They are both listed as equivalent to 746 Watts. However, does measuring the horsepower by the braking force required give a different answer to measuring according to the continuous power output. Horsepower and brake horsepower are different. Thus 16hp would be about 1.6 litres. However, a 1.6 engine wouldn't be a 16 brake horsepower engine because the measurements are made differently.
Brake horsepower is measured by taking a drum filled with a fluid and attaching that to an engine.
The power required to turn the drum is the brake horsepower - as the drum is 'braked' by the fluid.