CC-rating of a car is a useless quantity
Posted Sunday, July 1 2012 at 01:00
Kepha Bukindo asks three seemingly straightforward questions about cars: first, “what is ‘cc’ as used in 2000cc, 4000cc, 5000cc, etc, and which is the formula of finding it?”
Second, “what is the meaning of WHP and BHP? And finally, “what is the formula of finding horsepower?”
The answer to the first question is that “cc” stands for Cubic Centimetres or Centimetres Cubed.
This is the volume of one cylinder of the engine; that is, the combustion chamber where the fuel is burnt to produce motion energy.
The formula for calculating this volume is the cross-sectional area of the cylinder multiplied by the distance over which the piston runs during its up-and-down movements.
This explains why the figure is normally referred to as “displacement”: it is the volume displaced by the piston.
The only way to determine it independently is to dismantle the engine and take the necessary measurements.
If you are not a highly skilled mechanic, this is not something I would recommend! Therefore, you have to rely on the figures given by manufacturers.
Going to the second question; the letters HP stand for “Horse Power”. As explained in an earlier article, this is the rate at which the engine produces energy. One HP is equivalent to the working rate of a workhorse; not a racehorse!
BHP stands for “Brake Horse Power”. It’s the power generated by the engine when it is standing alone; not connected to anything (no gears, no differentials, no drive shafts etc).
WHP on the other hand is the “Wheel Horse Power”; that is, the power available at the wheels. Thus WHP takes into account the energy losses suffered along the transmission system of the car.
Now Kepha asks for the formula for calculating these HP figures. I assume he means in terms of the engines displacement (cc). The answer to that is No; there is no formula. Given the cc-rating of a car, it is impossible to calculate its power out.
The reason is that there are too many other variables that affect the power; for example, the number of cylinders (is it one or two or three or four or six or what?), how the engine sucks in air (naturally or via turbo), how it takes in fuel (carburettor or electronic injection), the cylinder’s aspect ratio (is it wide and short or narrow and long?) and so many others.
The power can only be measured experimentally in an engineering laboratory. Other than that, we have to rely on the manufacturer’s figures. But even these are for the prototype when new!
Nevertheless, the general rule of thumb is that the bigger the displacement the more powerful the engine … and the more fuel it consumes.
However, this assumption is not often wrong, very wrong: small engines can produce much more power than larger ones and also consume more.
The Mazda RX-8 sports car has a 1,300cc engine that produces more power than a 4,000cc conventional engine … and also consumes more fuel!
In a nutshell: the cc-rating of a car engine is a completely meaningless (and useless) quantity!