LateralG,
With forced induction, you are looking to elevate your wheel horsepower (obviously) by cramming air into your cylinders. Your wheel horsepower target is dependent upon a required air mass flow rate and the PSI needed to reach that flow rate. This is why people need to run certain boost pressures to make certain power levels. By adding headers, you improve the volumetric efficiency of your engine, in turn making your engine more efficient at inhaling and exhaling air and exhaust gasses respectively. If your engine is more efficient (see volumetric efficiency) in this regard , then the needed boost level to reach your horsepower goals is less since your engine can make more out of the given mass flow rate of air.
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