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Torque and Horsepower

I doubt any of us can look at a machine that has an engine and not wonder how much horsepower it generates. We are all curious about the number of horses hiding in there. The real question we should be asking, though, is, “How much torque does it produce?”

The interest in horsepower dates back to the 1800s, when James Watt (of electrical watts fame) tried to sell steam engines to farmers. He developed his steam engine to perform work, which is measured as torque. Every person he demonstrated it to would ask a question he could not answer: “How many horses can it replace?”

Eventually, Watt figured out a mathematical way to assign a value for the amount of horses an engine would substitute.  

torque vs. hp.

So is torque a substitute measure for horsepower? No. Torque is the measure in foot-pounds (ft.-lbs.) of the amount of work an engine can perform. Horsepower is the measure of how quickly that work can be done.

Let’s say you have two workers on the farm and you need to load a pickup with 50-pound bags of seed.

Worker A can carry one bag, and it takes one minute to load each bag. Worker B can carry two bags at a time, but it takes 3½ minutes to load both bags.

Worker A has less torque (can only carry 50 pounds) but can load faster, so that figures out to more horsepower. 

In contrast, Worker B has more torque (can carry 100 pounds) but takes longer to load, so that figures out to less horsepower. 

Determining engine capacity is more precise than the worker example due to the dynamometer. This device measures torque and uses Watt’s equation to convert to horsepower. 

Here’s the equation: hp. = torque × engine rpm ÷ 5,252 

If you have a tractor that generates 650 ft.-lbs. of torque at 1,200 rpm, that engine’s horsepower would be 650 × 1,200 ÷ 5,252 = 148.51 hp. at 1,200 rpm. 

If that same engine is modified to maintain 650 ft.-lbs. of torque at 1,500 rpm, it would now have 185 hp. at 1,500 rpm.

To convert horsepower to torque, use this formula: torque = hp. × 5,252 ÷ rpm

Using the engine mentioned above, 148.51 × 5,252 ÷ 1,200 = 649.978 ft.-lbs. of torque.

Every engine, regardless of design or fuel type, has horsepower and torque equal at 5,252 rpm. Because the numerator and the denominator of the equation are the same, they become one.

An engine needs cylinder pressure from combustion to produce torque and rpm to make horsepower. We all buy horsepower but drive torque in both the field and on the highway.

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