No, High Octane Gas Cannot Clean Your Engine - Why You Shouldn't Believe This Myth
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Most gas stations have three options that you can put in your vehicle: regular, mid-grade (plus), and premium. These labels are also often accompanied by numbers that represent the fuel's different octane ratings, with regular typically sporting the lowest octane level at around 87 and premium having the highest, usually sitting somewhere between 91 and 94. This is why Premium is often referred to as high octane gas.
That's all pretty straightforward, but there are a lot of people out there who might not fully understand exactly what it is that high octane gas does. There has been a persistent myth floating around for years that high-octane gas cleans your car's engine when you use it. It seems like this myth stems from the idea that high-octane gas burns hotter or cleaner, and that these properties allow it to somehow get rid of oil, dirt, and carbon deposits that have made their way into the cylinder block.
This is patently untrue, however, and it ultimately boils down to a fundamental misunderstanding of what high-octane gas is and how it behaves when it is burned inside the cylinders of a four-cycle engine.
What is high-octane gas?
A lot of people seem to believe that an octane rating is a metric that measures a gasoline's purity or combustive strength, but this isn't entirely accurate. The octane rating measures a fuel's stability under pressure and its resistance to auto-ignition, which is often referred to as knock. The rating number refers to an average of two measurements taken from different testing methods: the MOR, or motor octane rating, and the RON, or research octane rating. These both monitor the amount of pressure the fuel can withstand before spontaneously igniting without a spark introduced by the spark plug.
There are four cycles that take place in a gas engine: intake, compression, combustion, and exhaust. Auto-ignition can occur when fuel inside the engine combusts too early in this cycle, often producing a loud knocking sound. Not only is this obnoxious, but it can also cause serious damage to your engine over time. According to the U.S. Energy Information Administration, "this uncontrolled secondary combustion causes pressure in the cylinder to spike and causes the knock to occur."
This is the true purpose of high octane fuel. A higher octane number means that a fuel has been tested to be more stable under higher amounts of pressure, thus reducing the occurrence of knock. This is unnecessary in a majority of modern vehicles, but those who want to make sure they choose the right octane gas for their car should check their owners manual.
Why this myth may be harmful
Putting high-octane gas in a car or motorcycle that doesn't need it won't hurt the engine, per se. The fuel will still function the same way regular fuel would without causing any ill effects. But there are still a few reasons why this myth might be doing more harm than good.
The first and most direct issue is that of the fuel's price. Premium gas typically costs something in the neighborhood of $0.50-$1.00 more per gallon than regular. This might not seem like much on it's face, but it's still money down the drain which can be very detrimental to someone who is already financially struggling–particularly if they're having engine trouble and already concerned about looming maintenance costs.
The second, and more insidious reason that this big myth about gasoline can do harm is that it might cause people to delay maintenance for engine trouble that they've been experiencing. There are affordable and highly rated fuel additives that you can purchase, such as STPs Complete Fuel System Cleaner, that will assist in cleaning out your tank, fuel lines, fuel injection system, and engine.
One of these might be worth a shot if you believe your engine's internal fuel system may be dirty, but the belief that high octane fuel can clean the engine might cause the vehicle's owner to delay the correct maintenance for their vehicle, or perhaps move on to troubleshooting other potential causes for the issue without properly ensuring that their fuel system is clean.