How many times have you heard someone say, that car has such-and-such horsepower, and wondered what that actually means? One has to wonder what kind of horse is used as a benchmark and what that means when you step on the gas pedal. Although they are just about the only means for measuring engine power, horsepower and torque as they are currently measured, are actually very poor standards for comparing vehicles and their potential performance.
Horsepower is derived from a complex formula, but basically it is an amount of work over time. As it relates to cars, horsepower is most relevant to top end speed, not necessarily acceleration.
If you are using only one figure to compare vehicles, look at the torque rating, as it is the twisting force an engine can produce, and essentially how powerful the engine is.
Examine horsepower and torque ratings on high end sports cars, and you will find that they are also matched by impressive torque ratings. Extremely heavy duty vehicles that require a lot of power and not necessarily high top speeds, such as pickups designed for towing (especially those with diesel engines), tend to have torque figures that dwarf their horsepower rating. Muscle cars and diesel engines, which are known for brute power, tend to have similar horsepower and torque ratings, with many of the most powerful models producing around 300 horsepower and 500 pounds of torque.
However, horsepower and torque are not the only factors that determine a car's performance. Looking at only horsepower and torque, two vehicles may be similar, but one may be a muscle car and the other is a dump truck. Dividing horsepower or torque by vehicle weight will give you power to weight ratios, which has a big impact on acceleration and performance.
Even using power to weight ratios ignores one basic fact about how power ratings are obtained. Instead of measuring how much horsepower or torque gets to the wheels, these figures are measured on the back of the engine, which does not take into account how much energy is lost in the transmission. This discrepancy is magnified when comparing front wheel, rear wheel, and all wheel drive vehicles since each configuration requires a different amount of power to turn the wheels.
In addition, the gearing of the transmission determines how an engine's power is used. The same engine can be paired with different transmissions to produce faster acceleration or pulling power by varying the gear ratios. Because of this, a properly geared vehicle could out perform another with higher horsepower and torque ratings.
As you can see, the way horsepower and torque are measured provides numbers that do not necessarily reveal how a car will behave when you step on the gas. These figures would be much more accurate and helpful to consumers if they were obtained by measuring the power actually delivered to the wheels, and also using these figures to place power to weight ratios on the window sticker.
As you can see, the way horsepower and torque are measured provides numbers that do not necessarily reveal how a car will behave when you step on the gas.