Knock, Knock. What’s Octane?

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You’ve probably seen references to octane, octane grade, or octane rating or number when fueling up your vehicle, but many people are unsure of what it really means. In the simplest terms, the octane number is a measure of the fuel’s ability to resist engine “knock” or premature ignition inside your engine. You want the fuel to ignite, of course, but only at precisely the right time when the spark plug fires. If your engine requires Premium or higher-octane fuel to prevent early ignition (i.e., engine knock) and possible engine damage, it’s important to use the fuel recommended by the vehicle manufacturer (usually 91 to 93 octane in the U.S., numbers you’ll find on almost every modern gas pump). By the same token, no one wants to pay the extra cost of Premium-grade, high-octane gasoline for an engine that doesn’t need it.

So what’s engine knock? Engine knock is also called pre-ignition and occurs when the fuel ignites from heat created during the cylinder’s “compression stroke” rather than a little later when the spark plug fires to begin the “power stroke.” Those are the technical terms. Basically, like we said, engine knock occurs when your engine’s fuel burns at the wrong time. When that happens, in many cases you can hear an actual knocking sound from the engine compartment and, whether audible or not, damage can occur to the internal parts of your engine. 

The difference between “recommended” and “required.” Many modern vehicles have “knock sensor” technology built right into them that can detect when knock is likely to occur and dial back the performance of the engine to avoid it. So your vehicle manufacturer might say, for instance, that Premium fuel is “recommended” for your vehicle. That means it will run best on Premium-grade fuel, but your vehicle will still run on a lower grade without risking engine damage. If the vehicle manufacturer says Premium is “required,” that’s a different story. Even though your engine will likely still run on lower octane fuel, you run the risk of engine damage if you use lower-octane fuel*. If you’re unsure what octane grade is recommended for your ride, check your owner’s manual, or simply ask the pros at your local Jiffy Lube®

*Occasionally you may have no choice but to use a lower grade fuel than your vehicle requires. For example, you’re far from the next gas station and the station you’re at is out of Premium. In that case, consider using the highest grade available and only put in as much as you think will safely get you to the next gas station. Then fill up your tank the rest of the way with the right grade.