Engine cc, litres and horsepower: what the numbers mean

A 1.5-litre car, a 150 cc scooter, a 350 cubic-inch V8: engine sizes are quoted in different units around the world, and people often assume a bigger number means more power. Size and power are related, but they are not the same thing.

What engine size measures

Engine size, or displacement, is the total volume the pistons sweep inside the cylinders. It is a measure of how much air and fuel the engine can draw in each cycle — not of how much power it produces.

Converting size: exact

• 1 litre = 1,000 cc (cubic centimetres).
• 1 cubic inch = 16.387064 cc exactly.

So a 1,998 cc engine is 2.0 litres or about 121.9 cubic inches, and a 5.0-litre engine is about 305 cubic inches. Manufacturers round: most engines sold as 2.0 litres are between about 1,950 and 1,999 cc. The Engine CC to HP Converter converts displacement exactly.

Horsepower: only an estimate

Power depends on much more than size: whether the engine is turbocharged, how fast it revs, how it is tuned and what it is designed for. Two engines of the same size can differ several times over. What can be estimated is a realistic range from typical power per litre for each kind of engine:

• small scooter and commuter motorcycle engines: about 60–95 hp per litre;
• petrol car engines without a turbo: about 70–110;
• turbocharged petrol car engines: about 100–160;
• turbodiesel car engines: about 55–100;
• high-revving sport motorcycles: about 150–220;
• large truck diesels: about 25–40.

A 1,500 cc turbocharged petrol car engine therefore lands somewhere around 150–240 hp, typically about 180. A 150 cc commuter scooter makes about 9–14 hp. For a real vehicle, the manufacturer's rated figure is always the one to use.

hp, bhp, PS and kW

• hp and bhp usually both mean mechanical horsepower, 745.7 watts. bhp stresses that it was measured at the crankshaft.
• PS is metric horsepower, 735.5 watts — about 1.4% smaller, so the same engine shows a slightly bigger number in PS. It appears on many European and Japanese specification sheets.
• kW is the international unit. Multiply kW by about 1.341 for hp: 100 kW is about 134 hp, or 136 PS.

Why bigger is not always stronger

Modern small turbocharged engines often match the power of older, larger engines, because the turbo forces in more air. Sport motorcycle engines make enormous power for their size by revving very high. Truck diesels are huge and produce modest power for their size, but great pulling force at low revs. Size tells you something about an engine; it never tells you everything.

Electric cars

Electric motors have no displacement, so their power is quoted directly in kW or hp. Their running costs depend on energy use rather than engine size — see how much it costs to charge an electric car.