With gasoline prices in their summer roller-coaster mode, the thought of paying a premium for premium fuel may tempt some drivers to make a stab at the brake pedal.
But for vehicles designed to use it, premium gas makes their engines operate at intended horsepower and torque levels and maximum fuel economy. And it keeps those engines–generally engines with higher compression ratios such as 12:1 (the first number means the air-fuel mixture is compressed that number of times, e.g., 12) from being damaged by early combustion and knocking.
“Engines are designed to work with specific fuels,” said Gary Abusamra, chief engineer of engine management systems for Delphi Automotive Systems, Troy, Mich. “The combustion ratio and the combustion chamber design affect fuel requirements. Usually, the compression ratio is 9:1 to 10:l for regular fuel and 11:1 to 12.1 for premium. A higher compression ratio makes the engine more efficient. It also improves horsepower and torque.”
Premium fuel typically has a higher octane rating, expressed in numbers on the gas pump, he said. Compression ratios for midrange fuel usually falls in between regular and premium, although engines are typically designed either for regular or premium.
According to the American Petroleum Institute (API), octane measures a fuel’s resistance to engine knock, or the metallic pinging that can come from an engine experiencing detonation (a second spark) or pre-ignition (when combustion happens before the spark). Over time, engine knock can damage pistons and other engine parts, says API.
The most common octane levels are 87 (regular), 89 (mid-grade) and 93 (premium), and octane can be any level from 87 to 94. Vehicle age and mileage, driving habits, climate and geography may affect choice of octane, says the institute. But the octane level recommended by the vehicle’s manufacturer is a good starting point.
“If you put regular fuel in a vehicle with a higher compression engine, you will get worse fuel economy,” said Abusamra. Think in terms of dollars per mile instead of miles per gallon.
Dan Holt, technical editor for the Society of Automotive Engineer’s publications, says there’s no benefit to giving your car the occasional treat of a tank of super or premium gas. “Use what’s suggested in the owner’s manual,” he said. “If there is pinging, move to a higher octane level.”
Holt said higher-octane fuel, with its higher compression ratio, prevents pinging because it detonates slower than lower-octane fuel. (Ping occurs when the fuel in the combustion chamber ignites before the spark plug fires.) He also said electronic controls or knock sensors help engines adjust to any level of octane.
“Use the gas that works,” he said. “A little pinging won’t hurt your engine.”
API generally agrees with Holt, encouraging motorists to use the automaker’s recommended octane as a guide. But API spokeswoman Susan Hahn also says that as a car ages and accumulates mileage it may need a little higher octane to keep it from knocking or pinging.
“If your car is properly tuned and is getting engine knock, then you should try one level higher of octane,” Hahn said.
Different engines, fuel
Typically, premium fuel is recommended for higher performance engines and regular fuel is recommended for mainstream engines, said Toyota spokesman John Hanson in Torrance, Calif.
“Mainstream engines usually do not experience a `boost’ in output from using premium fuel,” he said. “However, as a general rule, Toyota/Lexus engines can run on either premium or regular fuel. When premium fuel is recommended, it is to achieve maximum output.”
Fuel octane is read by sensors that adjust timing and air-fuel ratio, Hanson said. In a Toyota/Lexus engine designed for premium fuel, the use of lower octane will reduce output only a little from peak. In most everyday driving conditions, the customer will rarely notice a difference in performance.
Though Honda’s Accord was the best-selling car in 2001, it was designed to use premium fuel when it got its first V-6 in 1996, said Honda spokesman Andy Boyd.
“So did the Odyssey minivan for the first two model years,” said Boyd, adding that this is no longer the case because the engines now have more sophisticated control systems that can adapt to lower-octane fuel.
The Honda S2000 sports car requires premium fuel, Boyd said. And vehicles equipped for towing, such as Honda’s new Pilot sport-utility, get better horsepower and acceleration at maximum towing capacity with premium fuel, he said.
Boyd said he sometimes alternates premium and regular in vehicles that require premium, and he can notice a difference in acceleration.
The Acura RSX Type S requires premium, but the base model can run on regular, he said.
Mercedes-Benz calls for premium fuel in all its cars “because it allows us to take the best advantage of our knock-sensor technology,” said Fred Heiler at Mercedes-Benz USA in Montvale, N.J.
“This technology optimizes ignition timing and compression ratio.”
A Mercedes engine will run reasonably well on regular fuel if premium is not available, Heiler said. In that situation, the knock sensors retard the ignition so the fuel doesn’t burn prematurely.
Expect poorer fuel economy with regular gas in a Mercedes, he said.
Buyers’ choice
So how do consumers feel about stepping up to the premium pump?
Pontiac Grand Prix, Buick Regal GS and Park Avenue Ultra intenders don’t change their mind when they find their vehicles require premium fuel, said Alvin Cruz at Anthony Pontiac-Buick-GMC in Gurnee.
“The kind of clientele that buy these vehicles–they cost $30,000 to $40,000–are not bothered by that,” he said.
“They will need to pay an extra 10 to 20 cents per gallon, but they know they need it to maintain vehicle performance.”
Octane and knock
The octane number measures a gasoline’s ability to prevent knock, a sharp metallic sound. The higher the octane number, the more resistant the gasoline is to knock. Therefore, a 93-octane gas will resist knock better than an 87-octane gas.
Octane ratings are determined through laboratory tests. They compare the burning characteristics of a mixture of iso-octane, which has a high anti-knock rating of 100, and heptane, which has an octane rating of zero. For example, 87-octane gasoline would burn the same way as a mixture of 87 percent iso-octane and 13 percent heptane.
