1994 Mustang GT Performance Chip Question ?
If your car is stock you definitely will *not* see a 20-30 HP increase. For a stock engine 10-15 HP is the most you can expect. By themselves tuners and chips don't do a whole lot. Where they do shine is when they are used in conjunction with other modifications, provided you have specific tune written or chip burned for your specific car. The vast majority of the time a chip or tuner will require the use of premium fuel, but this is to be expected since the chip/tuner typically changes the timing and/or spark advance, making 91 or 93 octane necessary.