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Using Torque & Horsepower - Kopycinski's Brain

Posted in How To on July 1, 2011
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We’re well aware that the two most talked about engine characteristics are torque and horsepower. But what do these two values really mean to us in the real world? Torque is defined as the tendency of a force to rotate an object about an axis, such as a tire spinning on an axle. Horsepower is defined as some amount of work performed over time. Both of these numbers are important in defining the performance we get from an engine and drivetrain.

Torque, simply put, is what you feel as you mash the accelerator to the floor. Torque often rises and hits a peak well before horsepower peaks. Horsepower will continue to rise rapidly past the engine speed of maximum torque. Horsepower can determine how fast you can get your vehicle from one point to another, given that the gearing is optimized. OR

(rpm x torque*)
Horsepower =__________________
5,252

(*torque is expressed in lb-ft)

Here's an example of horsepower and torque curves with respect to engine speed (rpm). Note that the maximum torque is developed at a relatively low engine speed and then maximum horsepower is developed at higher speed. These peak values and their locations determine a lot about how an engine performs. (Note: because of the math units or torque related to horsepower, their values will always cross at 5,252 rpm on the graph.)

In general, for a 4WD trail vehicle, superior performance occurs when we have maximum torque at a fairly low engine speed speed and maximum power at an engine speed that is at lest twice as fast or faster. The broad distance between these two peaks provides us with a comfortable driving behavior that allows us to tackle a wide range of vehicle engine loads without excessive gear shifting or throttle adjustment.

In contrast, a lightweight race vehicle that runs up higher on the throttle most of the time could better tolerate a maximum torque peak at a higher engine speed. However, the driver will work more (shifting, throttle) to keep the engine in a usable range.

Drivetrain gearing plays into the overall performance of the vehicle as well. Also note that maximum acceleration in any particular gear occurs at the point of maximum torque. Lower axle gearing (higher, numerically) can push the maximum torque point in the drivetrain lower on the engine speed scale. As significant weight and/or much larger tires are added to a vehicle, it becomes more important to consider axle gear changes to help keep the engine running in the powerband where torque and horsepower can best be used.

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