Don't be swayed by HP figures....Torque is what really counts.
04-26-14, 10:22 AM
#1
Lexus Fanatic
Thread Starter
Don't be swayed by HP figures....Torque is what really counts.
Like many members of my Baby Boom generation, I grew up in the 1960s and early 70s, during the Great American Muscle-Car Age....although, unlike some of my friends, I liked both muscle-cars AND big, plush, soft-riding luxury cars as well. Back then, especially in car-advertising, relatively little emphasis was put on engine torque. Indeed, many data and spec-sheets didn't include it at all. We all knew, for example (or at least those of us who where car buffs), that the famous dual-quad (dual four-barrel) 426 cubic-inch Chrysler Hemi produced an official 425 HP. But how many of us knew that also included some 490 ft-lbs. of torque? Heck, many of us kids didn't even know that torque WAS....and the auto companies often didn't publish it. Same with the famous Corvette Tri-Carb 427 c.i. V8 with the three two-barrel carburators. We knew it produced an official 435 HP, but, had no idea of what the torque was. In school and among us friends, the talk was always of the HP figures in the latest versions of the Road Runner, GTO, Chevelle SS, Mustang Mach 1, Charger R/T, etc...... but torque? People looked at you funny if you mentioned it, like...."What's That?" 
Well, of course, today, we know better. Torque, in most ads and auto reviews/write-ups, is now given the attention it deserves, with much less emphasis on horsepower. To understand why, I'll give a brief description of each. Complex math formulas, of course, are involved in determining each. But, as I am neither a physicist nor a mathmetician, I'll skip those and just explain the basics.
The term Horsepower generally comes from an old measurement, before the age of self-propelled autos, of the amount of work/physical power that it took the average farm horse to lift a 550-lb. weight one foot off the ground in one second. One horsepower was (roughly) equal to one horse lifting the weight one foot in that second. As it applies to vehicle engines, the HP figure is roughly the amount of power produced inside the cylinders (which can be measured on a dynamometer) as the fuel/air mixture burns and pushes down the piston.
But that is not (not directly, at least) what actually makes the vehicle accelerate when it is in gear and you push down on the gas pedal. For that, you need torque. Torque, although also a measurement of power, is substantially different from HP. Torque is the twisting, circular force that the output shaft of the engine (from the crankshaft) and its heavy flywheel actually produces going back to the clutch and transmission. It is usually measured in foot-pounds (ft-lbs.), one foot-pound being the leverage (twisting/cranking) force produced by a one-pound weight on a one-foot lever rotating around a central point. In other words, the HP in the cylinders produced by the air/fuel burning is converted, by the pistons/comnnecting rods and crankshaft, to the torque figure actually needed to reach the drive wheels, start the car up from rest, and accelerate. Many factors, of course (including transmission/final-drive gearing, number/type of differentials/final-drive units, and in car-based AWD vehicles, front/rear-wheel torque vectoring) will determine just how much of the engine torque at the output shaft will actually reach the drive wheels. But, in general, all else equal, the higher the torque figure, the faster the vehicle will accelerate. Horsepower, on the other hand, is more of a factor at high road speeds, and will help determine a vehicle's ultimate top speed against air resistance. That is why, for most American driving conditions, torque is a far more important figure......and this is generally reflected in the preference of the average American driver, who drives at fairly low engine RPMs and usually wants torque for acceleration. In some places, though, like on Germany's famous Autobahns where some sections of them have no speed limit at all (and people drive like it) HP is a more important figure, because it will determine, with air resistance, just how fast you can go in the left lane before you have to move aside for that Porsche 911 creeping up behind you.
There are also different patterns on a graph (known as HP and torque-curves) in the way power is produced in an engine. HP, in general, tends to be lower at low engine RPMs and steadily climbs with RPMs until reaching a maximum figure just short of the engine's redline on the tachometer (the maximim safe engine speed without damage). That's one reason, among others, why it's more of a factor at higher road speeds. Torque, on the other hand, usually tends to be concentrated in the mid-RPM bands of an engine (say, 2000-4500 RPM), and drops off at both very low and very high RPM. Some engines, though, have what is called a "flat" torque curve (which means that its output is fairly constant at both low and middle RPMs). Enthusiasts tend to like that, because it usually beans better acceleration from low speeds. Torque, rather than HP, is what actually made those old Road Runners, GTOs, Mustangs, etc.....do those dual-stripe burnouts from the rear tires and run those 14-second quarter-miles, but most of us dumb kids didn't really know that. We thought, at the time, that it was the HP rating, so that's all we talked about.
This, of course, applies primarily to conventional gas-powered piston engines, but diesels and electric motors bring a somewhat different set of rules. Diesel-engines, by their very compression/ignition design, use much higher compression ratios than regular gas engines, which, combined with diesel-fuel's different BTU energy content than gas, means (in comparison), lower HP figures and much higher torque ratings at lower RPMs. It is not unusual to see automotive diesels with twice the torque-rating of their HP. That generally means strong throttle response at low RPMs...but a diesel engine also runs and redlines at lower RPMs than a gas engine, so that response can out of steam fairly quickly.
Those of you with gas/electric hybrids and pure battery-powered electrics will also notice some different characteristics. Electric motors, unlike most gas engines, develop their maximum torque at VERY low RPS...theoretically, at 0 RPM. So, an electric motor starts out very strong at low RPMs and steadily weakens as RPMs build. There is, of course, no practical red-line for an electric motor,at all, as they spin at tens of thousands of RPMs. In fact, most pure-electric cars don't have a multi-speed transmission at all (and don't need one)....simply a one-speed device to transfer torque to the rest of the drivetrain.
So, in a nutshell, the next time someone brags about their engine and it XXX number of horsepower, your answer should be......."Now, tell me the torque figure and what it will REALLY do."
MM
Well, of course, today, we know better. Torque, in most ads and auto reviews/write-ups, is now given the attention it deserves, with much less emphasis on horsepower. To understand why, I'll give a brief description of each. Complex math formulas, of course, are involved in determining each. But, as I am neither a physicist nor a mathmetician, I'll skip those and just explain the basics.
The term Horsepower generally comes from an old measurement, before the age of self-propelled autos, of the amount of work/physical power that it took the average farm horse to lift a 550-lb. weight one foot off the ground in one second. One horsepower was (roughly) equal to one horse lifting the weight one foot in that second. As it applies to vehicle engines, the HP figure is roughly the amount of power produced inside the cylinders (which can be measured on a dynamometer) as the fuel/air mixture burns and pushes down the piston.
But that is not (not directly, at least) what actually makes the vehicle accelerate when it is in gear and you push down on the gas pedal. For that, you need torque. Torque, although also a measurement of power, is substantially different from HP. Torque is the twisting, circular force that the output shaft of the engine (from the crankshaft) and its heavy flywheel actually produces going back to the clutch and transmission. It is usually measured in foot-pounds (ft-lbs.), one foot-pound being the leverage (twisting/cranking) force produced by a one-pound weight on a one-foot lever rotating around a central point. In other words, the HP in the cylinders produced by the air/fuel burning is converted, by the pistons/comnnecting rods and crankshaft, to the torque figure actually needed to reach the drive wheels, start the car up from rest, and accelerate. Many factors, of course (including transmission/final-drive gearing, number/type of differentials/final-drive units, and in car-based AWD vehicles, front/rear-wheel torque vectoring) will determine just how much of the engine torque at the output shaft will actually reach the drive wheels. But, in general, all else equal, the higher the torque figure, the faster the vehicle will accelerate. Horsepower, on the other hand, is more of a factor at high road speeds, and will help determine a vehicle's ultimate top speed against air resistance. That is why, for most American driving conditions, torque is a far more important figure......and this is generally reflected in the preference of the average American driver, who drives at fairly low engine RPMs and usually wants torque for acceleration. In some places, though, like on Germany's famous Autobahns where some sections of them have no speed limit at all (and people drive like it) HP is a more important figure, because it will determine, with air resistance, just how fast you can go in the left lane before you have to move aside for that Porsche 911 creeping up behind you.
There are also different patterns on a graph (known as HP and torque-curves) in the way power is produced in an engine. HP, in general, tends to be lower at low engine RPMs and steadily climbs with RPMs until reaching a maximum figure just short of the engine's redline on the tachometer (the maximim safe engine speed without damage). That's one reason, among others, why it's more of a factor at higher road speeds. Torque, on the other hand, usually tends to be concentrated in the mid-RPM bands of an engine (say, 2000-4500 RPM), and drops off at both very low and very high RPM. Some engines, though, have what is called a "flat" torque curve (which means that its output is fairly constant at both low and middle RPMs). Enthusiasts tend to like that, because it usually beans better acceleration from low speeds. Torque, rather than HP, is what actually made those old Road Runners, GTOs, Mustangs, etc.....do those dual-stripe burnouts from the rear tires and run those 14-second quarter-miles, but most of us dumb kids didn't really know that. We thought, at the time, that it was the HP rating, so that's all we talked about.
This, of course, applies primarily to conventional gas-powered piston engines, but diesels and electric motors bring a somewhat different set of rules. Diesel-engines, by their very compression/ignition design, use much higher compression ratios than regular gas engines, which, combined with diesel-fuel's different BTU energy content than gas, means (in comparison), lower HP figures and much higher torque ratings at lower RPMs. It is not unusual to see automotive diesels with twice the torque-rating of their HP. That generally means strong throttle response at low RPMs...but a diesel engine also runs and redlines at lower RPMs than a gas engine, so that response can out of steam fairly quickly.
Those of you with gas/electric hybrids and pure battery-powered electrics will also notice some different characteristics. Electric motors, unlike most gas engines, develop their maximum torque at VERY low RPS...theoretically, at 0 RPM. So, an electric motor starts out very strong at low RPMs and steadily weakens as RPMs build. There is, of course, no practical red-line for an electric motor,at all, as they spin at tens of thousands of RPMs. In fact, most pure-electric cars don't have a multi-speed transmission at all (and don't need one)....simply a one-speed device to transfer torque to the rest of the drivetrain.
So, in a nutshell, the next time someone brags about their engine and it XXX number of horsepower, your answer should be......."Now, tell me the torque figure and what it will REALLY do."
MM
Last edited by mmarshall; 04-26-14 at 10:34 AM.
04-26-14, 12:32 PM
#4
Lexus Fanatic
That is not true. Horsepower is still very important, in fact extremely important. Horsepower and torque are related, but also important is the gearing and drive-train loss.
Horsepower and speed are related.
Both HP and torque are related and either cannot exist without the other.
More important to what mmarshall said is the rate at which the engine can create the torque. HP is the rate at which you can produce the torque, the higher the HP the quicker the torque is produced. To me, HP is very important.
Horsepower and speed are related.
Both HP and torque are related and either cannot exist without the other.
More important to what mmarshall said is the rate at which the engine can create the torque. HP is the rate at which you can produce the torque, the higher the HP the quicker the torque is produced. To me, HP is very important.
Last edited by Toys4RJill; 04-26-14 at 12:37 PM.
04-26-14, 12:49 PM
#5
Lexus Fanatic
Thread Starter
That is not true. Horsepower is still very important, in fact extremely important. Horsepower and torque are related, but also important is the gearing and drive-train loss.
Horsepower and speed are related.
Both HP and torque are related and either cannot exist without the other.
Horsepower and speed are related.
Both HP and torque are related and either cannot exist without the other.
04-26-14, 12:49 PM
#6
That is not true. Horsepower is still very important, in fact extremely important. Horsepower and torque are related, but also important is the gearing and drive-train loss.
Horsepower and speed are related.
Both HP and torque are related and either cannot exist without the other.
More important to what mmarshall said is the rate at which the engine can create the torque. HP is the rate at which you can produce the torque, the higher the HP the quicker the torque is produced. To me, HP is very important.
Horsepower and speed are related.
Both HP and torque are related and either cannot exist without the other.
More important to what mmarshall said is the rate at which the engine can create the torque. HP is the rate at which you can produce the torque, the higher the HP the quicker the torque is produced. To me, HP is very important.
04-26-14, 12:58 PM
#7
Lexus Fanatic
Thread Starter
True, but only to an extent. In a conventional gas engine, max torque is usually a couple of thousand RPMs below max HP, so the two aren't always that closely related.
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04-26-14, 01:06 PM
#8
Lexus Fanatic
Thread Starter
What actually moves the wall at high speed, when you it, is Kinetic energy, not torque. Kinetic energy is the energy stored in moving bodies of matter.......which is why, all else equal, large heavy vehicles offer more protection in a crash than smaller, lighter ones. With a wall, torque comes more into play if, say, you slowly back a 3500-series diesel pickup into it and attempt to physically push the wall down.
04-26-14, 01:14 PM
#9
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04-26-14, 01:19 PM
#10
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Yes and no.
Torque (rotational force) is a momentary measurement.
Power (or HP) is a measurement of power over a period of time.
1 HP = 550 ft lb / second. Yes, per second. Look it up.
This is why gigantic tractor engines produce huge torque, but accelerate at a snails pace, even if you're just counting the acceleration in rpm in neutral. These engines are slow, but create huge force.
This is why racing engines produce less torque than HP, and Rev happily to the moon and back.
Also, there is a virtually unlimited ability to magnify force (or torque) by using gearing. It's like using a longer wrench to multiply your torque to loosen a lug. If you apply HP over a long period, you can create virtually infinite torque (there is an eventual limit, but it usually won't be seen by normal automotive applications, and there is a point where it is not useful, practical, or cost effective). The same physics is at play when you use a scissor jack to lift a car.
Torque (rotational force) is a momentary measurement.
Power (or HP) is a measurement of power over a period of time.
1 HP = 550 ft lb / second. Yes, per second. Look it up.
This is why gigantic tractor engines produce huge torque, but accelerate at a snails pace, even if you're just counting the acceleration in rpm in neutral. These engines are slow, but create huge force.
This is why racing engines produce less torque than HP, and Rev happily to the moon and back.
Also, there is a virtually unlimited ability to magnify force (or torque) by using gearing. It's like using a longer wrench to multiply your torque to loosen a lug. If you apply HP over a long period, you can create virtually infinite torque (there is an eventual limit, but it usually won't be seen by normal automotive applications, and there is a point where it is not useful, practical, or cost effective). The same physics is at play when you use a scissor jack to lift a car.
04-26-14, 01:20 PM
#11
Lexus Fanatic
Because we are talking about the average user, a high HP engine is better suited than a low HP very high torque engine.
04-26-14, 01:31 PM
#12
Lexus Fanatic
Thread Starter
Originally Posted by dj.ctwatt
Also, there is a virtually unlimited ability to magnify force (or torque) by using gearing. It's like using a longer wrench to multiply your torque to loosen a lug. If you apply HP over a long period, you can create virtually infinite torque (there is an eventual limit, but it usually won't be seen by normal automotive applications, and there is a point where it is not useful, practical, or cost effective). The same physics is at play when you use a scissor jack to lift a car.
04-26-14, 02:29 PM
#14
Lexus Champion
Don't forget one extremely important fact. The old ratings (pre-1972) cannot be compared to horsepower ratings today. For example:
So when people look back fondly at the muscle car era and wax poetic about 400+ HP engines, if they were tested today in stock form then would not be putting down near those numbers. Not to mention those engines got 15mpg is you were lucky.
The ambiguity of gross horsepower ratings means that many pre-1972 American cars were actually a lot less powerful than the advertised figures would suggest. While the late sixties were a golden age of horsepower compared to the late seventies or early eighties, the differences weren’t quite as vast as they appear at first blush. For example, a 1967 Chevrolet Impala with the 396 cu. in. (6,488 cc) V8, rated at 325 gross horsepower (242 kW), probably had something like 220 net horsepower (164 kW) in pure stock form.
04-26-14, 02:30 PM
#15
Lexus Fanatic
Thread Starter
Have a nice evening.