Can You Mix 87 And 91 Octane? Here's What Happens If You Do

The answer is yes: You can mix 87 and 91 octane gasoline without blowing your engine to hell. With that out of the way, let's discuss what mixing different octane fuels does to your engine in terms of combustion and timing. Let's first figure out all the fuss about octane.

Octane rating is a fuel's resistance to spontaneously igniting at higher temperatures and pressures. The higher the octane rating, the more it resists catching fire before it's supposed to. To understand better, let's discuss combustion in a gasoline engine. A spark plug ignites the air-fuel mixture inside the engine's cylinder. The flame from the ignition burns it in a controlled and efficient manner, ensuring all the fuel in this mixture is burned off. This clean burn is important for the environment, too. 

Now, there's a chance that the fuel autoignites before it's supposed to, due to high temperature and pressure inside the cylinder. This phenomenon of the air-fuel mixture catching fire before the spark ignition is called knocking. In that case, you have two ignitions, one from autoignition and one from spark ignition. This puts excessive pressure on moving engine components and can damage them.

The 91 octane gasoline fuel resists engine temperatures and pressures better than 87 octane gasoline and keeps knocking at bay. That's why it's recommended in turbocharged and high-compression engines. It's why some modern engines require premium gas. 

What happens when you put 87 octane in a car designed to run on 91 octane? What should you do if you're running on empty and have a long way to go but the gas pump in the middle of nowhere serves up only 87? There are a lot of myths surrounding using premium gas.

For example, if your car is a 2018 Nissan Maxima, whose owner's manual recommends you use 91 octane, the manual also says it's okay to fill it up with 87 octane. Modern engines in cars like the '18 Maxima have this cool tech called knock sensors. A knock sensor identifies abnormal vibrations (knocking) within the engine and signals the car's electronic control unit (ECU), which adjusts the ignition timing to fix it. 

Timing is the precise moment a spark plug ignites air-fuel mixture in the combustion chamber. A car needing high-octane gas needs advanced ignition timing, where the spark is ignited way before the piston reaches top dead center. Cars that use low-octane gas needs delayed ignition timing, where the spark plug ignites closer to — but before — the piston reaches top dead center. A car's ECU sends a signal to the spark plug to ignite fuel at the precise moment the car requires, based on inputs from other sensors.

The difference in octane rating between 87 and 91 octane gasoline isn't that much, and modern engines can compensate for it. However, if you're driving an older car that doesn't have knock sensors, using a lower octane fuel than recommended could result in reduced performance and knocking sounds from the engine. At worst, you could be looking at expensive engine damage. What happens if your car requires 87 octane gasoline and you fill it up with 91 octane gas? No, that does not get you 50 extra horsepower. The truth is you won't even feel the difference except for an unusually lighter wallet, given the price you'd pay for premium gas. 

You can even get away with mixing fuels of slightly different octane ratings, provided your car runs fine on the lower octane gas or if you have a modern car that can handle the change in fuel octane ratings. Mixing two different octane gasoline fuels will give you gasoline with an octane rating somewhere in between the two. For example, if you fill your car with equal parts of 87 and 91 octane gasoline, you will get gasoline with around 89 octane rating. This is called mid-grade gasoline.It is still sold in gas stations, and used in some American cars as well, so mid-grade gasoline is not as pointless as it may seem.

So you can use a lower-octane gasoline in emergency situations provided you have a modern car, but it's a good idea to stick to your car's recommended octane rating. Far easier to get away with a higher-than-recommended octane rating.