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Octane is a hydrocarbon that raises a fuels resistance to auto-ignition. Auto-ignition occurs when the fuel in your engine explodes spontaneously out of sync with the spark plug. You know this has happened when you hear the characteristic sound of your engine knocking or pinging. GM designs its cars to run smoothly at a specific octane rating. Regular gasoline has an octane content between 85 and 88, mid-grade gasoline has an octane content between 88 and 90 and premium gasoline has an octane rating higher than 90.
87 Octane Requirement
Most entry-level and moderately priced GM vehicles have engines tuned to run on regular gasoline with an octane rating of 87, to help users save money on fuel. For instance, GMs Vortec 4300 4.3 liter, Vortec 4800 4.8 liter and Vortec 5300 5.3 liter all run on regular 87 octane fuel. Using higher-octane fuel on these engines will not improve their performance.
90+ Octane Requirement
GM engines that focus on high performance need premium grade gasoline to increase the power and efficiency of those engines. For instance, the Vortec 6000 6 liter Silverado will run on 87 octane but GM recommends you use premium fuel. The same is true of GMs Chevrolet Volt. It will run on regular fuel but is 5 percent more efficient if you use premium, 90+ octane fuel.
Flexible Fuel Engines
GM vehicles with flexible fuel engines require either 87 Octane or E85 fuel. E85 fuels contain a mixture of 85 percent ethanol and 15 percent gasoline. GM vehicles with flexible fuel engines include the Chevrolet Avalanche, GMC Savana, Cadillac Escalade and Hummer H2.
