Dear European Car,
What exactly is torque?
Sourena Javadpourvia e-mail
Torque and how it relates to horsepower is probably one of the most misunderstood concepts enthusiasts have when it comes to automotive performance. The chest-beating big-engine guys wax poetically about the virtues of big torque, while the low displacement, forced induction fans get all giddy about 8000rpm redlines and shifting right at the rev limiter. What most of them don't understand is that they both like the same thing, just in different flavors.
First let's talk about what torque is. Torque is force, one that's applied in a circular manner. The difference between a linear force and torque is that a linear force is the same whether you're pushing from one foot away or three feet away. Torque uses the force multiplier that a lever arm provides. Take a wrench turning on a bolt as an example. If you have a one-foot-long wrench and you are applying 10 pounds of force on the end of it, the bolt is experiencing 10 lb-ft of torque. Why is that? Because you have 10 pounds of force acting from one foot away. If you had a 20-pound wrench one foot away you would experience 20 lb-ft, and conversely if you had a 10-pound wrench acting from two feet away you'd still have 20 lb-ft. So, why aren't all wrenches two or even six feet long? Well, to get the 20 lb-ft of torque, using only 10 pounds of force you're going to have to supply that 10 pounds over a much greater distance, circumference being 2pr, where the circle's radius (r) is multiplied by two and by the mathematical value of p, roughly 3.14. As for how this applies to cars, we'd need to get into the ins and outs of gear ratios, which is a whole different column.
Now that we understand what torque is, what exactly is horsepower? First lets look at the equation. Horsepower = (torque x rpm) 5,252. As you can see, we have the force multiplied by engine revolutions, which throws in a unit of time. The 5,252 figure is simply a constant developed to convert from lb-ft/rpm to a more useable unit. The unit of horsepower was originally developed by Henry Watt when he wanted to compare the amount of power one of his steam engines could produce compared to an average horse. He determined that a horse could produce 33,000 lb-ft/minute or 550 lb-ft/second.
This explains what the numbers mean, but it is the real-world application that really matters. A car that can make 400 hp at a mere 2500 rpm is making an astounding 840 lb-ft of torque, while a car that makes 400 hp at 8500 rpm is only having to make 247 lb-ft of torque at 2500 rpm. The same horsepower, just made a completely different way.
A better real-world example is to think of it this way: You have two guys with barrels of water at Point A that they need to get to Point B. One guy is really big, think Hungarian World's Strongest Man competitor, very strong but not all that fast. The other guy is a sprinter, maybe not that strong overall, but very quick. The big torque monster is going to carry all the water at once from A to B, just very slowly. The sprinter, who is like a small high revving engine, is going to run back and forth with smaller buckets of water, making several trips in the same amount of time but ultimately accomplishing the same thing.
So why do enthusiasts argue the virtues of torque vs. horsepower all day long? Partly because that's what car guys do, argue over trivial things. The other part of it is because while they may think torque and horsepower are different things, in reality they're arguing more over engine characteristics than real physical properties. When someone says a car needs more torque, he's really saying that the car needs to develop more power in the low end of the rpm range. When people say they like the thrill of big high-end horsepower, they really like the noise and mechanical feel of the engine spinning away at astronomical rpm. Hopefully that cleared some things up for you, Sourena. Now go out and drive.