When several people asked for a conversion of engine capacity to brake horsepower I started collecting data on all types of engines - cars, bikes, pickups, racers, chainsaws, outboards, model engines, lawn mowers, etc. as seen in the table below. The list is not meant to be exhaustive - just enough data for a meaningful result.
The ratio between cc and hp was calculated and the result is the table below.
The data was then sorted and the results are on another page.
The increase in horsepower can be seen between an auto manufacturer's turbo, supercharged or blown model and the standard, naturally aspirated version. Most engines can be tuned with kits.

Size isn't everything. An 8.2 litre dragster produces nearly twice as much horsepower as the mighty 71.5 litre Pratt & Whitney Wasp Major, 28cyl radial aero engine - the largest production petrol engine ever built.

The largest diesel engine 'to buy' ( in 2002 ) is the Wartsila-Sulzer RTA96-C turbocharged two-stroke ships engine - the 14 cylinder, 1,556,002 cu. in. ( 25,498 litre ) model produces 108,920 hp at 102 rpm ( 234 cc/hp) weighs 2,300 tons and consumes 1,660 gallons of heavy fuel oil per hour.

The cheats of the engine world are the outboard motors. You will pay far more for a 70hp than a 60hp, but the capacity and basically the whole engine is the same and costs the same to make. A higher compression ratio and a change of carb jet size is often the only change.