A Lesson on Gas Grades

When you buy unleaded gas, you’re usually presented with three options – Regular, Midgrade, and Premium. Each of these ‘grades’ has two numbers prominently displayed. The first is the price…and that’s all we’re going to say about that, because you already know.

The other number is called the octane number – also known as the octane rating. Usually, the lowest is 87, then 89 or 90, and finally, somewhere between 91 and 94. But what do those numbers mean? Here are some lessons.

The Small Engine Lesson. You may remember that most cars have a four stroke gasoline engine. One of those four strokes is the compression stroke, during which a piston compresses air and gas before the spark plug ignites it.

It’s important to know that the compression from that piston can cause the air and gas to ignite – before the spark plug fires. This spontaneous combustion is called knocking, and it’s not good for your engine.

The Chemistry Lesson. Crude oil straight from the ground is made up of all different kinds of hydrocarbons, each with a different number of carbon atoms chained together. For example, methane has one carbon atom, butane has four. We’re interested in the two that have a chain of seven (heptane) and eight (octane) carbon atoms.

It doesn’t take a lot of pressure to make heptane explode, whereas octane can handle a lot of pressure.

The Numbers Lesson. This is where the numbers come in. 87 percent gasoline has 87% octane and 13% hectane. 93 percent has 93% octane and only 7% hectane. The higher octane fuel can stand more pressure before it knocks. Engines are designed for fuel not to combust spontaneously. Rather, fuel should burn through controlled combustion, and each engine is designed with a minimum octane rating. Higher performance and luxury vehicles often need a higher octane gasoline.

Am I going to need this lesson in real life? Yes. If your car needs a higher octane gas to operate correctly, you’ll have a notice inside your car’s manual, and inside the fuel door. This will tell the actual minimum number that you need to choose. Some cars require premium gas, and others only recommend it. If you only have a recommendation, you can usually use regular gas, but you may see a decrease in performance and gas mileage.

So here’s your homework. Find out what grade of gasoline your vehicle needs. Check out the owner’s manual – or ask one of the mechanics at Norris Automotive. Whether you need 87% or 94%, we can promise you that you’ll get a 100% accurate answer when you ask us. And – asking us is the smart thing to do!