About

Converting Horsepower (HP) to cubic centimeters (cc) is not a direct physical conversion because HP measures power output while cc measures engine displacement volume. The relationship depends entirely on engine efficiency, tuning state, and cycle type (2-stroke vs 4-stroke). Mechanics and hobbyists use this estimation tool to benchmark engine performance or identify replacement parts when specifications are missing. A standard lawnmower engine produces power differently than a high-revving chainsaw or a turbocharged car engine. This tool uses statistical averages from industry data to provide a realistic displacement range rather than a single theoretical number. Accuracy relies on selecting the correct specific engine configuration.

Formulas

The estimation relies on the Specific Output Ratio (R), defined as cubic centimeters required to generate one unit of horsepower.

{

cc ≈ HP × R_mincc ≈ HP × R_max

Where R varies significantly by engine architecture. For example, a racing 2-stroke might have R ≈ 8, while a generator might have R ≈ 40.

Reference Data

Engine Category	Specific Output (cc per HP)	Efficiency Note
High-Perf 2-Stroke (Racing)	6 - 10	Extremely high volumetric efficiency
Modern Sportbike (4-Stroke)	10 - 15	High RPM, advanced timing
Chainsaws & Cutters	15 - 20	tuned for torque at speed
Standard Car (Naturally Aspirated)	13 - 18	Reliability focused
Turbocharged Diesel	15 - 25	High torque, lower HP/cc ratio
Lawnmower (OHV)	25 - 30	Standard consumer grade
Lawnmower (Side Valve)	30 - 35	Older technology, low efficiency
Portable Generator (Low Tune)	35 - 45	Designed for steady low RPM
Industrial Stationary Engine	40 - 50	Maximum durability focus
Marine Diesel (Large)	50 - 80	Very high torque, low RPM

Frequently Asked Questions

Displacement is static volume, while horsepower is a function of torque and RPM. Two engines with 200cc can produce vastly different power figures if one is a low-speed generator and the other is a high-speed racing kart engine. The conversion requires context about the engine's design efficiency.

For naturally aspirated cars, the estimation is generally within 10-15%. However, for turbocharged or supercharged vehicles, the correlation breaks down because forced induction artificially increases the effective displacement (air intake) without changing the physical cylinder size.

No. Electric motors are rated in kW or HP but do not have displacement (cc). Converting electric power to internal combustion equivalents requires a completely different physics model involving torque curves.

Most residential 4-stroke lawnmowers and pressure washers fall into the 25cc to 30cc per HP range. Older side-valve engines are less efficient and may require 35cc to produce 1 HP.