Horsepower depends on much more than just engine size. For example, a well tuned up 246 cc engine will have more horsepower MORE?
The Kawasaki 454 LTD is a motorcycle produced from 1985 to 1990, also known as the EN450. It is the forerunner of the Kawasaki Vulcan. The engine was a precise copy of the Kawasaki Ninja 900s, with two fewer cylinders. The Kawasaki 900 had a 908 cc engine. Removing two cylinders from the 4-cylinder divided the number of cc's by two. (908/2 = 454, thus the name). Included was the liquid cooling, the bore and stroke, the double overhead camshafts, and four valves per cylinder. This gave it a great deal of power for its size, redlining at 10,000 RPM while delivering 50 horsepower. The 454 is considered a good starter bike for having a low seat height and light weight, as well as an efficient use of counterbalancing to limit vibration. The Kawasaki 454 is well known for its high rate of acceleration, having raced against a 454 ls big block Chevrolet Corvette and beating it to both 0-60 and the quarter mile by more than a second.
Despite the 440 being its predecessor in the sale of middle-size-displacement motorcycles designed by Kawasaki, the two shared almost nothing in design, as the Kawasaki 440 had a single overhead camshaft and had only two valves per cylinder, and was air-cooled, producing only 27 horsepower to the 454's 50 horsepower. Both motorcycles were highly reliable and low maintenance, but for different reasons, as the 440 was just a very simple machine to service, and the 454 needed little maintenance over time as a result of its belt drive. Land transport
The tax horsepower or taxable horsepower was an early system by which taxation rates for automobiles were reckoned in some European countries, such as Britain, Belgium, Germany, France, and Italy; some US states like Illinois charged license plate purchase and renewal fees for passenger automobiles based on taxable horsepower. The tax horsepower rating was computed not from actual engine power but by a simple mathematical formula based on cylinder dimensions. At the beginning of the twentieth century tax power was reasonably close to real power; as the internal combustion engine developed, real power became larger than nominal taxable power by a factor of ten or more.
The so-called RAC horse-power formula was concocted in 1910 by the RAC at the invitation of the British government. The British RAC horsepower rating was calculated from total piston surface area (i.e. "bore" only). To minimise tax ratings British designers developed engines of a given swept volume (capacity) with very long stroke and low piston surface area. Another effect was the multiplicity of models: Sevens, Eights, Nines, Tens, Elevens, Twelves, Fourteens, Sixteens etc. each to fit with a taxation class. Larger more lightly stressed engines may have been equally economical to run yet, in less variety, produced much more economically.