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Knowing What You’re Torquing About

If you’ve ever taken even the slightest look at any car review, some of the key bits that tend to be described and compared are the engine stats and specs.  Some specs are easy enough to understand – the 0–100 km/h sprint is a measurement of how long it takes the car to go from not travelling at all to going at full open road speed.  It’s a measurement that is easy enough to picture, and it’s easy enough to convert to and from metric units, if you’re not too fussy. If you want to get absolutely precise, 100 km/h is equivalent to 62.14 mph, so if you are presented with an Imperial measurement looking at the 0–60 mph sprint, it’s roughly the same as the usual nought-to-the-ton metric figure.  Get out your calculator if you want to be absolutely precise. 

However, some specs are a bit harder to get a mental picture of.  The key figure is power. Power is defined as the rate at which work is done, or else the rate at which energy is converted into motion.  In cars and vehicles of all types, the formula for calculating power is a little more complicated and the power output (measured in kW (kilowatts) using metrics) is the force times the velocity (physics-speak for speed!).  If you want a formula, it’s Power = work/time = (force x distance)/time. Force, of course, is derived from the mass, so small cars (and motorbikes) tend to have engines with fewer kilowatts of power – you don’t need as many kW to get up to speed.  All sorts of things go into the power, such as the number of cams (pistons) in the engine, the number of combustion chambers in the engine and the size of the combustion chambers.  Power used to be measured in horsepower, which was originally used to compare how well a steam engine or traction engine could work in comparison to a big Clydesdale.  And here’s some handy little figures so you can compare that European car that measures the engine power in kW against an American car that’s measured in horses:

  • 1 kW equals 1.34 hp
  • 1 kW equals 737.56 foot pounds per second
  • 1 hp = 550 foot-pounds per second (ft lb/s)
  • 1 hp = 0.75 kW

Power is closely related to torque.  Torque takes into account the fact that although power is produced in a straight line inside the combustion chamber(s), it is applied around an axis – the whole set-up with transmissions and drivetrains involve rotational motion (and is then converted back to straight-line motion when the rubber applies a force to the road). It’s measured in Newton-metres (Nm or, if you want to be really picky, N.m). 

Now, we all know that engines run at different speeds, depending on how far you press down the accelerator.  No engine produces the same amount of power or torque at all speeds.  You often see a graph that shows the level of torque produced by the engine at various engine speeds (measured in rpm (revolutions per minute)). The place where the graph reaches its highest point is the peak torque – and this helps you know where the engine should be running to get the maximum oomph to the wheels when you need it to accelerate or pull things.  Power curves exist, but the engines torque is what really counts when the old girl is fully laden with the family and towing the caravan to your camp site.  The idea of gears is to make sure that the engine purrs over at the right speed – the right number of revs or rpm – so the engine gives the right amount of power and/or torque for what you need. 

Newton-metres are the SI (metric) units used for torque.  The non-metric unit used for American cars is in pound-force feet, also known as pound-feet. Just to be confusing, pound-feet are used for torque, while foot-pounds are used for power, something that’s likely to drive you half dotty. The formula for converting is 1 pound-foot = 1.356 Nm.