Octane ratings for gasoline are a part of daily life, but very few people understand what they actually mean. Many believe that using gas with a higher octane rating than your car requires has a multitude of benefits ranging from increased power to decreased engine wear. Further clouding the issue, cars that advise the use of high-octane gas will generally run fine when filled with a lower-grade fuel. Deciphering this mystery requires understanding what an octane rating means and why it's important.
What is octane?
The octane ratings that you see on the gas pump indicate a fuel blend's ability to resist knock. Knocking is a pinging sound that is caused when fuel in your engine ignites prematurely, which can throw off the rhythm of your engine's combustion. Gas with a high octane rating requires more pressure to ignite, and certain engines take advantage of this quality. Low-performance engines generally do not pressurize the fuel-air mix inside engine cylinders as much as high-performance engines. This allows them to use lower-octane blends, because the designers don't need to worry about the gas lighting early due to pressure. In very powerful engines, however, the pressure in each cylinder is much higher, and gas that has too low an octane will cause the off-time ignition that leads to pinging and a loss of power.
So is high octane always better?
Many people who own cars designed for low-octane fuel choose to fill their cars with high-octane options. They believe that doing so will improve their car's performance by ensuring that the engine runs perfectly, and that this will also improve durability. While it's nice to think that a simple change of gas can act like an auto insurance policy and safeguard you from potential issues, save your money. While gas that is higher than a vehicle's recommended octane will work fine, it won't offer the miracle benefits an owner might hope for. The U.S. Federal Trade Commission said that high-octane fuel will not clean your engine better than its lesser counterparts, and a Car and Driver test found that at least one car designed to run on regular actually lost power when filled up with high-octane premium.
But what if my car requires a high octane?
If you have a modern car with a high-compression engine, you have a choice on whether to fill it with regular or premium fuel. This wasn't always the case; prior to the 2000s, filling a car designed for premium gas with regular could lead to knocking that would eventually damage the engine. Today, however, basically all new vehicles are sold with a knock sensor that can detect when the engine experiences premature ignition. When this happens, modern cars can adjust the piston's timing to accommodate the problem and prevent engine damage. While this allows owners to save some money at the pump without worrying about their vehicle, it is not a perfect solution.
While using lower-octane gas than recommended will not damage your car, it can have a detrimental affect on performance. As a car adjusts the motor's timing to prevent knock, it will generally lower the vehicle's power. The horsepower numbers you see on a car's window in the showroom are based upon the car functioning in ideal conditions with the recommended type of gas. While using something else will work, you will not be getting all the speed you paid for.
What if my old car starts to knock?
As a car ages, its compression ratio can change. The build-up of age and grime inside an engine can cause older low-compression ratio engines to prematurely ignite low-octane fuel. In these cases, the best recourse is to switch to a more expensive blend.