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What is Torque in a car?

"How do you measure Torque?"

Torque is a measure of power. It explains how much push a car gives or the turning power of the engine.

If you had  a lever attached to the center of a wheel the Torque is the amount of force you could use to pull on that handle to rotate the wheel. 

Torque is an angular force (although through an infinitely short arc it can be considered to be linear) but is still acting about a center of rotation.

Torque in a car is measured in nm (newton meters) or ft-lb (pounds per foot) - NB people quote torque as lbft, ftlb, lb-ft and ft-lb but these can be regarded as one and the same. All terms will interchange quite safely lbf likewise is fine, although possibly might be mistaken for pounds force which is a linear force.

What is the difference between BHP and Torque?

BHP shows how quickly you hit a wall and torque indicates how far through it you get.

Do not confuse Torque with BHP as they are different although both related to the power of a car. So what is Torque then and how does it compare with horsepower or BHP?

Horsepower = Torque x rpm/5252
Torque = Horsepower x 5252/rpm

Any Power/Torque graph you see will cross at 5252 provided you measure engine speed in revs per minute, torque in lbft and power in BHP. (*see below for an explanation of this.)

What does Torque tell me about an engine

In an engine the quoted Torque figure is just the peak figure the engine can attain. This is only a small part of the story.

220ft-lb at 8000rpm is the turning force that can be exerted at 8000rpm.

Let's look at a complete Dyno graph to get an idea of this measure of a cars power.

dyno-plot

You'll note the torque peaks at 4000rpm and then tails off. So the engine is pulling the hardest at this point. In some cars the power builds gradually, TorqueCars prefer a nice flat torque band which covers most of the RPM range, which means you have plenty of in gear acceleration.

As the torque tails off around 5500 to 6000 you should really change gear here to drop down the RPM range and hit the higher Torque Band.

Why does Torque and BHP cross at 5252 on the dyno graph?

To begin, we should start with gentleman named James Watt. He is the one who developed the entire idea of a "horsepower" and of course, the SI unit of power, the watt.

Watt suggested that per second the average horse worked at a rate 550 foot lbs, or 33,000 foot lbs per minute. Imagine a horse pulling a weight up a vertical mine shaft. This means that on average, a horse could move 33,000 lbs 1 foot distance in 1 minute. So 1 hp=33,000 foot lbs per minute.

Now, since we're talking about engines, work is being applied to a rotating crankshaft. This means that we need to talk about torque. But what does a ft-lb of torque mean? Well, it means that we have a 1 lb weight attached to a weightless bar 1 foot from the fulcrum.

Now imagine that bar is rotated a full turn 360 degrees. The distance traveled is the circumference of the circle, which is 2*radius*pi, which equals 6.28319 ft. So one rotation is 6.28319 ft in distance and we know that 6.28319 ft-lbs of work (torque) was done.

Watt figured out that 1 hp=33,000 ft-lbs of work per minute. If we divide 33,000 by 6.28319, we arrive at 5,252.1. This means that for every 5,252 revolutions per minute of 1 ft-lb of torque, we get 1 hp.

The 5252 mentioned is unitless, simply forming part of the calculation for determining power from torque and revs on a dyno graph it happens to be RPM.

To work out either HP or Torque :-
Horsepower = Torque x rpm/5252
Torque = Horsepower x 5252/rpm

So a Dyno measures Torque & then uses the equation to calculate horsepower. When you look at a torque graph you get a feel for where the power range of an engine is and can work out the optimum gear shift points.

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