Pulling your car up to the gas tank can be a time of questioning. “Sure 87 makes my car run okay, but will 91 or 93 make it run better? Will it give me better gas mileage?” Don’t count on it. As with anything related to cars, everything comes down to science—and whether or not your car will have a marked improvement with higher octane gas is a matter of combustion.
Higher Octane Doesn’t Mean Higher Speed—Probably
Let’s start from the beginning, what exactly does “octane” mean, anyway? Simply, octane is a hydrocarbon. A very combustible and flammable hydrocarbon. When it comes to gasoline, the definition isn’t exactly talking about the hydrocarbon itself, but rather a higher octane number assigned to the gasoline indicates that it is less likely that the engine will knock when using it.
As a reminder, the way an internal combustion engine works is that fuel is injected into the cylinder, the engine then compresses a mixture of this fuel and some oxygen, which is then ignited at a precise time by the spark plug, forcing the piston back out, and powering the car. The multiple pistons in the car are ignited in precise timing to get the engine to run smoothly. An engine knocks when an explosion happens caused by any leftover air and fuel in the engine after the usual spark plug combustion. When a high powered car is running at speed with lower octane fuel, premature ignition can also be caused by heat and pressure alone without any ignition from the spark plug. The more the fuel can be compressed in the cylinder before combustion happens, the more energy will be produced from it. A higher octane can help ensure optimal timing before combustion. Both of these types of unwanted explosions aren’t timed properly, and are not great for the engine.
Driving a car with higher octane gasoline doesn’t mean that the gas will produce more power, but rather that it is less likely to combust in the engine at any time other than when the spark plug ignites. There is a lot of heat and pressure in a car’s engine, and factors like the car’s engine type, the temperature, air pressure, elevation and a whole host of other things can effect how likely a car is to knock with lower octane fuel.
What Cars Need High Octane Fuel to Avoid Engine Knocking?
The easiest way to answer this question without a doubt is to look at your owners manual. A general rule of thumb, however, is that the higher performance that your car is, and the more power that the engine pulls, the more likely it is that you will want to be running a high octane fuel. Any car with a turbocharger—for example—is much more likely to require premium gas. High-performance import cars like BMWs and Porsches are recommended by the manufacturer to be run with premium gas.
What If I Run Premium Gas In My Less Than Premium Car?
The short answer is: not a whole lot. Car and Driver took the time to test drive a few cars with different gas types. They put premium in a Honda CR-V—which definitely doesn’t need premium—and it had very little difference in performance between 87 and 93 octane fuel. The same can’t be said from putting 87 in a tank that calls for premium, however. They tried lower grade fuel in a BMW M5, a Ford F150 and a Dodge Charger and found each of them to be wanting with the lower grade fuel. They used 87 in the Ford and the Dodge, but couldn’t bring themselves to put anything under 91 in the BMW. All of the cars that required premium gas did better on nearly every test—such as wheel power, wheel torque, fuel economy and acceleration—with the premium gas. If your car doesn’t call for premium, don’t waste your money. But if it does—especially if you need that extra torque while trying to haul something—it is worth every penny.
But What About Knock Sensors? Won’t That Fix the Problem?
Well yes, but also no. Knock sensors are built into many newer engines to help detect and prevent knocks. They will detect when explosions happen before they are supposed to and (with the help of the computer) adjust the timing of the spark plugs to make sure that it detonates at the perfect time—before any other unwanted explosions. So this may help to prevent the damage that would be caused by engine knock, but it will also tank your performance. The spark plugs on your car’s engine are designed to go off at the perfect time for maximum efficiency and output for the specific type of gas that your manufacturer recommends. So if you are using 87 in a car that requires 93 for peak performance, the piston won’t be able to compress as far before the spark plug is forced to fire. Which simply means you won’t get as much power. You may be unlikely to be actively harming your engine when you run 87 in a car equipped with a knock sensor that requires 93 (please don’t do it unless you have to), but you probably won’t like the lower overall performance output.
Buy the Gas That’s Best For Your Car
The general rule of thumb here is just to buy whatever gas your car manufacturer recommends for your car. If it says that your Volvo can run on regular gas just fine, you should take their word on it. But if your Porsche says it requires premium (and your Porsche will definitely require premium), go ahead and fill it up with the good stuff, you and your car will be happy that you did.
