What is the horse power of a 390 4 barrel mercury engine?

The horsepower of a 390 cubic inch 4-barrel Mercury engine varied depending on the year of manufacture and specific model. There's no single answer. However, a typical range for a 390 4V (4-barrel) Mercury engine from the late 1960s would be in the 260-335 horsepower range. To get the exact horsepower for a specific engine, you'd need to know the year and model of the car or boat it was in.