How many cc equaL 21hp?

The conversion from horsepower (hp) to cubic centimeters (cc) isn't straightforward, as it depends on the engine's design and efficiency. However, a rough estimate is that 1 hp is approximately equivalent to 15 cc for a small internal combustion engine. Therefore, 21 hp would be roughly equal to 315 cc. Keep in mind that this is a general estimate and actual values can vary.