Despre PUTERE

Highly regarded technical experts have often been heard to say that power is not important and 'it is torque that counts'. Somewhat controversially Stage-Rally is going to disagree with this viewpoint and offer one of its own.

Torque and Power are related by a constant: Torque(lbft)=Power(BHP)*5251.4/RPM (equation 1) thus if you have torque then by definition you have power.

The area where Stage-Rally and the experts agree is that Peak Horsepower is basically meaningless, this is a commodity which can win pub arguments and on-paper marketing battles but when it comes to the open road peak horsepower is as much use as the proverbial chocolate fireguard. A 'spread' of torque is what makes a car driveable and this is the basis of the experts' hypothesis. Driveability is as important as any other factor when it comes to getting from A to B quickly as it is this spread of torque which allows the driver to avoid unnecessary and time-consuming gear changes.

Where Stage-Rally's view departs from conventional wisdom is that power is far more important than torque. Why? - because of the gearbox. This is because power and torque figures are normally considered primarily at the flywheel whereas what actually motivates a car to go forwards is the torque at the wheels.

The effect of the gearbox is shown by two rules:

1. Velocity Ratio (V.R.) and

2. Mechanical Advantage (M.A.).

M.A. is the inverse of V.R. thus the lower a gearbox's V.R. the better its M.A. For example if a gearbox has a ratio of 3.5:1 the engine speed will be slowed down 3.5 times (divided by 3.5) but the torque will be multiplied by 3.5. By the time the torque is fed through the crown-wheel and pinion (CWP), with a ratio of say 4:1 the flywheel's torque is multiplied 14 times and the drive shaft's rotational speed is one fourteenth of the flywheel rpm.

The above is the main reason why a car accelerates much better in the lower gears (e.g. typical 1st gear ratio 3.7:1) than in the higher gears (e.g. typical 4th gear ratio 1:1) and obviously why the speeds attainable in the lower gears are so much less than the high - but it is also the reason why in many production cars maximum speed is attainable in fourth gear rather than fifth (which is often an overdrive - typically around 0.85:1).

This can further be expanded by rearranging equation 1. Power(BHP)=Torque(lbft)*RPM/5251.4. As you can see if torque remains constant but RPM increases, the BHP will increase.

To explain the application of this principle here is another example:

A car is producing 150 lbft of torque at the flywheel and is doing 70 MPH, it has 205/60R13 tyres (which have a rolling circumference of 181cm) - therefore the driveshafts are rotating at 1037rpm (There are 1037*60*1.81/70 metres in a mile!). If this torque is produced at 5500rpm (overall gear ratio=5.3) the torque at the wheels will be 795lbft (150*5.3). If however the engine is modified to produce the same torque at 6500rpm and the gearbox is modified to maintain the same drive shaft speed (an overall ratio=6.27) the torque at the wheels will be 940lbft (150*6.27) and the car will have a higher rate of acceleration. Torque at the flywheel has not been increased but by increasing the rpm at which it occurs, more power is produced and hence more torque at the wheels.

This is what the engine tuner constantly seeks to achieve when modifying a car, by porting the cylinder head, increasing valve sizes and installing a bigger set of carburettors or throttle bodies the volumetric efficiency of an engine can be increased and hence the ability of an engine to rev. higher and produce more power - even if the torque still remains the same. Once the efficiency of the engine has been improved the cams will ideally be changed to optimise the engine's performance at higher rpm. This tuning will have a more significant benefit if the gearbox and/or diff.ratio are then altered to take advantage of the higher rpm. This is known as 'Powertrain Matching'.

The latter statement does not hold true if the purpose of tuning is to increase the car's ultimate performance - i.e. make it go at a higher maximum velocity. For rallying an increase in maximum speed is not usually the objective, in fact, a rally car's ideal top speed is usually around 110-120MPH - far lower than a performance road car, a lower set of overall gears is therefore the vital ingredient in optimising a car for rallying.

Various other factors need to be considered when optimising the powertrain - such as aerodynamic effects, rolling resistance and frictional losses (in the transmission) but the overall key is appropriate gearing and the right sort of power delivery.

For most rally cars (unlike race cars) the engineer is usually faced with a limited selection of ratios for the gearbox, a slightly wider selection for the back axle and a reasonably large selection of tyre sizes. For this reason when tuning the engine it is necessary to aim for a power curve which makes best use of the available gear ratios, ignoring this and going for maximum peak power (or torque) can lead to a car which becomes slower after it is modified.

How To Use The Desktop Dyno

The linked file (which is an Excel spreadsheet and requires Microsoft Excel '95 or later to run) is a 'desktop simulation' of a car's dynamic performance. It is very simple to use and allows the rally car engineer to adjust certain factors such as gear ratio, differential ratio, tyre size, vehicle weight, and torque (power) curve. It also allows the user to experiment with different gear change points in order to maximise acceleration - plus a maximum rev. limit as in first gear the engine will want to rev. as high as it can - this is not always practical when considering engine life. Get The Desktop Dyno (261KB)

The input sheet provides spaces for all vital information to be entered, by default the torque curve included is from a mildly tuned Astra 2.0l 16valve engine on Twin 45 Webers, the gear ratios are from a Getrag (Manta) gearbox and the diff. is a standard Manta ratio. tyre sizes are entered in conventional format (e.g. 205 60 13) and from this the rolling radius is calculated. The weight must be entered (including driver and co-driver if necessary) - remember it is no use calculating how fast a car will accelerate with no crew on board! Transmission efficiency is set at 85% which is a fairly typical value (fiddle factor), frontal area is 1.7 metres squared (about right for an Escort-sized saloon) and Cd (drag coefficient) is 0.4 - perhaps a little pessimistic but rally cars are not particularly aerodynamic especially with front air dams and rear spoilers - which aim to generate downforce/reduce lift, not reduce wind resistance.

The input sheet includes a 'quick calculator' which will convert a bhp figure at a particular engine speed into a torque figure. This may be of use if you have a dyno sheet for your car which gives bhp rather than torque.

By clicking on the tab marked dynamic forces at the bottom of the screen two graphs are displayed. The first shows torque at the wheels as would be experienced under dyno conditions - with no aerodynamic effects. The second shows force at the wheels (force=Torque(Nm) divided by rolling radius of wheel in metres) less aerodynamic forces. This second graph is particularly interesting as it shows that at certain speeds the aerodynamic forces acting on the car are greater than the force at the wheels.

There are three states a car can be in whilst travelling in a straight line: (n.b. rolling resistance has been factored in with transmission losses)

1. a car will accelerate when the force at the wheels is higher than the aerodynamic forces.

2. a car will remain at a constant speed when the force at the wheels is the same as the aerodynamic forces.

3. a car will decelerate when the force at the wheels is less than the aerodynamic forces.

Looking at the graph we can see that if the curve is above zero the car is accelerating, at zero the car is 'cruising' and below zero the condition (speed) is unattainable.

When changing gear if you change too early the torque at the wheels may be less than it was in the lower gear whereas if you change gear too late acceleration may be hampered as more torque at the wheels would have been available in the higher gear. The ideal time to change gear is when the two lines on the graph cross - hence the same amount of torque is available in each gear, acceleration will remain constant at the gear change point but whilst the available torque from the lower gear diminishes, the torque at the wheels in the higher gear will increase.

Another departure from conventional wisdom is introduced at this point. As the car goes faster, aerodynamic effects make it necessary to change gear at a lower engine speed, hence Stage-Rally questions the value of a shift light as a performance aid. The value of a shift light is the same as that of a rev-limiter - to protect the engine.

By choosing the optimum gear change points from the dynamic forces graph we can enter these in the input sheet and hence calculate 0-60 times, vmax and time taken to achieve vmax. Examples are already included, they can just be overtyped. The calculations are worked on a point-to-point basis thus they are of a slightly 'rough and ready' nature this can lead to some small steps on the input sheet's acceleration curve graph, errors experienced might be as much as 10% but the calculations are good enough to give the rally engineer a good idea of how changing a car's set-up can influence its performance. Additionally once the torque available at the wheels has reduced below the point where meaningful acceleration is attainable the simulation assumes that maximum attainable speed has been reached.

Have fun playing around with the programme - further notes will be published as and when bugs are detected and improvements are made. Stage-Rally would appreciate your feedback on how you've got on with using this tool whether it's just-for-fun or in developing your rally car. (Dan Hart 30/10/00)