Low end torque is low end horsepower

[Stelth]

Not sure why this never really clicked in my mind, probably because of all the weird theories and superstitions I've heard over the years about torque and horsepower, but the recent thread about flat torque curves did it.

I've heard so many people say so many different things: "OK, when you're starting from a dead stop, that's when you need torque." Or, "For towing, you really need an engine with lots of torque." IOW, for some things you need horsepower, for other things you need torque. Well, please pardon me if I say, "Horsefeathers."

When we talk about engines, torque IS horsepower, because of the formula torque x rpm /5252 = hp. Engine torque is calculated at a particular rpm which means it's not static torque.

I believe the current 6.7L Dodge/Cummins diesel makes 350 horsepower at 3013 rpm. It makes 800 ft. lbs. of torque at 1600 rpm. That's a lot of low end torque, but it's also 243 horsepower. 243 horsepower at 1600 is also a LOT of low end horsepower.

I remember years ago seeing a rating plate on a Cummins diesel in an over-the-road tractor. It said the engine made 350 hp at 2000 rpm and 1400 ft. lbs. at 1200 rpm. That's about 320 hp at 1200 rpm.

So whether you say you want low-end torque, or low-end horsepower, or low-end grunt or whatever, it's all the same thing.

 

[friendly_jacek]

The difference about low RPM torque peak vs high RPM torque peak is how much downshifting you need to do while driving, thus low RPM torque is very valuable in daily driving. As for high speed driving, power is all one needs to have.

 

[mechtech2]

Turbo truck diesels skew RPM numbers vs. passenger cars.
They operate at low RPM and with a limited redline.
So 1,200-1,400 is not low to them.

Most street cars love low end torque. Road racers like mid torque and top end HP.
Drag racers need high RPM HP.
Engines are indeed biased or compromised.

 

[Food Scientist]

Originally Posted By: Stelth

So whether you say you want low-end torque, or low-end horsepower, or low-end grunt or whatever, it's all the same thing.


That is so far from the truth. Low end torque is not low end horsepower. Have you ever seen high horsepower cars vs high end motorcycles? Its never from a dead stop (at least on a street race) because the cars make so much torque compared to the motorcycle at low rpms the motorcycle will get beat off the line. An easy way to tell there is a difference is get a vehicle that makes more mid range torque vs a vehicle that makes low end torque and compared the 0-60 times and the 60 ft times and the low end torque will be better with everything being equal.

Think of this way - big rigs need a lot of torque to get 80,000 lbs moving from a dead stop, which is why diesel pickups that make a lot more horsepower than the 18 wheeler can never move the load because they lack the vast amount of torque that a 18 wheeler can generate.

AC electrical motors make all of their torque at the low end (I have seen motors that generate 100% of their torque from 0-400 rpm on a 1750 rpm motor) and very little horsepower and these have no problems moving large loads when first started, which proves that torque and horsepower regardless of rpm are not the same.

The rule of thumb for racing is use a motor that produces high amounts of torque at low rpm (usually a big block) and use a cam that produces high horsepower because torque gets you started, but horsepower is what counts when the vehicle is up to speed.

 

[yonyon]

The benefit of a flat torque curve depends a lot on the type of transmission used. With a trans that provides close ratios and quick shifts the flat curve becomes less of an issue. A slippy thing like a torque converter can make low end torque and a flat curve less important compared to a traditional manual with a high first gear provided it can shift snappily. If your trans can't quickly get the engine speed close to optimal and keep it there, a peaky torque curve looks less pretty.

 

[morris]

i cant say the above posts are wrong cause they are not. but to best under stand horse power, the key is in its name. look up james Watt. he set down what horsepower is. just read his work.

 

[Stelth]

Originally Posted By: SVTCobra
Originally Posted By: Stelth

So whether you say you want low-end torque, or low-end horsepower, or low-end grunt or whatever, it's all the same thing.


That is so far from the truth. Low end torque is not low end horsepower.


It is the truth. Horsepower is a function of torque vs. rpm. That's it.

Quote:
Think of this way - big rigs need a lot of torque to get 80,000 lbs moving from a dead stop, which is why diesel pickups that make a lot more horsepower than the 18 wheeler can never move the load because they lack the vast amount of torque that a 18 wheeler can generate.


No, they don't need torque, they need power. Torque is force. If you put 1000 ft. lbs. of force against something and it doesn't move, there's no horsepower. In fact, with a torsion bar suspension, torque is what's holding the car up. However, with the car sitting still, that torque isn't making any power. On the other hand, if you're driving, and hit a bump, then power is exerted into that torsion bar, and the torsion bar exerts power pushing the wheel down (and the car up) afterwards.


Quote:
AC electrical motors make all of their torque at the low end (I have seen motors that generate 100% of their torque from 0-400 rpm on a 1750 rpm motor) and very little horsepower and these have no problems moving large loads when first started, which proves that torque and horsepower regardless of rpm are not the same.


I'd really like to see the torque and power curves on the motors you're talking about. Nonetheless, if the motor makes the same torque at 1750 rpm that it makes at 400 rpm, it's making more power at 1750 rpm.

Quote:
The rule of thumb for racing is use a motor that produces high amounts of torque at low rpm (usually a big block) and use a cam that produces high horsepower because torque gets you started, but horsepower is what counts when the vehicle is up to speed.


Well, I'm not a racer. But the Chevy 302 from the 1969 Z-28 certainly didn't follow this rule, with a torque peak at 4200 rpm. But they did well in the Trans Am series.

 

[Shannow]

Originally Posted By: mechtech2
Most street cars love low end torque. Road racers like mid torque and top end HP.
Drag racers need high RPM HP.
Engines are indeed biased or compromised.


For any normal(ish) sort of engine, the torque is related to the capacity, and BMEP...that's largely constant.

Moving the torque higher in the rev range produces more power from that torque.

 

[Stelth]

Originally Posted By: Shannow
Originally Posted By: mechtech2
Most street cars love low end torque. Road racers like mid torque and top end HP.
Drag racers need high RPM HP.
Engines are indeed biased or compromised.


For any normal(ish) sort of engine, the torque is related to the capacity, and BMEP...that's largely constant.

Moving the torque higher in the rev range produces more power from that torque.


Yes. A Formula 1 engine that makes 740 hp at 19,000 rpm does it with 205 ft. lbs. of torque. If you had an engine that made 205 ft. lbs of torque at 10,000 rpm, you would have 390 hp. At 5000 rpm, that 205 ft. lbs. would give you 195 hp. And so on. So at any rpm, the more torque you have, the more horsepower (or kilowatts for our European friends) you have.

 

[OVERKILL]

Originally Posted By: Stelth
Originally Posted By: Shannow
Originally Posted By: mechtech2
Most street cars love low end torque. Road racers like mid torque and top end HP.
Drag racers need high RPM HP.
Engines are indeed biased or compromised.


For any normal(ish) sort of engine, the torque is related to the capacity, and BMEP...that's largely constant.

Moving the torque higher in the rev range produces more power from that torque.


Yes. A Formula 1 engine that makes 740 hp at 19,000 rpm does it with 205 ft. lbs. of torque. If you had an engine that made 205 ft. lbs of torque at 10,000 rpm, you would have 390 hp. At 5000 rpm, that 205 ft. lbs. would give you 195 hp. And so on. So at any rpm, the more torque you have, the more horsepower (or kilowatts for our European friends) you have.


Certainly, that's because RPM is the RATE at which work is being performed.

Torque is force.
Horsepower is work.

The rate at which an amount of force is applied to perform work is RPM. The faster the work is being performed, the less force that is required to perform the same amount of work in a given period of time.

Of course one can have force without work. If you've got an electric motor that makes 500lb-ft of torque, but is locked dead against a mass that requires 550lb-ft of torque to move, you are applying force, but no work is being performed (no HP is being generated). I believe another poster touched on this earlier briefly.

 

[Cujet]

Stunningly simple concept. Impossible for some to describe succinctly

 

[Stelth]

Originally Posted By: Cujet
Stunningly simple concept. Impossible for some to describe succinctly


Yes. That was my epiphany - after decades of hearing that torque is one thing, and horsepower is another, seeing the plain, simple relationship.

 

[mechtech2]

The final determinate is the gearing in a car.
It's the torque at the wheels that counts [for acceleration].
It is why a CVT trans car at full throttle keeps the RPM steady at the torque peak, and varies the gearing.
And it's why we downshift to multiply wheel torque.

 

[yonyon]

True that.

With a CVT and drive-by-wire throttle the accelerator pedal becomes "power demand" control for input to the computer instead of a direct throttle control. With modern engines with variable valve timing and maybe even a turbocharger it becomes possible for the computer to deliver power on demand and still achieve excellent fuel economy for that particular requested power.

I'll have to try to remember that the next time I start cussing about these #$%$^~@!^ automotive engineers putting that *#@ bolt right behind that bracket that I can't remove until said bolt is already out.

 

[Ben99GT]

Originally Posted By: mechtech2
It is why a CVT trans car at full throttle keeps the RPM at the torque peak, and varies the gearing.


Nope, a CVT at WOT will rev to the point of peak horsepower everytime.

 

[Ben99GT]

For example:



Altima 3.5 - 270 HP @ 6000 rpm, 258 LB-FT @ 4400 rpm



Duratec 3.0 - 203 HP @ 5750 rpm, 207 LB-FT @ 4500 rpm



Subaru 2.5 - 170 HP @ 5600 rpm, 170 LB-FT @ 4000 rpm

 

[Kiwi_ME]

Originally Posted By: OVERK1LL
...that's because RPM is the RATE at which work is being performed.
Torque is force.
Horsepower is work.

The rate at which an amount of force is applied to perform work is RPM. The faster the work is being performed, the less force that is required to perform the same amount of work in a given period of time.
to clarify:
1) RPM does not imply that work is being done. It's happy all by itself.
2) Horsepower is work done per unit time. Work done is simply a fixed amount of energy.

 

[Stelth]

Originally Posted By: Kiwi_ME
Originally Posted By: OVERK1LL
...that's because RPM is the RATE at which work is being performed.
Torque is force.
Horsepower is work.

The rate at which an amount of force is applied to perform work is RPM. The faster the work is being performed, the less force that is required to perform the same amount of work in a given period of time.
to clarify:
1) RPM does not imply that work is being done. It's happy all by itself.
2) Horsepower is work done per unit time. Work done is simply a fixed amount of energy.


Let me see if I understand you correctly. In 1) an engine could be spinning away in neutral at any rpm, and beyond what's taking place in the engine, no work is happening, right? But at the same time, there's no torque measurement, because that would imply a load.

In 2) horsepower is like wattage? IOW, the amount of electrical power used is measured in watt-hours, so the amount of IC engine power used would be horsepower-hours? (or possibly watt hours, depending on what scale you're using)

 

[Shannow]

Originally Posted By: Ben99GT
Originally Posted By: mechtech2
It is why a CVT trans car at full throttle keeps the RPM at the torque peak, and varies the gearing.


Nope, a CVT at WOT will rev to the point of peak horsepower everytime.


Yep, having all of the available power, and sufficient gearing to match that RPM to road speed gives maximum tractive effort.

(I would assume that light to moderate would be at max torque 'though, as that's most efficient point in the BSFC Map usually...dunno, haven't driven that many CVTs to know 'though).

 

[OVERKILL]

Originally Posted By: Kiwi_ME
Originally Posted By: OVERK1LL
...that's because RPM is the RATE at which work is being performed.
Torque is force.
Horsepower is work.

The rate at which an amount of force is applied to perform work is RPM. The faster the work is being performed, the less force that is required to perform the same amount of work in a given period of time.
to clarify:
1) RPM does not imply that work is being done. It's happy all by itself.
2) Horsepower is work done per unit time. Work done is simply a fixed amount of energy.


OK, but then this is where we determine how pedantic we want to be here

1. In order to have RPM, we must have force (torque) and if any force is being applied and it causes movement, we are performing work, no matter how minute that amount of work is to create that RPM.

2. Certainly. In the case of unit time with a gasoline engine, we are speaking of RPM. 200lb-ft applied at a rate of 3,000RPM for example. This would yield 114HP.