The Difference Between Horsepower & Torque - How It Works
What’s the Difference Between Horsepower and Torque? The Answer Might Surprise You
From the July, 2011 issue of Chevy High Performance
By Stephen Kim
Try this for kicks. The next time you’re surrounded by your gearhead buddies, ask them to explain the difference between horsepower and torque. The participants will immediately divide into “pro torque” and “pro horsepower” camps, and at the end of two hours—or two years—worth of arguing, all you’ll have to show for it are a bunch of lame clichés.
Raging debates and a healthy discourse of divergent opinions is part of what makes building hot rods so much fun, but the key issue at hand here is the lack of knowledge or history per se. Few people seem to truly understand the difference between the two units of measurement, and those who do can be hard pressed to explain it in a way that most people can understand. Fortunately, we know some really smart dudes who have both the knowledge and ability to lay it all out in a palatable fashion. Our cast includes Jon Kaase of Jon Kaase Racing Engines, Judson Massingill of the School of Automotive Machinists, Scott Shafiroff of Shafiroff Racing Engines, and Harold Bettes of Power Technology Consultants. Depending on your preconceived biases, the truth might be difficult to accept, but the experts have spoken once and for all.
Harold Bettes: An engine produces a twisting force at the flywheel, typically measured in lb-ft of torque. One lb-ft of torque is equivalent to the force of a 1-pound weight pulling down on a 1-foot-long lever. Horsepower, by definition, is torque times rpm divided by 5,252. In a running engine, you can’t have torque without horsepower and horsepower without torque because they’re interrelated. The horsepower formula was invented by Scottish engineer James Watt in the late 18th century to help him market his newly invented steam engine. Since his new invention was competing with horses, he had to come up with a way to relate the output of his steam engine to horses. The problem was that his engine produced a twisting force, or torque, while horses performed work. In other words, horses performed work and therefore made power, not torque. If he told potential buyers that his steam engine made more torque than their horses, no one would have known what he was talking about. Watt determined that the typical horse could pull 33,000 pounds of weight 1 foot in one minute, and he had to figure out a way to relate the rotary motion of his steam engine to the linear motion performed by horses. To do this, he divided 33,000 by 6.28, since the circular distance a 1-foot lever travels in one revolution of a crankshaft is equal to 2 pi. By dividing 33,000 by 6.28, Watt came up with the constant of 5,252 that’s now used universally in the horsepower formula. Consequently, an engine that’s producing 33,000 lb-ft of torque per minute—or 550 lb-ft of torque per second—is producing 1 hp, and the constant 5,252 is used to convert the rotational motion of a crankshaft into linear motion.
Judson Massingill: There is a distinct difference between force and work. You can push against a wall with incredible force, but if the wall doesn’t move then you haven’t performed any work. You may have worn yourself out, but you still haven’t performed any work. The same applies to an engine. Torque is just a force, and horsepower is a unit of work that measures the rate at which torque is applied. That means that an engine that produces half as much torque as another engine needs to turn twice as many rpm to produce the same amount of horsepower. For instance, let’s say I need a ditch dug on both sides of my desk. On one side I have Arnold Schwarzenegger with a big shovel, and on the other side I have a skinny guy with a small shovel. If Schwarzenegger throws out 3 cubic feet of dirt with each scoop, and the skinny guy takes out 11/2 cubic feet of dirt with each scoop but works twice as fast, they’ll both dig a ditch the same depth in the same amount of time. Therefore, they’ve accomplished the same amount of work.
This example shows why you can have a big engine that makes much more torque than a smaller engine, but doesn’t produce any more horsepower. Torque is important, but it’s not nearly as important as the rpm at which that torque is produced. To illustrate the point, let’s compare a ’70 Buick 455 to a ’70 LS6 big-block Chevy. The Buick made 510 lb-ft of torque compared to the Chevy’s 500 lb-ft. If you thought that the Buick could outrun the Chevy because it made more torque, you were living in a dream world. Since the Chevy made peak torque at 3,600 rpm instead of 2,800 rpm, it made 450 hp compared to the Buick’s 350 hp. Torque is directly related to displacement, and in the last 40 years, torque output per cubic inch has only gone up 10 percent at most. On the other hand, horsepower per cubic inch has gone up dramatically, nearly 30 percent, in that time. That’s because today’s engines aren’t producing that much more torque, but they’re maintaining that torque at a much higher rpm before it drops off.
Jon Kaase: Horsepower is just a figure that’s calculated from math. When you put the gas pedal to the floorboard, torque is what you feel. In a street car, you want lots of torque. Let’s say you have a small-block V-8 that makes 250 hp at 5,500 rpm and a small Japanese V-6 that makes 250 hp at 7,000 rpm. If you drove both of them back to back, there would be no comparison on how much faster the V-8 would feel because the smaller motor makes far less torque. Having too much torque is like having too much money. You don’t want any less of it. You just have to learn how to manage your account better. If you’re making so much torque that your car is blowing off the tires, either get some bigger tires or manage the power more efficiently with better electronics.
Judson Massingill: The 433ci LS small-block in our ’99 Camaro drag car makes 1,050 hp naturally aspirated, and has an 8,000- to 9,600-rpm powerband. We don’t even know what the peak torque output is because the motor never turns that low rpm. The reason I quit paying attention to peak torque in race motors is because the vast majority of them operate at rpm that are above peak torque the entire run, whether it’s in NHRA Pro Stock or circle track. If peak torque was more important than peak power, we’d be better off putting our turbo diesel in our race car instead of in our tow rig since it makes more torque than our race engine. If torque is all that matters, why not put some taller gears in our Camaro so the rpm drops down to 7,000 rpm between shifts instead of 8,000? Our 433 small-block might make more torque at 7,000 rpm than at 8,000, but it makes far less power at 7,000 so the car would fall on its face and slow way down. Gearing multiplies torque, and turning more rpm enables you to run shorter gears. In many road racing and circle track applications, a high-rpm motor with lots of gear will make more torque at the rear wheels than a low-rpm motor with less gear since they’ll be turning more rpm and getting more torque multiplication coming off of a corner.
From our racing experience, it’s our opinion that the minimum rpm at which you can go wide-open throttle without breaking the tires loose is the most important part of the power curve. With our Camaro, that point happens to be right around 8,000 rpm. In some respects, the horsepower it makes at 8,000 rpm is more important than peak horsepower. That’s because if the engine speed drops down to 8,000 between shifts, the motor better have some beans at that rpm to keep the car moving down the track. At the end of the day, I am a “bottom end” kind of guy. I just refer to it as horsepower instead of torque.
For people who say that they would rather have torque than horsepower, you have to think about where in the rpm range that torque is produced. Torque multiplication with the transmission in Reverse is greater than in any of the forward gears. If that’s the case, a car will produce the most torque at the rear wheels while in Reverse, so shouldn’t you just leave it in Reverse and mash the gas? Of course not. Likewise, if you prefer torque over horsepower, maybe you should try leaving a car in First gear all the way down the dragstrip. Let’s say you have a motor that makes 400 lb-ft of torque that’s matched with a 2.50:1 First gear ratio and 4.00:1 ring-and-pinion set. That equates to 4,000 lb-ft of torque that’s being applied to the axles at the engine’s torque peak. However, since the transmission gear ratios get taller with every upshift that means the torque applied to the axles drops significantly in each successive gear. Obviously, if you leave the car in First gear just to maximize the torque that’s applied to the axles and wheels, you’re not going to go very fast. This example clearly illustrates that it’s not torque that pushes a vehicle down the road, but rather horsepower.
Judson Massingill: In the racing world, the motor that makes the most explosions going down the track is going to win the race. In other words, the motor than turns the most rpm is probably going to win. Again, that’s because torque per cubic inch is difficult to increase, so the next option is turning more rpm. Not surprisingly, unless the rule book restricts it, engines in every form of racing turn more rpm every year. With today’s cylinder head technology, torque curves stay flat even at high rpm so turning more rpm is the key to making power. In most race engines, horsepower will continue climbing even after peak torque, and as long as rpm increases faster than the rate at which torque drops off after its peak, horsepower will continue to increase. An extreme example is Formula 1. Since the rules limit displacement to 2.4 liters, the motors barely make more than 200 lb-ft. By revving them up to 18,000 rpm, however, they make an incredible 800 hp.
Jon Kaase: Horsepower is torque multiplied by rpm, so if you have a drag car with flat torque curve, the horsepower will keep climbing for a while because of the way the math works out. In other words, if an engine produces lots of torque at high rpm, it will make a ton of power. A Pro Stock motor makes about 1.7 lb-ft per cubic inch, while a very healthy street engine can make as much as 1.5 lb-ft per cubic inch. The big difference is that a race engine will continue to maintain high torque output even after its torque peak. All they’re doing in a Pro Stock engine is raising the rpm where torque is made, and they don’t make that much more peak torque than they did six to eight years ago. Most engines will usually turn a couple thousand rpm past their torque peaks. If horsepower drops off any quicker than that, it usually indicates a problem with the motor. Either the heads, camshaft, intake manifold, carb, or headers are too small, or it’s running out of valvetrain. Scott Shafiroff: In racing, rpm is critical. If you think about it, the greater the number of explosions per second an engine produces, the better it will accelerate a car down the track. In race cars, you look at the horsepower an engine makes and the rpm it turns, and gear it accordingly. You never want a motor to be below peak torque, which requires keeping the rpm up. On each gear change, you want the rpm to drop down to the torque peak and then accelerate beyond power peak. Engine design is about making compromises. You don’t want to trade a lot of torque for only 2 hp, but you do need to sacrifice some torque in order to make horsepower up top.
Harold Bettes: People like to argue over torque and horsepower, but in the process they often overlook the area under the curve. That refers to the average horsepower an engine produces throughout its operating range. Let’s say one engine produces a power curve that looks like a church steeple, while another engine has a power curve shaped like the top of a balloon. Even if both engines produce the same peak power, the engine with the broader curve will be both more fun to drive and substantially faster at the track. That’s because when you shift gears, the engine with the balloon-shaped torque curve will be making more horsepower. For this reason, the area under the curve, or average horsepower, is a much more effective way of gauging how well an engine will perform than peak torque or horsepower.
Scott Shafiroff: People want to see a big peak horsepower number, but average horsepower is much more important. It’s not just the peak number that counts, but rather the horsepower that’s produced over an engine’s rpm range. Some people think that building street motors is easier than building race motors, but that’s not always the case. In some ways it’s harder to build a street motor since they have to operate in a broad rpm range. Whether it’s a street motor or a race motor, the goal is to maximize the average horsepower output.
Judson Massingill: In America, we tend to look at low-rpm engine output in units of torque and high-rpm output in units of horsepower. However, the truth of the matter is that you can’t separate the two since horsepower is a derivative of torque that’s mathematically calculated from torque and rpm. People are familiar with the formula horsepower equals torque times rpm divided by 5,252. So if you have more torque at, say, 5,000 rpm, than the guy in the other lane, you’ll have more horsepower as well. Likewise, if an engine is producing lots of torque at 3,000 rpm, it’s also producing lots of horsepower at 3,000 rpm. Another way to look at it is that if you’re building an engine to maximize torque output at low rpm, you’re also building an engine that makes lots of horsepower at low rpm. People tend to get very passionate about the horsepower versus torque topic, and some people are torque guys while others are horsepower guys. I’ve been at engine conferences where people got so riled up about this debate that they had to be escorted out of the room by security. The funny thing is that they’re arguing about the same thing. CHP