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Mass airflow through the carb or throttle body
Stock engines ~75–85%, ported/built ~90–100%
Cooler air = denser air = more power
Performance: 12.5, stoich: 14.7, lean: 15+
From Airflow: A good rule of thumb is roughly 1.6 CFM per horsepower for a naturally aspirated engine at standard temperature and pressure. Air density corrections for intake temperature improve this estimate.
From Displacement: Using the engine's displacement, RPM, and volumetric efficiency, you can estimate the airflow the engine can move, then convert to HP. Compression ratio further refines the result.
From Boost: Adding boost pressure increases the air density in the cylinders, which increases power roughly proportionally. Every 14.7 psi of boost approximately doubles the naturally aspirated horsepower (assuming the fuel and engine can handle it). Compressor efficiency and intercooling affect the final number.
From Torque: The classic formula — HP = (Torque × RPM) / 5252 — gives exact results when you have a dyno sheet or known torque figure.
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