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Reader help: Why don’t we see torque/weight figures?

A reader wonders about torque, power and weight and why car companies always talk about kilowatts per kilogram…

DOES MEASURING a car’s weight per lb-ft or Nm of engine torque serve any useful purpose? Is it a valid measurement like lbs per HP / kg per kW?

Since weight:HP tells you the load each unit of power has to carry, I’d think knowing how much load each unit of torque carries would be useful too. But you never see weight:torque specs, I guess because torque is just a force that twists the drive wheels, so the weight of the car doesn’t matter? Thanks to enlighten me, and sorry for using HP, I suppose in Australia it’s kW.

Thanks for your question,

Humans like simple, easy measures even if the measures do not properly reflect the value of what they’re meant to be measuring. A while back it was all about megapixels in cameras. Before that it was Mhz in computer CPUs. And for a long time it’s been power and torque figures for cars, particularly performance cars and 4X4s.

The problem is that all these measures are too simplistic. The quality of a camera’s photo is not defined by megapixels; you can have a lot of megapixels but a poor lens and camera processing unit will deliver a poor picture. And for cars the power and torque figures are also misleading.  Peak figures often mean less than the spread of torque and power over the rev range, and there’s gear ratios too.

The mass of a car does indeed make very big difference to all aspects of its performance, and its torque relative to mass is also important as evidenced by the famous equation F=MA, or force = mass x acceleration. Reworked, that’s A=F/m, or acceleration = force / mass.  

In other words, the heavier a car is the slower it accelerates for a given force.  And yes, quoting a torque/mass figure would be more meaningful than a torque figure alone, and I do calculate and quote them in some reviews where appropriate. The better the torque/mass ratio, the quicker the car will accelerate…all else being equal such as gear ratios, torque curve and so on.

You see how this gets complicated now, and that’s why it’s best to just look at the end result figures. Want to know how quick the car accelerates? Look at the 0-100km/h times, although those have been ‘gamed’ a bit by some manufacturers.

There’s more at these links, including explanations of power and torque:

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Robert Pepper

Robert Pepper