knock.

Fun combustion facts about detonation

• Flame spread during detonation can reach 700-MPH

When an engine fires the spark plug, the flame front moves smoothly across the combustion chamber, traveling at 45 to 90 MPH. Late-model engines with high-swirl combustion chamber designs have faster airflow and faster flame spread.

However, when you use regular-grade (87-octane) gas in a high-compression engine that was designed for 91 or 93-octane, detonation occurs. The combustion gasses in that same scenario can reach a speed of over 700-MPH!

That kind of rapid expansion increases combustion temperatures and can damage pistons, gaskets, valves, and cylinder heads. The metallic knock you hear is the force of the combustion slamming against the piston while the piston is still moving upwards on its compression stroke.

How does detonation differ from pre-ignition?

Preignition can be caused by using a low-octane fuel in a high-compression engine designed for high-octane fuel. The heat created during the compression stroke can ignite the fuel before the spark plug so you wind up with two colliding flame fronts. The collision is referred to as ping.

Pre-ignition can also be caused by hot carbon deposits in the engine that ignite the air/fuel mixture before the spark plug fires. The result is the same as detonation–it causes multiple flame fronts; one from the hot carbon and one from the spark plug.

Premium gas MYTHS

• Premium gas provides more power and better pickup— False. A gallon of higher-octane (91 or 93) fuel contains no more energy than a regular (87) fuel. The available energy may vary slightly from brand to brand due to the brand’s proprietary gasoline formula. However, within the brand, regular and premium have the same energy. If your engine doesn’t require a high octane fuel, you won’t get any more power or acceleration, and you’ll be wasting money. There’s a small exception to that; if your engine has carbon buildup problems and experiences ping during hard acceleration, then using a higher octane fuel can help get more power. It’s not because the fuel has more power, though. It’s because you have a carbon buildup problem that’s robbing your engine of power.

• Premium has more additives and better detergents— FALSE. Certain brands may add more detergents than others, but as a general rule, premium fuel doesn’t contain more additives than regular. Adding premium will NOT clean out your engine better than regular.

• Premium gas is always ethanol-free— NOPE. Some stations sell ethanol-free premium gasoline. But premium gas can contain ethanol. See the labeling on the pump if you want to fill up with ethanol-free premium gas; it’s not a given. Over 95% of all gas (regular and premium) sold in the U.S. contains 10% ethanol.

•Premium gas burns slower—NOPE. Premium gasoline burns as the same rate as regular

• Premium makes starting harder when used in an engine designed for regular—NOPE. A lower-compression engine doesn’t produce enough heat during the compression stroke to ignite premium gas. But the spark always provides enough energy to ignite both regular and premium gas.

Can any car get better MPG with premium?

No. In fact, the EPA mandates that engines certified to run on regular CANNOT show a fuel economy increase of more than 3% by switching to premium. That ruling is designed to prevent car makers from setting factory spark timing too late so that adding premium results in advanced spark timing. If car makers were allowed to alter their software this way, it would create an unnecessary high demand for premium gas.

©, 2015 Rick Muscoplat

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