Fuel does not provide the power in a combustion engine, oxygen does. The fuel is just a catalyst to make the oxygen burn more easily. Fuel also has a cooling effect. A car that runs rich, runs cooler, at the expense of mileage of course.
That's why if you up the air into the car while leaving the fuel the same, you are actually making more power, but it's at a higher temperature making the car suseptable to pre-ignition (aka knocking/pinging/detonation/bad things).
Most turbo cars are tuned to run a little rich simply to combat detonation... by changing the intake w/o tuning for it, you get closer to stoich (when all the fuel and all the air are burned in the combustion reaction) which is "ideal" for mileage purposes, but is actually a little too hot. If the car starts knocking, then the ECU retards the timing to prevent detonation, which is why you ultimately make less power than you would with the stock intake.
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