Torque & Horsepower
by Brian Brown
To Analyze a car's performance by it's specs, it's necessary to know the power/RPM curve of the engine (not just the peak horsepower), the car's gearing, and the weight of the car. Torque and Power are related mathematically by engine speed. A peak torque spec is also fairly useless for comparison by itself.

Torque is a measure of twisting force.

Power is a measure of the rate that energy is being used or supplied.

The unit of measurement for torque is foot-pounds (ft-lb), or in metric is Newton-meters (Nm).

The unit of measurement for power is Horsepower (HP), or in metric is Watts (W).

One of the functions of a car's wheels is to be a force conversion device. They convert twisting force (from the shaft) to linear force (at the contact patch and the wheel bearing). Linear force (from the contact patch through the wheel bearing) is what pushes the car forward.

When the linear force exceeds the rolling resistance and air resistance, the car will accelerate. The larger the force, the faster the acceleration. (Also, the lighter the car, the faster the acceleration. Weight is another HUGE factor in determining a cars performance).

In order for the car to continue to accelerate, the force needs to continue to be applied. Energy is being transferred into the car's motion (kinetic energy). If the device supplying the energy (the engine) can't continue to supply the force fast enough (supply enough power), the acceleration will diminish.

Power is force times speed (or torque times RPM for rotational systems). (There are also conversion constants in the math to make the units work out).

The power and torque ratings for an engine are usually quoted as peak values at the RPM where these peaks occur. The actual levels of torque and power vary with speed. These can be measured and plotted out on a graph. The acceleration of the car in a given gear corresponds directly to the *power* graph. In other words, the acceleration corresponds to rate that energy is being transferred to the car's kinetic motion.

As a very loose rule of thumb, an engine only produces about half of it's peak power when spinning at half the speed where the peak power occurred.

Torque can be multiplied by gearing. Power can't.

On the other hand, gearing can be used to match the engine speed with the load so that the engine can be in the speed range where it produces the most power.

As another loose rule of thumb, for a given engine displacement and compression ratio, an engine can be tuned towards producing more low RPM torque or more peak horsepower. Generally, somewhere between these two extremes is best for street cars.

When an engine is referred to as being 'torquey', people usually mean that it has a lot of low RPM torque. This type of engine is *easier* to accelerate with casually, because downshifting isn't as necessary. This helps make it seem more responsive, and can be very practical for driving in traffic. If a similar engine was tuned for more peak power, it will accelerate quicker. It does requires more work on the part of the driver (and the right gearing).

For maximum acceleration, an upshift should occur when the engine would produce the same level of power after the shift as before the shift.

To a point, the more gears a transmission has, and the closer they are together, the better the car's acceleration will be. This is because the engine can spend more time near it's peak power RPM.

To analyze a car's performance by it's specs, one would take the area under the engine power curve that is used in each gear. Dividing the sum of all of these areas by the car's weight would provide a performance index number that would be a pretty accurate comparison to other cars. The only thing that this method doesn't reflect is variations in launching technique from a stop.

Accurate power/RPM curves are sometimes hard to come by.

If you want simple numbers to compare, measured acceleration times (0-60, 1/4 mile, and in gear times) are much more direct and meaningful. They look directly at the end result.


Most of the above is just looking at a car's maximum acceleration. There are other important engine considerations: Part throttle tip-in response, the power bandwidth (an engine with a 'dead' low speed range is hard to drive), inertia, smoothness, weight, and fuel economy all are very important factors that people have to spend more time living with than flat-out acceleration.

Regards, Brian.
September 17, 1999