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Bren wrote:

This one has bothered me forever, and I'm not even sure how to articulate it.

I guess the question might be:

a) What is the relationship between torque and horsepower?

or maybe

b) How are torque and horsepower the same, and how are they different?

or perhaps

c) How is it possible for an engine's peak horsepower to be reduced by an adjustment which increases its maximum torque, and vice-versa?

I understand that torque is a measurement of the ability to twist a lever (I think) and power is a measurement of the ability to do work (I think), but how is it that these are different things?

How is it possible that various diesel-engined Volkswagens, for example, have almost no horsepower, but mountains of torque? Conversely, how do you explain the relatively high horsepower ratings on various Acura and Ferrari motors, despite their relatively low torque ratings?

It seem intuitively logical that a motor with high torque would also naturally have high horsepower, but that's not always the case. I've read explanations of what each term means, but I'm still confused. Can anybody 'splain it to me in terms that will make sense to my little brain?

Thankew.

One rule of thumb is that other things being equal, diesel torque > gasoline torque and a larger engine will put out more torque than a smaller engine.

Tallgeese, I know those things already. That doesn't address my confusion, but thanks for making the effort.

Mo' ron, that helps; I've never seen it explained quite that way. I think I'm still confused, though. When a manufacturer releases a given car's horsepower rating, they don't say it has X amount of horsepower in one gear or another, which I think you've explained. They just say it has X amount of horsepower at X rpm.

But how this is the same as torque divided by time, I do not know. Very confusing.

Last edited by Bren (2009-08-29 11:33 pm)

When you shift gears, your horsepower isn't going to change. Instead, what you are changing is the ratio of torque and RPM. If you have a higher RPM, you're going to have less torque. That is, it will turn faster, but it can't move as much mass with a single turn. On the other hand, if you have a lower RPM you're going to have more torque. It will turn slower, but more mass can be moved with a single turn.

At the end of the day, regardless of your gear ratio you can get the same theoretical amount of work done. (since the horsepower is the same) However, there are cases where you want the balance of torque vs. RPM in one direction or the other. For example, when going up a hill, you want the higher torque to make sure you can haul your car's mass since less torque won't get the job done. On the other hand, when you're cruising on the freeway, you don't need much torque so it's better just to have a higher RPM to allow you to go faster.

Note that in actual real-life driving, you are also varying the amount of horespower you're using based on how much you press on the gas pedal.

I still don't understand how or why a given engine might have a whole lot of horsepower, but very little torque, say, in the case of the various Acura Integra GS-R engines, while another engine, such as that found in many varieties of diesel VW Jetta in bygone years, might have a really healthy amount of torque, and very little HP.

Shouldn't high torque and high horsepower be inextricably linked? Shouldn't a motor with very little torque also have very little HP?

This is clearly not always the case, but I don't get why.

ADDITIONALLY

A diesel, cubic inch for cubic inch, typically has a longer stroke and smaller bore than a gasoline engine. Also, the high compression ratios for a diesel engine are the root of its high torque numbers.
One thing to remember, though, is that the torque curve for a diesel engine is highest at the lowest RPM and starts to drop off at higher revolutions. Diesels have superb low-end torque but it really drops off in the top end. Gasoline engines have a power band closer to the middle of the operating range.

It's been a while since I took physics, but the way I think about it is that torque moves the car and horsepower moves it fast.

Before I took the Buick across the country, I put in a new transmission and rear end. I upped the final ratio from 2.41 to 3.42, boosting my torque by about 70%. When I crossed the mountains, I had to downshift just to maintain 50 MPH up some of the 10 mile 7% grades. The horsepower was the same and it was still slow as smurf. However, if I hadn't boosted the torque it just wouldn't have been able to haul up those grades at all (okay, maybe at 30). The torque got the mass moving, but the horsepower wasn't there to get it moving quickly.

Lets put it this way Bren. Try towing a boat with your imaginary Ferrari, you get loads of HP and little torque. Now lets take a Dodge Ram Diesel. Here you have loads of torque and modest HP. Which car will be able to tow the boat easily. The answer is the Ram. That's because of the increased torque that has the ability to move more weight. More horsepower means more speed. If you have an engine with lots of torque and HP, like the Cayenne Turbo, you can tow the boat easily and at a quick pace.

If my little sister can undertand that explanation then you should be able to understand it too.

gd wrote:

Try towing a boat with your imaginary Ferrari, you get loads of HP and little torque. Now lets take a Dodge Ram Diesel. Here you have loads of torque and modest HP.

This isn't quite correct. The difference isn't so much the horsepower, but whether the engine and gears are set up to give you more torque or RPM. Think of horsepower as RPM times torque. Therefore, you can have one car with 2x the torque but 1/2 the RPMs of another car, but their horsepower will be exactly the same.

Higher torque is going to give you the ability to haul more weight, while more PRMs is going to let you move faster. For example, a pickup truck can haul a lot of weight, but can't go very fast. Its engine is set up to provide more torque, but less RPMs. On the other hand, a Ferrari is built to go fast, and the car itself is light. Since it has to carry very little weight in comparison, they can tune the engine to give you more PRMs and less torque.

Note that you still have to have a decent amount of torque to accelerate quickly. While higher RPMs lets you go faster, it will take longer to get to your maximum speed. For a car such as a Ferrari, they have to balance it carefully so you get high enough torque to accelerate quickly, but also give a high enough RPM so you can have a faster maximum speed.

To give a quick summary:
- horsepower = PRM x torque
- RPM allows you to go faster.
- torque gives you more force, which gives greater acceleration and the ability to haul more mass. (because force = mass x acceleration)

Speed and acceleration are often used synonymously, but they are very different. In this case, if you have high torque but low RPM, you can have a very high acceleration, but your top speed isn't going to be very high. On the other hand, if you have a high RPM but low torque, you're going to have a much higher top speed, but it will take you a while to get there.

In case you didn't understand all of that, I found an article online that explains it. (http://craig.backfire.ca/pages/autos/horsepower)

Your's were but akb's weren't

Bren wrote:

Tallgeese wrote:

I put in a new transmission and rear end. I upped the final ratio from 2.41 to 3.42, boosting my torque by about 70%.

Wait a minute. I understand that you're saying this change allowed you to put more torque to the ground, but did it really boost your torque? Manufacturers of traditional cars like your Regal were known to offer different final drive gearing as options, but ordering the optional gearing didn't alter the factory's torque rating.

... why'dya think drag/street racers always picked ratios in the 3.73:1-4.56:1 range? It doesn't alter the engine output, it increases the torque thru the driveshaft, to the wheels. (It's their- suspension, tires etc. job to transmit that to the pavement). The cost is shifting your whole power band incl. top speed downwards a corresponding amount.

2.42:1 is very long, mileage-oriented gearing. 3.42 in much more moderate- lose some highway mileage, gain a good deal of moxie around town.

Maybe I can help--(or maybe not).  The relation between torque and horsepower is like the relation between speed and acceleration.  (Torque and horsepower is slightly more complicated, one is not the always derivative of the other, but while using the same gears, they are)

Just as a vehicle with higher acceleration can change speeds more easily, a vehicle with higher torque can distribute horsepower better.

EDIT: Or to reduce it to practical terms, a tractor needs a ton of torque, its carrying or pushing huge loads across and through dirt, but it doesn't need a lot of horsepower because its not going to go fast.

Last edited by Metacell (2009-08-30 3:11 pm)