Originally Posted by rsilvers
I did not realize the Focus was 12:1. I now accept that it probably will benefit from 93 octane fuel, especially considering that the owner's manual says that it will.
So now I just want to know how much it benefits, so I can see if it is worth using.
If 93 fuel costs 15% more, and I gain less than 15% increase in MPG, then it is not worth it for fuel economy.
Now if it gains HP, then that is another matter, as I can justify it by acceleration gains rather than on economy gains.
Top gear tested what - a Mustang? How much HP gain and what was the HP? We can then scale it to the Focus HP and come up with a guesstimate.
Ford quotes about 400hp for one of there Mustang models on 87 octane and over 420hp on higher octane, 93 I think. I would not be surprised if 93 octane gas resulted in about 10hp more in the 2012 FF than 87 octane. But, adding ethanol will also increase the hp because it has additional oxygen that acts somewhat like Nitrous Oxide to increase power -- thought that increase in hp comes at the price of lower mpg.
The ideal way to test mpg would be to drive at Daytona Speedway on a calm day then switch back and forth, several times, from one octane to another (87 then 93 then 87 then 93). Even better if you could use a graduated cylinder to hold a precisely known quantity of gas then drive it until it goes from the top mark to the bottom mark.