But because compression creates immense heat, a fuel must be able to withstand extreme temperatures, otherwise it can misfire prematurely. An octane rating is simply a measure of how heat resistant a fuel is in order to prevent knocking. If you hear knocking, you could be using the wrong octane for your ride. Which octane rating should you use? You should always use at least the minimum octane rating recommended by your vehicle manufacturer. Using a lower-octane fuel than required can cause knocking and will prevent your vehicle from meeting its stated fuel economy.
The trend we're really living is the story of smaller engines working harder, in everything from family crossovers to six-figure autobahn barges. Downsized but hardly diminished, many of these shrunken engines are more powerful than their predecessors, thanks to turbocharging, variable valve timing and lift, direct injection, and the advanced computer controls tying these all together. Today's engines are so sophisticated that even mainstream nonperformance vehicles can benefit from running on higher-octane premium fuel.
Vehicles such as the Ford Escape and Mazda 6 are advertised with power figures made on octane fuel, although both companies are quick to note that these vehicles will happily run on What automakers rarely say is what, precisely, are the benefits of paying for premium.
That ambiguity can be expensive. Raising the octane rating also known as the anti-knock index doesn't change the energy content of a gallon of gasoline. A higher octane rating indicates greater resistance to knock, the early combustion of the fuel-air mixture that causes cylinder pressure to spike. When higher-octane fuel is flowing through its injectors, the engine controller can take advantage of the elevated knock threshold and dial in more aggressive timing and higher boost pressures to improve performance.
To understand how higher octane affects acceleration and efficiency, we assembled a four-wheeled quartet sampling a broad spectrum of the market. The Honda CR-V stands in for a swath of affordable crossovers and sedans with its turbocharged 1.
At the intersection of effortless speed and opulent luxury, the BMW M5 Competition squeezes horsepower from its twin-turbo 4. Ford's F is America's best-selling vehicle and is equipped here with its most potent engine, the hp EcoBoost twin-turbo 3.
We performed acceleration runs, mile fuel-economy loops at 75 mph, and dynamometer pulls, running each vehicle on two different fuels and completely draining the tanks in between. The differences likely would have been exaggerated by extreme summer heat, which exacerbates engine knock, but we sniffed out differences even with the engines huffing cool midwestern spring air.
Even as it's sucking down as much as Honda asks for 87 octane and makes no claims that raising the fuel octane will lift performance. Based on our testing, premium fuel might as well not exist in the CR-V's world.
We could see this coming. During a similar Car and Driver test 18 years ago, an Accord powered by a 3. The modern CR-V, with half the displacement but rated at just 10 fewer ponies, makes the same argument: don't waste your money on premium.
Switching from 87 octane to 93 yielded a 7-hp gain on the dynamometer, but that advantage was lost in the noise at the track. There, the CR-V's zero-tomph and quarter-mile times both tracked a tenth of a second slower on the expensive stuff. While fuel economy at 75 mph ticked up from Honda built its reputation on a line of unassuming, egalitarian motorcycles in the '60s.
Nearly 60 years later, the company's identity is still predicated on the same sensible and modest ethic, right down to the fuel that you put in the tank. BMW explicitly warns about engine damage from doing so, and while that seemed unlikely in such mild temperatures, using the cheap stuff would have been wildly out of touch with what an owner would do at least until this M5 reaches its fourth owner sometime in BMW requires octane fuel at a minimum, with 93 recommended, so we did just that, switching between the common forms of premium gas depending on which state you're in.
Are Californians, with their watered-down premium, leaving something on the table? Pentane has five, hexane has six, heptane has seven and octane has eight carbons chained together. It turns out that heptane handles compression very poorly. Compress it just a little and it ignites spontaneously.
Octane handles compression very well -- you can compress it a lot and nothing happens. It spontaneously ignites at a given compression level, and can only be used in engines that do not exceed that compression ratio. Cheaper grades of gasoline could be made usable by adding TEL. This led to the widespread use of "ethyl" or "leaded" gasoline. Unfortunately, the side effects of adding lead to gasoline are:. When lead was banned, gasoline got more expensive because refineries could not boost the octane ratings of cheaper grades any more.
Airplanes are still allowed to use leaded gasoline known as AvGas , and octane ratings of or more are commonly used in super-high-performance piston airplane engines.
In the case of AvGas, is the gasoline's performance rating, not the percentage of actual octane in the gas. The addition of TEL boosts the compression level of the gasoline -- it doesn't add more octane. Currently engineers are trying to develop airplane engines that can use unleaded gasoline. Jet engines burn kerosene , by the way. Sign up for our Newsletter!
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