22nd Jan 2011, 15:58
That's not quite how it works. Higher octane doesn't mean a more efficient burn, it means a slower, more controlled burn. High compression engines (and some highly worn engines) require premium because if the burn is too rapid and uncontrolled, the fuel will detonate improperly. It's just to prevent detonation and knocking, there's no other purpose to it. The compression ratio in an I4 Fusion just isn't enough for the engine to need premium gas, and besides; It's been tuned by the factory to work at a regular octane rating. Unless the compression has been altered, or the engine is really old and starting to knock, you're wasting your money.
22nd Jan 2011, 23:02
Here's the idea: The reason turbochargers, superchargers, intakes, etc, work is because they force more oxygen or denser air into the engine. When this happens, the combustion can't be rapid, or the engine will knock, detonate, or worse. The same goes for high end, high output cars like Porsches, BMWs, Ferraris, etc, because they have high compression engines. Higher octane rating or content means that the fuel-air combo burns slower, not that it will release more energy when burning. A 4-cylinder Fusion (or maybe even the V6, for that matter) won't have high enough compression to make a noticeable power difference.
As for efficiency, the ECU determines how much fuel goes in, not the fuel itself. Just because you used 91 octane doesn't mean it will magically decide to use less of itself. 1-2 MPG, by the way, is barely enough to register as a margin of error; I could get that same increase by taking off my roof racks.
14th Jan 2011, 15:05
Maybe you should also consider that if an engine is built and tuned to run on a specific grade of fuel, you shouldn't mess with it. To run super unleaded, means you are burning at a hotter temperature, which causes more wear and tear on vital engine parts designed to run on regular gas, which can lead to premature failure.
Also, with complete computer control of the fuel system, the engines of today run at optimum levels on the recommended fuel. If you change from regular to premium, your engine will adjust to it and still run about the same. You aren't going to gain a whole lot of anything really. The only law of physics you are abiding by is how fast your wallet shrinks when you use premium fuel in any modern car that doesn't require it. It is purely and simply a waste of money.