Understanding the power output of an engine compared to its physical size can be confusing. Whether you are looking at a lawnmower, a motorcycle, or a small go-kart engine, you might find yourself needing to convert horsepower (HP) to cubic centimeters (CC). Use our simple tool below to get an estimate.
How to Convert Horsepower to CC
The first thing to understand is that there is no direct mathematical formula to convert Horsepower to CC. This is because Horsepower measures the work an engine can do (power), while CC (Cubic Centimeters) measures the volume of the engine's cylinders (displacement).
However, for most small, four-stroke gas engines (like those found in lawn equipment), enthusiasts and mechanics use a general rule of thumb:
- 1 Horsepower = 15 to 17 CCs
- 15 CCs = Approximately 1 Horsepower
Why isn't there an exact formula?
Two engines can have the exact same displacement (CC) but vastly different horsepower outputs. This discrepancy is caused by several factors:
- Engine Design: Overhead valve (OHV) engines are generally more efficient than older side-valve designs.
- RPM (Revolutions Per Minute): An engine that spins faster can generally produce more horsepower from the same amount of displacement.
- Fuel Injection vs. Carburetion: Modern fuel systems optimize the combustion process, extracting more power.
- Two-Stroke vs. Four-Stroke: Generally, a two-stroke engine produces more power per CC than a four-stroke engine because it fires once every revolution instead of every other revolution.
Common Conversion Reference Table
For quick reference, here is a table based on the standard 15cc per 1hp ratio commonly used for small engines:
| Horsepower (HP) | Displacement (CC) |
|---|---|
| 1 HP | 15 - 17 CC |
| 3.5 HP | 125 CC |
| 4.5 HP | 150 CC |
| 5.0 HP | 160 CC |
| 6.5 HP | 196 CC |
| 10.0 HP | 300 CC |
| 13.0 HP | 390 CC |
The Difference Between Torque and Horsepower
When looking at engine specs, you will often see "Torque" listed alongside CC and HP. While CC tells you the size and HP tells you the power at high speeds, Torque tells you the "grunt" or rotational force the engine provides. In applications like lawnmowers or tillers, torque is often a more important measurement than raw horsepower because it determines how well the machine handles heavy loads (like thick grass).
Summary
While the 15-17cc per HP rule is a great starting point, always check the manufacturer's specifications for the most accurate data. If you are buying parts or comparing engines, displacement is a fixed physical size, whereas horsepower is a dynamic measurement of performance.