* Specific supercharger: Different superchargers have different displacement and efficiency. A small supercharger will add less horsepower than a large one.
* Supercharger configuration: The way the supercharger is installed and tuned significantly impacts the outcome.
* Engine build: The condition and modifications of the 426 Hemi itself (camshaft, heads, intake manifold, etc.) will drastically affect how it responds to forced induction. A highly modified engine will see a larger gain than a stock one.
* Fuel system: The fuel system must be able to support the increased power demands. Insufficient fuel will limit the horsepower increase.
* Tuning: Proper tuning of the engine management system is critical to maximize power and prevent damage.
You might see anywhere from a substantial increase of 100-200 horsepower on a relatively stock 426 Hemi with a well-matched supercharger and proper tuning, to significantly more (300+ hp) with a highly modified engine and a larger supercharger. However, some builds could fall outside of that range.
To get a more precise estimate, you'd need to specify the exact supercharger, engine modifications, and planned tuning approach.
