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Horsepower vs Torque

This is the best explanation I’ve seen yet.


I love that!

Love Johnathan Winters, too.

He was gut busting funny without profanity.

I’ve noticed that for some reason even though HP = Torque X RPM, which involves all sorts of weird unit conversions, when plotted together they tend to have a similar range of values; i.e. they two curves can be plotted vs rpm using the same numeric scale and fit on one chart.

I’ve also wondered what engine specs tend to get you high torque, and what tend to get you high HP? Seems engine designers get these parameters to meddle with

  • compression
  • number of cylinders
  • total displacement
  • displacement per cylinder
  • cylinder diameter
  • piston stroke length

Engine size or induction increases torque ,for whatever reason diesels have more torque then gas engines esp at lower revs,low enfd torque is what gets things moving the wildest most hairy engines in the world usually have little torque at the start due to induction difficulties ,contrary to popular belief multi cylinder engines with a lot of power pulse overlap have the best low end go (see the old 16 12 ,cyl motors,takeoff in high gear no problem,but the old large lungers would shudder and grumble off pretty well too(if they were big enough ) There is a formula that calculates HP were the torque and hp meet on the curve,nothing beats the torque of an electric motor ,but the ICEs do pretty good when you get them up to speed,the characteristics of the ICE engine make for some pretty complex drivetrains ,( clutches ,torque converters ,multi speeds ,what have you ) if you could solve the energy storage problem for electric drivetrains ,they would be the winner hands down.
Torque gets you moving ,HP gives you the ability to move that resistance fast .

Actually, it’s power, not horsepower that’s the product of angular velocity and torque.

Let’s say we have a torque of one newton-meter acting on a shaft that is rotating at an angular velocity of one radian per second. This delivers exactly one watt of power to the load.

If we want to use imperial units, then torque in ft-lb times angular velocity in radians per minute gives us power in ft-lbs per minute but since a horsepower is defined as 33,000 ft lb of work per minute, we have to divide by 33,000 to get horsepower.

So, horsepower equals radians per minute X ft-lb / 33,000
But we usually measure angular velocity in revolutions per minute so we have to convert revolutions to radians by multiplying by 2π or approximately 6.28. When you divide 33000 by 2π, you get the familiar constant of 5252.113122 which is usually rounded off to 5252.

There is a common misconception that torque and power are mutually exclusive. You need both torque and rpm to make a lot of power.
Torque is mostly tied to displacement and volumetric efficiency. The bigger the engine is, the more torque it makes, the more completely it reloads the cylinders, the more torque it makes. A closed throttle reduces the torque of an engine by preventing the cylinders from reloading completely during the intake stroke. A supercharger increases torque by stuffing in more air/fuel mix than the engine would normally fill the cylinders with.
Formula 1 engines make a lot of power because they make both a lot of torque and they can make that torque at very high rpm’s.
Torque is how well the engine breathes. Power is how much air the engine breathes. The torque tends to peak at the rpm where the engine most completely fills the cylinders with air measured in cubic feet per revolution. The power tends to peak at the rpm where the engine pumps the most air, measured in cubic feet per minute.

@GeorgeSanJose, the last four on your list are closely related.

How the power is applied to the road includes gearing, both transmission and rear end. Newer cars take advantage of the power band by using more forward gears, allowing for smaller engines without much loss in performance.

It depends on what type of electric motor you are talking about. Series wound DC motors have lots of starting torque, that’s why they are the motors of choice for locomotive traction motors and car engine starters.
Low slip AC induction motors often have very low starting torque. Permanent split capacitor single phase motors have such weak starting torque that a dry bearing can prevent them from starting. Once they pull up to synchronous speed, they can be very powerful though.

Actually, the king of zero rpm torque is the steam engine. It’s why they were used in locomotives for a long time, no gearbox needed, the driving wheels were the crank shaft.

The shape of the torque curve is at least as important as the peak values of torque and horsepower (that’s spec’d) when it comes to driveability of a street vehicle.
Unlike internal combustion engines, electric motors can have a torque curve that’s ruler flat from near zero rpm to the limit, which can eliminate the need for a multi-ratio transmission and create exciting thrust.

The torque of an electric motor is only limited by the current the wires can stand without overheating. 48 volt fork lift motors are fed 400+ volts and over a thousand amps to make the KillaCycle electric drag bike do the quarter mile in less than 8 seconds, and the motors can stand this abuse for about 8 seconds, usually.
The motor that starts your car engine puts out a surpisingly large amount of power, for only a couple of seconds, then it sits there and cools off before it has to start the car again.
That’s why it’s a bad idea to use a car starter for an electric vehicle, unless you significantly de-rate it, easily done by running it on six volts instead of 12. These motors were designed for intermittent duty, as are many hydraulic elevator motors.
I read the nameplate of one of those motors and it claimed a duty cycle of 30 minutes per hour and 120 starts per hour.
These motors are very small for their horsepower but live because of the intermittent nature of their use and the fact that the entire motor and its windings are submerged in the hydraulic oil that they pump. That’s why they can get away with #15 gauge magnet wire while drawing 45 amps from the line.

The big trouble with the chugging steam cylinders was in the fact they were fairly hard to control an electric motor is a lot better for starting a load on steel rails then steam cylinders ,watch the old movies when those steel driving wheels spin and slip due to the uncontrollable torque,the primary reason electric drive replaced steam locomotives wasnt because of cost,but because it was simply better.A locomotive is a lot better if you can ease it up(ask any engineer that had to carry a coupling down a string of rolling stock to replace a broken one,you need a little slack not a jerk )

High torque engines are responsive at low RPMs. Most drivers are not gear heads and have no desire to rev engines up through the gears to get them to perform, they just want the engine to quietly and smoothly give them the power to merge, pass, whatever, to get them where they are going. Today’s technology has been somewhat successful in giving us adequate torque and horsepower from relatively small engines by using variable timing and control of the ignition and fuel injection plus multi speed transmissions that are computer controlled. Gears turn horsepower into torque when demanded.

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One thing you got to remember is that when a lot of people talk about “torque”, they don’t mean it literally. What they really mean is that the engine has a wide range of rpm’s where it pulls strongly. Or as some would say, "if the engine is running, it’s in its powerband.
Another thing that many of us translate as “torque” is rpm stability under various loads. A four stroke engine with the throttle cracked open just a little for a fast idle has a very high intake manifold vacuum when unloaded. This very high intake vacuum limits the no load rpm, if the rpm increases, the vacuum becomes even higher and the torque drops to zero, but if the load slows the engine down, the vacuum decreases (manifold pressure increases) and the torque automatically increases in response to the slow down. This makes engaging the clutch very easy. The engine doesn’t race if you don’t engage the clutch enough or die if you engage it a little too much.
When you have a racing cam, you have a much lower idle vacuum and you have an engine that wants to race if you don’t engage the clutch enough and die if you engage the clutch too much.
Two stroke engines, with their very low manifold vacuum also like to race or die when you try to engage the clutch.
Diesels have no inherent rpm stability because there is no throttle to create manifold vacuum so idle governors are used to artificially give them idle rpm stability. This gives the impression that diesels have a lot of torque.

The thing that gives the impression that diesels have a lot of torque is…Well, they have a lot of torque. The school bus that I drive has a 409 CID straight six turbo diesel which exerts 560 lb. ft. of torque at 1,600 RPM. It can put out 240 HP and does not rev over 2,600 RPM. Turbocharged diesels tend to put out quite a bit of torque. The reason they are used in large trucks and buses is the large amount of torque, which is usually available at fairly low RPM, allows them to generate adequate horsepower at useable RPM. Larger straight-six turbo diesels (14-16 liters) in tractor-trailer combos will start to pull really hard at about 1,200 RPM.

560 ft-lb from 409 CID is only 1.37 ft-lb per cubic inch of displacement. NASCAR engines are putting out about 520 ft-lb from a mere 357.65 cubic inches, or 1.45 ft-lb/cubic inch, without a turbocharger.
Formula 1 V-8 engines put out around 214 ft-lb or torque from an engine that displaces 146.457 (2.4 liters) or 1.46 ft-lb/cubic inch, without a turbocharger.

Sounds to me like diesels make a lot of torque mostly because they have a lot of displacement.

Diesels typically have more torque and less peak hp than similar-sized gas engines. For example, the 5.0 Cummins diesel in the new Titan has 310 hp/555 lb-ft, compared to 390 hp/394 lb-ft in the 5.6 gas engine.

We’re still comparing turbocharged diesels to un-turbocharged gasoline engines.

For the VW 2.0 TSI (turbo) gas: 197 hp, 207 lb-ft
TDI 2.0: 181 hp, 280 lb-ft

Edit: Multiply these numbers by 1.5 and you get the specs for the Audi 3.0 gas and diesel engines. Same result.

Another reason diesels produce more torque is due to the high compression ratio (and thus cylinder pressures). This may also be why race engines can be more torquey (high compression with race fuels).

Also, many diesels are designed with longer strokes.

I have to correct you a few things about electric motors. Yes, when AC motors are hooked to raw AC power they will have low start up torque, but have high breakdown torque once the motor gets up to speed. However, with a vector speed drive an AC motor can be modified electronically to produce max torque at zero RPM just like a DC motor. That is why trains are now going AC because the electronics have made it possible to make an AC motor behave like a DC motor.

The the old adage that DC is stronger at start up is no longer true anymore thanks to advancement in electronics.

@ZipZap is correct. AC motors can produce high torque at zero RPM, not max, but then no one wants that anyway. No locomotive nor electric forklift wants to spin the wheels at startup. That causes damage. But variable frequency AC drives can give smooth, powerful, controlled torque and that’s what all the electric car makers are using as did GM’s EV-1 and other electrics from the 90’s.