BlasterMaster555 wrote:Both engines are running 14.7 fuel air mix. If the car makes double the horsepower, it should halve the fuel economy regardless. The only time you MPG goes up with a turbo is on a diesel, where fuel air mix is not a constant.
Fuel consumption works in the following way.
An engine has what is called a brake specific fuel consumption, represented by the fuel economy stat and graph in the engine creator.
The graph is, IIRC, 1:1 the amount of fuel used per HP at each RPM at full load.
The fuel/hp is the important part.
If you have two engines with the following power curves, which are completely unrealistic, but should be fine for my explanation:

The Y axis represents the power in kW, the X axis represents the RPM
Now, let say both have a fuel economy of 100g/kW*h at all RPMs, and that due to drag and mechanical losses you need 50hp to cruise at 100km/h.
Since IIRC fuel economy is maxed out at maximum engine load for piston engines, the optimal gearing would be to have the engine cruising at under 2500 RPM.
Therefore, since both engines are producing the exact same amount of horsepower at the optimal gear for cruising, and that they have the exact same fuel economy, they will use exactly the same amount of fuel, even though engine 2 produces 3 times the horsepower.
OFC this example is completely unrealistic but it should be good enough to explain that an engine making twice the peak horsepower can burn just as much fuel while cruising. It will probably burn twice the fuel in racing though, as you will be going more in the high RPMs.
As for your other problem, did you change anything other than compression when making the engine 91 octane? Gearing might need to be adjusted for maximum fuel economy.