Torque is a rotational force, measured at the crankshaft. It represents how much power the engine can produce. Horsepower is a measurement of how quickly an engine can work, and is based, as the name suggests, on the amount of work a horse can do. A car’s horsepower is calculated from its torque and RPM.
The motor trade and auto magazines tend to put a lot of weight on the importance of torque and horsepower, with more always being supposedly better, you generally see the maximum figures quoted on new vehicles ready for you to test drive on trade plates.
So, what do the figures really mean?
In practical terms, high torque allows a vehicle to accelerate quickly from a standstill and move heavy loads, like a trailer. High horsepower increases top speed and means faster acceleration once the vehicle is already moving.
Diesel engines are popularly perceived as sluggish, and less fun to drive than petrol cars. Several factors contribute to the way diesel cars perform. One is that diesel engines are heavier, meaning that more power is required simply to get moving, petrol engines revolve faster, producing higher maximum revs.
There’s also a difference between the two fuel types that affects performance. Petrol releases energy more quickly whilst diesel produces more power per litre, giving a petrol-powered vehicle the advantage when it comes to acceleration yet a disadvantage when it comes to MPG.
The result is that diesel vehicles have a narrow power band of peak performance, while petrol engines spread the power over a wider band of revs. The lower horsepower and higher torque of a diesel car, usually driven at lower revs than a petrol motor, gives it more grunt but less speed. Just think of them as the tortoise and the hare!
The power output of an electric car can be measured in kilowatts but is often given in horsepower to provide a useful comparison with petrol and diesel engines. The nature of an electric motor means that it is different again in terms of performance.
In a conventional petrol or diesel engine torque and horsepower build to a maximum in a particular rev band. An electric motor’s full power is available as soon as it is switched on, giving it the maximum torque instantly and throughout the journey. This gives electric cars their impressively speedy acceleration from a standstill but notably lower MPG, this is due to the battery capacities still being developed.
Horsepower figures tend to be lower for electric cars, but they make good use of the power they have, needing less horsepower to reach their top speed quickly.
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Electric vehicles are often smaller and lighter than their petrol and diesel counterparts. This can allow an electric car to beat even a high-end sports car off the line, although it may not stay in the lead for very long!
For drivers making a purchasing decision on new and used cars, horsepower and torque are just two of the many factors to take into consideration. Other elements of a decision include how economical a vehicle is to run, how much an insurance policy will cost, and what it will be used for.