How much hp will 5 psi of boost add to a 165hp engine?

The horsepower increase from adding 5 psi of boost to a 165 hp engine can vary based on several factors, including the engine's efficiency and design. Generally, a rule of thumb is that each additional psi of boost can potentially add about 10-15% more horsepower. Therefore, 5 psi could add approximately 15-25 hp, resulting in a total output of around 180-190 hp, assuming the engine can handle the increased boost without other modifications. However, this is a rough estimate, and actual results may differ.