The stock SVT calibration is set up for typical "premium" fuels (91-94 octane) under all conditions. There are some conditions where 89 will work as well, but there is a "safety net" so to speak, in the knock snesor, the knock sensor will catch detonation, and retard timing to eliminate it. This gives a sort of an "emergency" capability of the engine to use lower grades of fuel or guard for the infamous "bad tank of gas".
That said, run only the fuel your engine is designed for if you can help it. For the SVT this means 91-94 (any of the typical premium fuels, sorry for oyu poor guys out west
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and on the 104 stuff, don't waste your time or money, increasing octane is only intended to keep pre-ignition (detonation) at bay, and if you are not having detonation issues, higher octane will not help you. not to mention some of the higher octane "race" fuels are leaded, which is bad news for your O2 sensors and cats.
