Power and Torque Curves

Power - we all want more power, it makes a car go fast! But what do we really want when we say we want more power? How can we compare 2 cars and see which has the better, more powerful engine?

When we look at a GT car one of the first things the game shows us is its power. This value is the peak power of the car. That is, the maximum power it can generate at any stage. But there is a lot more to power and the engines capabilities than just peak power. Here you will learn a bit more about power and torque. This will help you to understand your car and learn how to maximise its performance.

Power

Power is a measure of how quickly your car can use energy. That energy is what makes the car go - so power = go!

So we know the peak power of the car, but what does that tell us. It tells us that is the best the car will do. It doesn't tell us when it goes that well, or how well it goes at other times.

The power of the engine is related to :

the throttle position
the revs (rpm)

Naturally more throttle means more power. So when examining the power of a car it is assumed that the throttle is on full - often called "wide open throttle (WOT)". So we can ignore the throttle and just look at the relationship between power and revs.

Engines generally make more power as the revs increase, until some limit when it starts to drop off. So we can assume that more revs is good, which is handy since for any gear revs increase as speed increases.

Why does more revs mean more power?
To answer that we need to look at torque...

Torque

Torque is like force (technically it is a twisting or turning force.)
Thus torque is what pushes the car.

So why is that different to power? Well power is not the push, but how quickly that push is being done. An example may help...

You are lifting some dumbbells - the amount you can lift is a measure of the torque of your arm. How quickly you can do it is then a measure of the power. You can get the same power by lifting less weight quicker or more weight but slower.

So power = torque x revs (which is the speed of the engine)

So if torque stays the same as revs increase, the power will increase. In fact, even if torque decreases, power can still increase, as long as the torque decreases less than the increase in revs!

Now we are ready to have a look at torque and power curves.

The Curves

Since an engine has to work across a large range of revs, it is important to know the torque and power values at all revs, not just the maximum values. The easiest way to show these is on a curve. Often both values are graphed together - this is how GT displays them in several of the tuning menus.

We will consider torque first, because it is really the defining item. Power is usually considered to be derived from torque.

It's a fact of how an engine works that at both low and high revs that torque will be low. Ideally through most of the rev range of the engine torque will be large, dropping slowly at high revs.

The shape of the torque curve depends on the engine, turbo and normally aspirated engines tend to have quite different curves.

A typical normally aspirated torque curve has high torque across a wide range of revs.
It looks like...
Graph to be done

Look at the Viper or Cerbera for examples.

The power curve would then look like:

A turbo engine tends to have torque increase with revs (due to the turbo effect at high revs).
This looks like...
Graph to be done

Look at a Supra or GTR with a stage 4 turbo for examples.

The power curve would then look like:

Describing These Curves

There are many ways of describing curves, but there are some common terms used when describing torque and power curves.

Torque curves can be:

fat - indicates it has high torque over a large rev range.
flat - another good sign, torque that jumps up and down is not as desirable as a constant amount of torque.

Power curves:

peaky - high power is all in a small rev range, usually at high revs.
narrow - another term for peaky.
flat spots - not good! A flat curve is good, but flat spots are bad. These are bits were the power doesn't increase with revs, hence the curve is flat. These are actually dips in the torque curve. So flat torque is good, but flat power is not. This can be felt when driving when the acceleration decreases in the flat spot and increases again past the flat spot. The Nismo GTR has a flat spot at around 4000rpm.

Powerband

Powerband is the rev region in which the engine is producing most of its power. There is no strict rule, but roughly it's the rev range in which the engine produces 75% or more of its peak power.

It also tends to correspond to the high, flat region of the torque curve.

Ideally when driving the car you want to use as much power as possible. So that means keeping the engine in the powerband. This sounds easy, but as the car accelerates, the engine's revs change and the car can move out of the powerband. No problem, change gear. But at what revs does the new gear put us? Hopefully in or near the start of the powerband!

If the next gear lands well into the powerband, then we have wasted useful revs. If it is too far below the powerband, then we will not be using the best part of the engine's power. So we can see that gears have a big influence on this. But there is only a fixed number of gears. All things being equal, it would be better to have a bigger powerband!

More gears can help you get the most out of a peaky engine, by allowing the engine to stay in the powerband. The peakier the engine, the more important it is to get the gear ratios right!

Typically turbo engines are a lot more peaky than normally aspirated.

Check the power curve on the CRX LM (small turbo engine) against that of the Cerbera LM (large na engine).
Although both produce about the same power (around 570HP) the Cerbera has a much larger powerband and hence is a much more usable engine. This reflects in how easy they are to drive. With typical gearing, the Cerbera will always be in the powerband and will feel strong. The CRX will often drop out of the powerband and will hesitate and feel weak at times.

Without the Curves

Sometimes you don't have the power and torque curves available, such as when you are in the dealership. The car's specs are available, with the maximum power and torque, but you can't see the powercurve without buying the car. Now you want to get an idea of the powerband of the engine. How can you get around this and make a guess at its powerband?

Well there is some info that is given in the cars specs (available in both the garage and the showroom floor). That is the maximum power and torque figures, and more importantly the revs at which these maximums will happen.

The quick estimate is to see how close in revs these two maximums happen. Note, the power maximum will never be less than the torque (why? remember power=torque x revs).

Get the difference between the max power revs and the max torque revs. The larger this number, the larger the powerband!
It's that simple.

There are times when this can be wrong. This is most often when it hints at a narrow powerband, but it is actually quite wide. This is because the torque curve is wide and flat, but just happens to peak at high revs.

However, the reverse won't happen. It will never lie about a good powerband. If it shows a wide powerband, then it must be a wide powerband.

Here is a quick example

power torque (metric!)
CRX LM 563@ 8800 47.7@ 8800 diff=0
Cerbera LM 581@ 7500 61.1@ 5500 diff=2000rpm

This shows that the Cerbera has a powerband that is at least from 5500 to 7500, probably even wider. During this rev range the engine has plenty of torque and will pull strongly. This car will have a strong powerband of probably 3000 or more revs.

The CRX has max power and torque at the same revs (and very high revs.) This indicates an engine that probably only has a small usable rev range (could be less than 1000rpm) and that is very high.

Check it yourself

Have a look at a few cars. Look at their max power and torque figures and check the rpms for both. When you tune the car up, check the power curves.

Then drive these cars and see how they perform across their rev range. The "fatter" engines will pull strongly from lower revs, giving a strong but constant feel to the acceleration.

The "peaky" engines will feel weak, slowly accelerating, until the revs build, then the engine will quickly become stronger and accelerate much harder. If the gears are set badly, the engine will go weak again after changing gear.

Play around with gear settings (see Setting Gear Ratios) to get a feel for how to use your knowledge of power and torque to improve your car.

Terms

Torque - turning force, what pushes or accelerates the car.

Power - torque (or force) at revs, it gives the car speed.

Power Curve - a graph of power against revs. Tends to increase with revs.

Torque Curve - a graph of torque against revs. Should be both fat and flat!

Powerband - range of revs for which the torque is high, hence the power is good.

Fat Powerband - a powerband over a large range of revs. GOOD!

Narrow or Peaky Powerband - a powerband over a small range of revs. BAD!

Out of the powerband - when the engine revs are outside the powerband (usually too low revs)

Gears - things to keep the engine in its powerband across a range of speeds.