I've done quite a bit to the PCM in my own car and quite a few others as well. I can tell you not to expect what the chip "makers" hype about their products: 21HP on the 2.5L V6 from a chip? No way, not in a million years.
Chips can be tuned to give better response via spark and fuel changes, to alter VRIS points (V6) to match altered intake components, alter cooling fan on/off points and to change other more esoteric calibration parameters (an example, throttle tip-in timing reduction elimination, knock retard attack/decay etc...) but in the end, you'll be tweaking, not moving more air through the engine which is what ultimately makes a big power. This is one reason that turbo cars respond very well to chips: with their computer controlled wastegates, they can get more air into the engine via a boost increase just with a PROM mod. For example, it's easy to get 30HP+ in a Volkswagen 1.8T with a simple PROM upgrade. Not so with well-tuned normally aspirated motors like ours.
If (and it's a big if) you can find a place to customize your PROM for your own setup (and tune it on a dyno etc), expect maybe 5HP. If you buy an off-the-shelf generic PROM from, say, Superchips, I wouldn't expect even that much. When reading chip makers ads, I offer four words of wisdom: Don't believe the hype.
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