Quote:
Originally Posted by airforceret
Because the air is less dense, less air flows to your car's engine. This translates to a slightly richer air-fuel ratio, and deters engine knock. For that reason, you'll find that regular gas carries an 85 octane rating here in Denver and throughout Colorado, Wyoming, Utah and Montana.
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True the air is less dense, but your engine has a one bar Manifold Air Pressure (MAP) sensor and an Initial Air Temperature (IAT) sensor that allows the Engine Computer Unit (ECU) accurately determine the actual air density and inject the correct amount of fuel for the computed density. The feedback from the three O2 sensors ensures the fuel air ratio is correct. The reason that 85 octane rated fuel is available is the air density effectively lowers the compression ratio sufficiently to allow the engine to run successfully at altitudes 4,000 ft ASL.