Brands that aren’t on Top Tier’s list aren’t selling Top Tier-certified gas, the website says.
Debates over whether cars designed for 87 octane run better when given 89- or 93-octane gas are easy to find on the internet, and 87-octane adherents make strong arguments.
Knocking is caused when fuel in an engine’s cylinder ignites before the piston reaches top dead center. The misfired piston then is thrust toward the back of the crankshaft against the momentum of the engine.
Ignition in modern cars are controlled by the cars’ computers, they point out, and those computers are calibrated to detect knocking and advance the ignition timing to eliminate it.
In an undated post, AAA said it found no increase in power or fuel economy when running 93 octane in cars designed for 87 octane. But those tests were conducted with cars that were just two years old.
As cars age, their engines become more susceptible to carbon buildup, which also contribute to knocking and pinging by reducing the volume of space in the cylinder, creating heat-retaining “hot spots” that ignite fuel prematurely, changing the fuel-to-air mixture, or impeding regular movement of valves.
Filling up with premium helps clean up that carbon and other byproducts of burning cheaper gas, dos Santos said.
Improvement is real
Experts’ advice notwithstanding, drivers who report feeling better performance when upgrading to high octane gas aren’t imagining things, he said.
Drivers “will feel more improvement if they drive cars with smaller four-cylinder engines than larger engines,” he said. “But it also improves performance of six- and eight-cylinder cars. Older cars might feel more improvement because they typically have more residue and carbon built up over the years.”