* The camshaft itself: Different cams have vastly different profiles (duration, lift, lobe separation angle). A mild cam might add 10-15 horsepower, while a more aggressive cam could add 20-40 horsepower (or even more with supporting modifications). Aggressive cams often require other engine modifications to work effectively.
* Other engine modifications: The gains from a cam are heavily influenced by other components. A better intake manifold, headers, exhaust system, carburetor/fuel injection system, and cylinder heads will all significantly impact the final horsepower increase. An aggressive cam in a stock engine might not even yield a net horsepower gain.
* Engine condition: The starting condition of the 305 engine is crucial. A well-maintained, properly tuned engine will see more benefit from a camshaft upgrade than a neglected one.
* Dyno testing: Accurate horsepower gains can only be truly determined through dyno testing, which measures the actual output of the engine.
In short, you could see a range anywhere from a negligible increase to a significant boost depending on the factors listed above. To get a reasonable estimate, you would need to specify the exact camshaft and other engine modifications being used.
