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posted on December 27th, 2017

It’s not that dumb of a question

Does premium fuel make your car perform better? Is it bad to use regular gas if premium is recommended? What does 91 octane even mean?

Leveling up

I’ll throw you a bone up front. Unless your owner’s manual explicitly recommends premium fuel, no, your car does not perform better running on high-octane. Not only does this waste money, but the premium fuel isn’t even better for your engine.

Years ago, premium gasoline contained higher levels of additives and detergents to stop carbon deposits from building up in the engine. But thanks to regulations aimed at curbing carbon emissions, most major gasoline brands now put plenty of additives in all grades to both protect engines and cut pollution. Premium fuel no longer gives an advantage in cleanliness.

Upgrading to premium won’t turn your Toyota Camry into Dominic Toretto’s Charger either. If your engine isn’t designed to take advantage of a high-octane fuel, the higher grade means nothing. Any performance boost will be negligible and not worth the higher cost.

Downgrading

In combustion engines, the flame to burn the fuel comes from the spark plugs firing. If the mixture of air and fuel fired off from a particular spark plug experiences too much heat or pressure before its time to shine, it will ignite spontaneously. This small self-ignition makes a loud bang, and is known as the “knock.” When fed with low-octane fuel, engines designed to run on the primo stuff have a tendency to knock, and knocking is bad. Knocking wears down an engine over time.

The higher the octane rating, the more knock-resistant the fuel. Octane refers to the ratio between two specific hydrocarbons in a petrol recipe — isooctane and n-heptane. Isooctane is particularly resistant to exploding under heat, while n-heptane is susceptible to self-ignition. If the mixture is 87 percent isooctane and 13 percent n-heptane, the fuel has an octane rating of 87. Typically, regular-grade gas is rated at around 87, mid-grade is at 88-90, and 91 and higher is premium.

Higher-octane gas doesn’t necessarily contain more energy, but it allows for more aggressively designed engines to extract more oomph from every gallon. This is why the highest-performance engines require high-octane fuel — racing fuels are rated at 100 octane and up.

The good news is, virtually all new cars from major manufacturers have tiny sensors mounted on the engine block that sense vibration patterns and alter cylinder spark timing to avoid knocking. So if your premium-requiring car is new or sufficiently sophisticated, filling up with regular gas won’t damage the engine. Its power output, however, might drop by five or 10 percent.

The takeaway is easy: Use the lowest recommended grade. Don’t use a higher-than-needed octane fuel, but don’t skimp if your car demands premium. It’s just not worth it.

Steven is an avid car guy and content specialist at Turo. Between Golden State Warriors games he can be found getting lost somewhere in California.