Gas Octane Ratings Explained

December 8th, 2022 by

More manufacturers recommend Premium, high-octane gasoline in their new vehicles than in previous years. As consumers, you see the higher cost of Premium gas at the pump, but many don’t understand the difference beyond price. So many drivers want to save money and wonder if using lower-octane gas will cause harm or save them money. To help, our fuel experts delved into the topic to educate you on why your car needs Premium fuel and the potential results of using a lower grade of gas.

What Is an Octane Rating?

engine bay with a carburetor

Image by Chris Phutully | Licensed with CC BY 2.0

You’ve seen the octane rating at the pump. Usually, you have the choice between 87-94, with 87 being the lowest and 94 the highest rating. These octane ratings measure the stability of the fuel at combustion. Specifically, octane ratings tell you the pressure at which fuel will spontaneously combust.

How Does Fuel Get Its Octane Rating?

Fuel companies determine the octane rating by calculating the average of two octane rating methods. The two methods are motor octane rating and research octane rating. These methods differ only in operating conditions. So, the higher the octane rating a fuel has, the more stable it will be during combustion.

You have three primary octane grades to choose from, starting with Regular. Regular gas usually measures an 87 octane grade and costs the least. Mid-grade octane fuel can range from 88-90 octane rating, and the price varies depending on how high the octane rating is. Premium gas has a rating that falls between 91-94 octane.

Depending on the gas station, you’ll see different names for these ratings. Most call the low-grade fuel Regular, but you might see it called Unleaded. Mid-grade fuels can go by the name Super or Super Unleaded. Top-grade fuels go by Ultra, Premium, Super Premium, and Ultra Super Premium. Gas companies created these names for marketing purposes. You only need to remember to check the octane rating, as this number tells the story.

What Does Octane Do To My Engine?

Your engine was manufactured to burn fuel in a controlled combustion environment. For example, the spark plug provides a tiny flame that ignites the fuel inside the cylinder. The burning fuel forces open the cylinder, turning the crankshaft that rotates the drive shaft, eventually transferring rotational force to the wheels.

Sometimes, spontaneous combustion of the fuel occurs. Experts also call this auto-ignition, detonation, and knock. Spontaneous combustion happens when the engine’s high operating temperatures or increased pressure cause unburned fuel to ignite. As a result of this uncontrolled combustion, the engine emits a knocking sound.

Auto-ignition also puts high pressure on the piston as it enters the power stroke in the cycle meant to generate power. This unintended pressure can cause damage to the cylinder as the energy of burning fuel disperses unevenly.

Before the advent of electronically controlled ignition systems, knocking frequently occurred, causing significant engine damage. Mechanics would find damage to engine parts such as o-rings, seals, and even bent rods. Repairing an engine involves more labor than most routine maintenance, making it an expensive problem. Modern engines now have sensors that detect spontaneous ignition.

In addition, these electronic systems can delay the intended ignition from your spark plug. This delay allows the controlled combustion to occur when the pressure lowers, thus saving your engine from damage. However, even though the delay prevents the knock from happening, it does result in lower efficiency.

How Octane Levels Are Measured

Fuel companies test the gasoline’s octane level with an octane-testing engine. This practice works the same way scientists measure the mass of an object by comparing it to things with a known mass using a scale. First, testers use primary reference fuels (PRFs) with precise octane levels, such as iso-octane, heptane, and toluene. Then, they bracket fuel samples with the PRFs and compare when the same knock occurs. Then, by adjusting the cylinder height of the octane testing engine, they change the compression ratio to pressure in the engine until the knocking occurs.

The process involves two tests — a research octane test and a motor octane test. The research octane test uses an engine at idle with low air temperature and slow speed. The motor octane test happens on a different engine under the stress of high speed and warmer temperatures. Then, testers take the average of the two results by adding the research octane number and the motor octane number and dividing this total by two. The equation at the gas pump reads (R+M)/2 on that yellow label.

Is It Okay To Use a Lower Octane Fuel In My Chevrolet?

The answer is yes and a resounding no. As we explained above, using a lower octane fuel in your Chevrolet can cause damage. Damage doesn’t happen immediately but over time. How much time is anyone’s guess and depends on many factors. If you tow and haul heavy loads, your engine already operates under significant pressure. Adding unstable fuel increases the risk of damage you want to avoid.

However, if you’re low on fuel and stop at the gas station and they only have Regular Unleaded, and your Chevrolet Silverado calls for Premium, it’s okay to use the Regular fuel. We don’t recommend filling your gas tank with it, but you can add enough to get you to another station where you can find the proper gas.

What Does Engine Knock Sound Like?

If you hear your engine knocking, you’ll know it immediately. Typically, it sounds like an extra pop in the normal firing of your engine. A standard engine has cylinders firing in sequence, over and over, creating a stable and constant hum. A knock disrupts that pleasant sound. Your engine may not knock with every crankshaft rotation, but it’ll happen often enough to let you know you have a problem.

Have Your Engine Inspected Today

At Bachman Chevrolet in Louisville, Kentucky, we want your Chevy to run in tip-top condition. If you hear a knock or suspect you may have engine damage, we recommend you schedule service today. Let one of our factory-certified technicians test your engine. You might have a problem with knocking, or it could be something else entirely.

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