# 4WD: Assess torque and power needs

RAY CULLY, 4WD, The West Australian Updated November 11, 2012, 10:48 am

Torque is king when pulling heavy loads across challenging tracks. Picture: GBTW)

When considering a new four-wheel-drive, what should you be looking for? More Newton metres of torque or more kilowatts of power?

To keep it simple we'll avoid detailed technical data.

Torque is turning force, which is described in Newton metres (Nm). Newtons are a unit of force and at the surface of the earth a 1kg object will exert a force on the ground of 9.8N, due to gravity.

This provides the boffins in white coats a measurement for determining how much "work" your 4WD can do.

The amount of torque generated is equal to the force applied multiplied by the length of lever used. In its simple form: Torque = Force x Distance.

On the other hand, kilowatts (kW) is a measure of power that indicates how "fast" your 4WD performs the work.

How much power an engine produces is determined by the torque multiplied by the engine revolutions per minute (rpm) divided by 9549 (a constant number worked out by efficient scientists with large clipboards).

To demonstrate the application of torque versus power let's look at two very different vehicles.

The marketing spiel for an Aussie performance car always includes the kW numbers as its representation of performance. With a 6.3-litre V8 Holden GTS producing 325kW @ 6000rpm, it's bound to be quick.

So why not use such a car to pull heavy loads up hills? Surely it would remove the frustration of being behind a slow-moving truck because the GTS produces 550Nm of torque at 4600rpm.

Now it gets interesting.

Remember I said "speed" of work versus "amount" of work.

Our high-performance thoroughbred produces high torque at high rpm, which equates to a lot of power - ie. the rate of work is "fast".

Let's look at a high-spec 4WD. The Range Rover has a smaller 4.4-litre diesel V8 that produces a more modest 230kW at 4000rpm, but a barn-flattening 700Nm of torque from 1500rpm, which is perfect for heavy haulage.

But the Rangie is still no slouch, punching from 0-100km/h, and I've seen a GTS pulling a hefty car- trailer with a racing outfit.

Both these impressively powerful vehicles will perform and execute a large range of tasks but their core strengths are different, highlighting the impact of the two measurements.

Once you have an idea of how these measurements influence the driving characteristics of the vehicle you can assess your style of driving. What percentage of the vehicle's driving will involve tugging a heavy load like the 6m fat-bottomed princess of a caravan, camper-trailer or horse float?

Will the 4WD be more the daily transport for regular family duties, spending most of its time negotiating traffic on the blacktop with the annual camping trip away a few weeks of the year?

Keep this in mind when listening to the polished sales pitch to make your own decision on what balance of power and torque will best suit your intended usage of the vehicle.

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