The horsepower of a 305 cubic inch Chevy small-block engine varied significantly depending on the year of manufacture, the specific model of the vehicle it was installed in, and any modifications made to the engine. There's no single answer.
You'll find horsepower ratings ranging from approximately 145 hp to over 200 hp across different years and applications. To get a precise horsepower figure, you need to specify the year and model of the vehicle the engine came from.
