How do you Convert cc to hp horsepower?

The formula that works best only applies to the small newer normally aspirated (carbureted) 4 cycle engines. The ratiOS change slightly worse with altitude and slightly better with fuel injection. Most American mowers and the newer 4 cycle outboard motors fit into one of these two formulas: Typical new carbureted engine well tuned: 1 x HP per each 25 cc Fuel injected: 1 x HP per each 22 cc Notes: -Most enginesare tuned down to a lower HP for better engine life. -It is common for the same cc engine to be built in 3different HP versions with the highest rated being the maximized HP per cc version. That is how you see price and HP changes on the same cc engine made by the same company. -Some of the newest (2008 and newer) engines from Japan perform slightly better (10-20%). A typical 150cc motor should equate to between 3.5 - 6 HP+ depending on tuning with 5 HP being the adv.
A Typical 1200cc Motor should produce between 50 - 75 HP depending on tuning. Max/High Performance engines for motorcycles and racing do not fit these formuls and can vary up to double and more. These are standard use numbers. The actual formula plots as a curve and is not a fixed linear formula.