The power of an engine depends on a number of factors, of which the size or volume is only one factor. The other major factors are the type of engine and the tuning of the engine, with a variety of secondary factors (e.g. gearing, type and quantity of accessories).

For example, in the case of an average modern passenger car (e.g. Ford Escort 1.6 liter), each horsepower requires about 15cc (or 1 cubic inch) so there is a ratio of 15cc:1HP. However, a Formula 1 sports car will generate about 1 horsepower for each cc of engine, giving a ratio of 1cc:1HP. Many examples can be found at cc to horsepower.

Aside from engine size, other factors that affect the cc to horsepower ratio include:

  • Point of Measurement. The horsepower depends somewhat on where the measurement is made. For example, horsepower for a car is about 15% to 25% higher at the crankshaft than when measured at the wheels. See types of horsepower and BHP Horsepower for details.
  • What is Included. Related to the above point is the question of what is included. For example, a measurement of the engine itself will give a higher horsepower than a measurement which includes associated components (e.g. engine cooling, muffler exhaust).
  • Type of Engine. Different engine types (e.g. reciprocating piston or rotary) will vary somewhat in power, for a given size. A 2-stroke engine will not have the same power or efficiency as a 4-stroke engine. 
  • Fuel Type and Quality. The type of fuel on which the engine is based (e.g. gasoline, diesel, kerosene) affects power, as well as the quality of fuel (e.g. octane rating).
  • Engine Design and Tuning. Engines can be designed for high performance, which gives a higher power to size performance, but typically at a cost (e.g. lower fuel efficiency, reduced engine reliability or engine lifespan). Alternatively, the tuning for an engine can be modified to achieve higher performance, but may have the same issues (fuel efficiency, engine reliability, engine lifespan).
  • Design Date. Modern engines are more powerful and efficient than older engines, due to improvements in design. 
  • Performance Accessories. Devices such as turbochargers and superchargers can be used to increase the horsepower to size ratio, although normally at a loss of efficiency and a potential loss of reliability.
  • Engine Condition. Due to wear and tear on various components, engines tend to decrease in power as they age. This is partly due to increased friction as worn components move less efficiently and partly due to various mechanical losses (e.g. pressure loss around worn valves, pistons, etc.).

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