HOME OF -VIRTUAL STOPWATCH- THE VEHICLE ACCELERATION AND LAP TIME CALCULATOR

Follow Competition Car Engineering on for all the latest updates

TORQUE, ACCELERATION and POWER

You will often hear a motoring journalist (or mate down the pub) exclaiming that the latest version of a new engine generates a huge amount of torque.  These "people in the know" will tell you that torque is what accelerates a car... that more flywheel torque equals more acceleration.  However, when you start to look at the hard facts that simple statement starts to fall apart....

Take for example the 70-90mph acceleration of the 5.0 litre V8 Mustang tested by Autocar in February 2016:

The car is clearly quicker from 70-90mph in 3rd gear than it is in 4th or 5th gears, but inspection of the Mustang's torque curve shows that the engine generates similar torque in all three of those gears, in fact it generates most torque in 4th gear:-

Why doesn't the car accelerate fastest when torque is highest? Because:

The flywheel torque figure does not account for the torque multiplication effect of the transmission system

So what determines the magnitude of the transmission's torque multiplication effect?

At any given road speed the transmission's torque multiplication effect is proportional to the engine speed. More engine speed = greater torque multiplication.

To take account of the transmission's torque multiplication effect we adjust the torque curve in porportion to the engine speed in accordance with the diagram below. The result is the "Power" curve.

When applied to the Mustang's Torque curve the conversion looks like this:-

So the Power Curve is merely a copy of the Torque Curve which is adjused to take into

account the transmission's Torque multiplication effect

Using the Mustang's "Speed-Adjusted Torque Curve" (The Power curve) it is easy to see why the car accelerates quickest in 3rd gear: the average Power in 3rd gear is higher than the average Power in the other gears:

COMPARING THE PERFORMANCE OF DIFFERENT CARS

The Power curve is not only useful for determining which RPM range will accelerate the car fastest: it can also be used to accurately predict the performance of different cars quickly and easily.

The chart below has been constructed from actual road tests conducted by Autocar Magazine on 27 different cars ranging from a Porsche 911 to a Volkswagon supermini, and most types of car in between.  As you can see there is no appreciable relationship between acceleration and the peak torque:weight of a selection of cars:-

 

Now if we display the Power:Weight vs acceleration of the exact same set of cars tested by Autocar Magazine it can be seen there is a very clear relationship between the car's power/mass and its acceleration, in fact, if we know nothing more than the car's power output and its mass we will still be able to estimate it's 30 to 70mph acceleration  with a high degree of accuracy, something we couldn't do before from the torque/mass figure (in fact the torque figure just confuses things).

 

FACTORY STANDARD GEARBOXES

It is clear from the above that gearboxes play an important part in vehicle performance;  The more gears a car has the better able it is to keep the engine in a rev range where it generates high power (remember: that means the rev range where the engine exhibits the best combination of gear ratio and flywheel torque).  Typically road car manufacturers give their cars 5 or 6 gears, but motorbike engines and race car engines may have 7 or 8; this allows them to stay in the "power band" (that's the optimum rev range) for longer.

HOW TO COMPARE ENGINES

Using the power curve you are able to make very accurate predictions about car performance.  For example the 5.0 litre Mustang engine above generates nearly 190bhp at 3000rpm: About the same as a hot-hatch engine at high RPM.  Sure enough, the Autocar road tests tell us that the Mustang accelerates 30-70mph in 4th gear about as fast as a Peugeot 208 GTI driven flat out.  At high RPM the Mustang generates around 400bhp and it therefore accelerates much faster than the GTI when it is also driven flat out.

Hopefully this has shown you that when comparing the output of different engines it would be better for manufacturers to quote a peak power figure (representing its accelerative ability when driven hard) and another power figure at say 50% of maximum revs (representing its ability to pick-up speed when you floor the accelerator without down-shifting when cruising on a motorway). 

You can use the converter below to help you quickly determine the equivalent power from the flywheel torque and engine rpm:-

Flywheel Torque: lbft, at an engine speed of rpm

Equivalent power

 

SUMMARY

An engine which produces more power (regardless of whether that power comes from more

flywheel torque, more flywheel speed, or a combination of the two) will be able to accelerate a car quicker.

 

Peak Torque figures sell diesel cars, a good Power Curve wins races!

 

You can see the affect on acceleration of various engines with different torque characteristics using the Virtual Stopwatch calculator (click Image below):-

 

 

COPYRIGHT 2012 Richard Machin. All Rights Reserved