The Stovebolt Site Forums The Shop Area Engine Shop torque

I'm still dense about the term torque and what it means in practical application. Maybe someone can enlighten me. By the way this is a great site!

A simple example: torque is the force you put out to turn a bolt with a wrench multiplied by the length of the wrench. For a given length wrench, the more force required to turn the bolt (the harder you try to bend the wrench), the highr the torque. Others on the site will correct me if I'm wrong, but power should be the speed at which you can turn that wrench at a given torque.

So as a result, engine torque gives you acceleration, but horsepower gives you top speed. Many other variables affect this relationship: rear ratio, trans, tire size, willingness to rev the engine until it explodes, etc. The amount of torque and power an engine produces is affected by displacement, stroke, cam profile, spark timing, engine breathing, etc, etc, etc. That's why there are sooooo many choices for engine mods out there!

I hope this made the waters a little less murky!

As stated above, torque is a force. Without movement whether linear or rotational, torque alone does NO work, nothing. Torque with some motion (linear or rotational) does work. Torque combined with rpm is horsepower. So I don't think that it is accurate to say that torque gives acceleration and horsepower gives top speed. Horsepower is responsible for all.

If you have a 1 foot long wrench, put it on a nut horizontally, then set a one pound weight on the end of the wrench you are applying one foot pound of torque.

Before I am corrected I realize that the weight of the wrench would have to be considered to be totally accurate. That is the explanation that made me understand it too many years ago.

My 2 Cents..................................Larry

Torque is a Force applied to cause rotation.

1 ft-lb of torque is equal to one pound of force applied 1 foot from the center of rotation and is equal to 12 in-lbs (inch pounds)

1HP = 1 lbft of torque applied at 5252 rpm for 1 minute.

Any measure of power requires a force x velocity over a unit of time.

Torque is converted to force when it is tranformed from rotary motion to linear motion through the use of gearboxes and tractive devices (tires) -

F=ma is a basic newtonian physics relationship, and (F)orce is derived directly from (T)orque through the gearbox and tires. (m)[censored] is how much your truck weighs and (a)cceleration is basically what you are after.

The longer you can apply acceleration, the faster you will go and the faster you go, the more horsepower you require to overcome drag and friction.

Trucks that haul stuff benefit from having low end torque for accelerating heavy loads from a dead stop. Race cars benefit from having torque at higher speeds where the engine will spend most of its time. A big truck with a 500 HP motor rated at 1800 rpm max would be able to move 40 tons easily while a race car rated at 500 HP at 6000 rpm may not even be able to budge the load.

HP is the work to keep a vehicle going at speed.

Torque is the neccesary force to get it going in the first place.

Torque is all but meaningless. Ken you got the definitions right but fail the fundamentals. Proper gearing negates any difference in where the HP peaks. Put a 3:1 gearbox begind the 6000 rpm engine and you have for all practical purposes the same thing as a 500 HP 1800 rpm engine. Would you argue that a gas turbine with 500 HP at 20 krpm couldn't move as much as an equivalent HP engine operating a 1800 rpm??????

Jason, I will have to agree, Ken blew it on the fundamentals here.

Long winded, but here goes:

In order to understand torque and horsepower, you have to first look at the fundamentals. The most fundamental unit in mechanics (in the physics sense, not the car repair sense) is force, typically measured in pounds-force, and usually referred to as simply 'pounds'

In any machine, a certain force is required to overcome static friction and "get things going". After that, the force may be just sufficient to overcome running friction (example being the force applied by the drive axle to maintain a fixed "speed"; it is the force required to overcome friction and wind resistance.). Or, it may be sufficent to provide acceleration. No matter what scenario you analyze in automobile dynamics, it is the force applied to the vehicle by the drive axle that creates all changes in velocity. Large forces create rapid changes in velocity (high acceleration), lower forces create slower changes in velocity (lower acceleration). The net force available to change the vehicle's velocity is the force applied, minus the forces eaten up by friction and wind resistance.

What determines how much acceleration a given (net) force produces? Simply the mass (commonly called weight) of the vehicle. This is where f=ma comes in.

Now, if the machine is powered by a rotating prime mover, torque provides the force. Without torque (which is ultimately converted to a forward force where the "rubber hits the road"), the vehicle can experience no change in velocity. Any fixed value of torque developed in the engine results in a fixed forward force. The relationship between the torque and the force is determined by the transmission and rear axle gearing as well as the tire diameter. We will assume that the tire has enough grip to transfer the force effectively.

Since torque determines force, and since force and weight determines acceleration, torque and weight determines acceleration. Period.

So where does horsepower fit in? Well, so far we have ignored work. Work is simply a force applied over a given distance. If our engine's torque provides a 100 pounds of forward force for 10 feet, the engine has done 1000 foot-pounds of work. The torque provided the effort, but didn't care for how long or how far. It didn't even care if the vehicle moved or not, the torque and force were still developed. If the car didn't move, though, the engine didn't do any work (except for the oil pump, alternator, water pump, etc.).

Horsepower is simply a measure of fast an engine can perform a given amount of work. It is defined as 550 ft-lbs/sec. As an example, (ignoring losses), 100 horsepower can move an object 100 feet each second while delivering a force of 550 lbs. Stated another way, it is how fast the force can move along.

So heres the relationship:
Torque determines the force, horsepower determines at what rate the force can be moved along. Horsepower is the result of torque and motion, and does not exist unless both are present. You can think of it as a measure of how effectively the torque is being put to good use.

Two engines capable of the same horsepower may develop it in different ways. The truck engine delivers a huge amount of torque and provides a huge amount of force. It can do a lot of work, but its low rotational speed limits the rate at which it can perform the work. A high performance car engine delivers much lower torque and force, but can move that force along at a much greater rate. Geared down, it will proved the same amont of torque, hence force, hence acceleration as the truck engine. And vice versa.

Guys - since when does a race car have the gearing to move a semi trailer filled to a legal load? :confused:

Just trying to keep things simple for y'all wouldn't want you to hurt yourself thinking too hard.

nice explanation dukw45

AD - you need to go put some polish on that Taiwanese grill before it starts to rust! :p

Thanks, Ken.

I do agree with you; the horsepower may be the same, but I'd like to see a 400 HP "race engine" pull an 80,000 lb load down the highway for 300,000 miles, or pull the same rig through Wolf Creek Pass without melting down into a glob of steel and aluminum!

I have seen a few cases where a high revving sports car moved a semi, a couple of feet. In all cases the car driver and car did not survive. I could comment on the affect on the gene pool, but I won't!

Well Washmart,
In a nut shell there it is! Now when you go to your next dinner party and the subject comes you can repeat what these guys said and know you now have a full understanding of the subject.

OR

You can find me over in the corner and we'll talk about what colors of paint we like on old trucks.

Buddy in AK

I must add that I appreciate all the explanation there guys! I could not explain it to someone else at this point but the picture is more clear. I'm going to read through it a couple of more times.

Thanks,

Buddy in AK

Watch out now................The experts here jumped all over me a while back when I said a 454 BBC engine could not pull 40 tons up a grade if installed in a semi-tractor..........even with a 13 spd. Road Ranger.
I was told that with low enough gears; it would work.
This may be true, but it would be a slow truck and would burn a lot of fuel for the effort it would have to provide. What about engine longevity?........

Now watch me get shot down again! Jason? Ken?

gmc248: If it were the same HP it would pull as well AND as quickly. As you guessed, it probably wouldn't last as long. Since OTR trucks have lots of engine torque, most assume that it is necessary to move a load. The only place you need torque is at the rear wheels. You can get that torque by having a low rpm engine (which to get reasonable horsepower at low rpm HAS to have high torque) with little gear reduction in the rear, or use a higher rpm engine with proportionately greater gear reduction. The military uses gas turbines because they have the horsepower and are light in weight. They just use a lot more gear reduction. Either way it is the same thing for the same HP. But, horsepower is what moves the load and gives you acceleration. Torque is just a force. You can take a torque wrench, clamp it in a vise, and see that you can have torque and do no work. Strap a rock to the end of the wrench if you want to take a step back. Unless you can apply that torque through rotation, no work is done (your sweat doesn't count!). If there is rotation, work is being done, a truck is moving. The important note is that whenever you have revolutions and torque (any physically realizable system) you have horsepower.

Jason, I agree. I was assuming we were discussing running engines..........
Even an engine at idle has torque.......it is rotating, although the "potential" is low......
I think most people don't realize: Semi-tractors often only have 3.73 to 1 axle ratio's.........all the reduction (Underdrive, Direct and Overdrive) is in the Transmission.........with 2000+ lbs. of torque "potential", this is reasonable; though not so hot at the drag strip................RPM range is limited, with some engines only able to tolerate 3500 RPM max. Fuel economy is the greatest gain of the diesel engine.....considering the displacement of these monsters............You guys ever see a piston from a D8 Caterpillar? That's a big dog........
Running on gasoline, it would be "gallons per mile" (or hour, on heavy equipment) not MPG.............

I think the 454 would do better at the strip, especially with 4.11 gears

Torque wrenches don't do much "work", but I can't work without one!