I understand that torque is what gears the truck moving and horsepower is how fast it it works but I have two questions...

1. When an engine reaches it's peak torque, does it stay at that torque(the engine itself, i'm not talking abou the effect of gears) throughout the rest of the rpm range(as it revs higher) or does the anmount of torque actually start decreasing?

2. Let me see if I have this straight./..If a truck has 300 lbs of torque at 2,500 rpms but only produces 100 horsepower, this means it will take off quick but not be able to pass well right? whereas an engine with 300 lbs of torque and 300 horses will be quick even when passing right?

Thanks for the help...I really wanna learn this stuff....

 

When an engine reaches it's peak torque, does it stay at that torque(the engine itself, i'm not talking abou the effect of gears) throughout the rest of the rpm range?
No.
As the rpms increase, so does friction inside the engine. The maximum torque rpm is the most efficient combination of combustion cylces within a given time period, rotating mass velocity AND piston speed.
Maximum Torque tapers off once past its quoted rpm.

Let me see if I have this straight./..If a truck has 300 lbs of torque at 2,500 rpms but only produces 100 horsepower, this means it will take off quick but not be able to pass well right? whereas an engine with 300 lbs of torque and 300 horses will be quick even when passing right?
The vehicle with 300hp and 300lbft will always be faster, due to the fact it can make more torque through a wider rpm range.
A vehicle with more torque but less power is better for moving heavy loads, more power and less torque is better for speed.

Although the best scenario is more torque AND more power...

 

Lacking a scientific way to phrase my answer, I'd say it like this.

Torque is your grunt. You need torque to make horsepower, but something without horsepower is not going to be fast regardless of the torque.

If you have a truck that is 300 pounds of torque at 1500 rpm, but only 100 hp, it'll tow well from a stop, and it will crawl well. You may be able to start in 2nd or 3rd on a 5 speed without giving it gas. But it won't do anything quickly...

Growing up my family had a 93 cummins dodge 4x4 with a 5 speed and 4.10 gears. It would tow anything as if it didn't have a load. It would go 75 mph, and it would get there VERY slowly. But it would do it just as quickly loaded as not. It had 160 hp and I don't remember how much torque. (A lot.)

 

I've had some practice at explaining this, so hopefully I do a good job.

Horsepower = how fast work is done
Torque = how much work is done

Horsepower, is not a physical measure of any force. Torque, is the physical measure of a twisting force, measured in ft-lbs. Horsepower, is a mathematical calculation, like a formula, with torque and RPM in it. Math is involved here.

Whenever a vehicle is modified to produce more horsepower, what you are actually doing, is increasing either the torque, or the redline RPM, or both, enough so that overall, it's higher then before.

A diesel engine, redlines in the 3500 rpm area (depending on which one), they are known to produce lots of torque, but little horsepower. A tractor trailer diesel, puts out only about 400 hp, but about 1200 ft-lbs of torque. Notice how the low horsepower number goes along with the low RPM redline.

My snowmobile engine has 140 hp, but 95 ft-lbs of torque. It's a 2 cylinder 700 cc engine pushing a 700 lb vehicle. The saturn Ion has 140hp in a 4 cylinder, but that 140hp snowmobile engine would be dieing if it were trying to push that saturn. It doesn't have the torque, the saturn motor has more torque.

An electric table saw....lots of power right? Wrong, it has lots of horsepower, but very little torque. Try to turn that saw on with a piece of wood touching the blades, it doesn't have enough torque to turn the blade when it has a big load on it. When it's on, it's spinning really fast and it uses that momentum to power through cutting a piece of wood. Cutting the wood doesn't require a lot of torque, it requres speed.

See if you can find a dyno graph. A dyno graph will show you what horsepower and torque the engine makes at a particular RPM. An engine like in our trucks, makes different power outputs at different RPMs. At 1200 rpm, my engine may only put out 100 ft-lbs of torque, or even less, but at 3500 rpm it puts out 300 ft-lbs. With a standard transmission, its easy to feel. Say you put it in 2nd gear at 1200 rpm, and burry the pedal. It'll take off kinda slow and lug a little, but as the RPMs climb, so does the amount of torque the motor can produce, so it actually acclerates faster AS it accelerates. Thats the torque band you're feeling.

Every engine has it's own "personality" because of the way it's designed. The cam has a lot to do with it, but depending on how the engine is setup, they run quite differently. With some parts changing I could easily turn my engine into a high hp screamer or low RPM torque monster...

 

Thanks, that's kinda the Idea i had...so why do mud boggers seem to want a high horsepoer engine to spin the tires through mud? I thought torque would better spin heavy tires...if they rev high, they are losing there torque and it seems it would then get harder to spin the tires.

Thanks again yall....

 

One other little piece of trivia: Horsepower will always equal torque at a specific rpm (I think it's 5280rpm). This has to do with the formula used to determine hp and is independant of type of engine. If you could spin a briggs & stratton to 5280 it's torque output would equal it's hp output. This is one way you can tell if a hp/tq dyno graph is correct if someone shows you one during a bench racing session or in a magzine.

Since hp is a function of rpm, the faster you spin the engine the more hp it will make. Since tq dropps off at higher rpms, the tq curve is usually dropping off at 5280 while the hp curve is rising. But hp is also a function of torque. So as tq dropps of at higher rpms, it will start to pull down the hp curve.

 

As mentioned previously, horsepower is just the multiplication of torque of speed.
If you have high torque and high horsepower, you can gear the vehicle down (part the reason mud boggers run rediculous gear ratios, also because of their tires), to effectively create more torque at the wheels, they spin at a lower speed however.

 

Mud boggers run high gears to make up for their tire size, and running a lower (numerically higher) gear over stock will improve power.

Whenever you think horsepower, think acceleration. Whenever you hear torque, think power, strength, brute force.

A tractor trailer can't get out of it's own way, because it only has about 400hp. But it can pull that heavy load because it has 1200 ft-lbs of torque.

A monster truck has about 1500 horsepower, probably around the same in torque. A 10k lb monster truck can go 0-70 in 4.5 or so seconds...it can be that fast because it has so much horsepower and torque.

My snowmobile does the 1/4 mi in 12.7 (about as fast as a viper) using 140hp and 95 ft-lbs of torque. Like I mentioned, if I put that snowmobile engine in a saturn Ion....it won't acclerate that ion very well. It'll try...you'll hear the motor lugging trying to raise it's RPM against that load, but it would struggle. It would struggle because it doesn't have the torque necessary to pull that heavier car (vs the 700lb snowmobile).

Make more sense?
'
Mud boggers want high horsepower and high torque, because they need the torque to have enough power through the manouver, and they want the horsepower so when they crack that throttle, those RPMs get up and the wheels will spin.

Typically, you'll see high hp low torque engines where the load isn't very high (light/quick/nimble vehicle). And you'll see low hp, high torque engines where acceleration isn't important (big rigs, tow trucks, RVs, wood chippers, generators, boat engines etc). Wood chippers never really need quickness in RPM acceleration, since you're just at a constant RPM doing work....so they don't really need a lot of HP.


Not very many people fully grasp the concept of Horsepower and Torque. Horsepower has just somehow become the most familiar to people.

 

exacly why big rigs aren't at drag strips. with around 400 hp and 1800 ftlbs, they will tow tons and tons...and tons, but wont have a good 0-60.

Its pretty basic, horsepower=speed and torque=towing/pulling power. thats why truck motors put out more torque than horsepower, and car motors have around the same hp/toque ratings, generally.

oh yeah happy 21st mustangGt221, find a designated driver for the f150 on 35's!

 

Actually it's 5252 rpm, buy hey who's counting


I think its good that this thread is here. A lot of people just don't understand.
I think its joke the way the car manufactures throw HP numbers around for big trucks and SUV's. I would like to see them show where their truck is producing it's power and what kind of torque is there. Hp is good for mustangs and such, but the "work" vehicles.


Torque is also known as a "moment of inertia". The act where a force will cause a rotation about an axis by another equal opposing force.

 

Actually it is 5250. Torque is a measured value, most often expressed in lb-ft. Horsepower is a calculated value, and is RPM dependent. The formula is:

HP=(Torque x RPM)/5250, where torque is measured in lb-ft., and RPM is revolutions per minute of crankshaft speed at that torque value

So, an engine manufacturer will measure engine torque on a brake at various RPMs, this produces a torque curve for that particular engine. Horsepower is then calculated via the above formula for certain RPMs, and from that set of data a horsepower curve is then plotted. The manufacturer will then typically select the highest calculated HP value, and report that as the horsepower rating of the engine at that RPM.
Now, of course, torque curves can be altered by such things as compression ratio, camshaft profiles, valve sizes, port size and configuration, fuel delivery systems, etc. So,for a truck type application, a manufacturer may select certain parameters which result in maximum torque at a low or lower RPM, to achieve greater "pulling" power at lower vehicle speeds. One can see from the formula, that if torque is maximized at lower engine speeds, horsepower will be lower by virtue of the multiplication implicit in the formula.

 

I'm glad this is here too as apparently I don't understand it well either. So assuming my truck has about 425HP and 770 lb torque it's the HP that is launching me past those ricers and Chevies at red lights? I always thought the torque was the cause of that launch while the HP allowed me to sustain high speeds.

Guess I need to go back and study this again

 

Acutally, HP is RPM as well as torque dependent. In order to see a HP increase in an engine, either the torque or rpm have to change, or both (postively of coarse).

Torque is a measured value but I prefer to say it's a measure of a physical force, where as horsepower is not a measure of a physical force.

Instead, if your truck had 700hp and 770 ft-lbs or torque, your truck would accelerate a heck of a lot faster, but pretty much pull the same load. Thats the reason why I don't like diesels, they're torque biased and I prefer an engine (like a gas engine) that produces near equal torque and near equal horsepower. BOTH are important, you can't say either one is more important than the other.

Horsepower is not what allows you to sustain high speeds. Torque is what does that. Torque is the twisting force of the engine, if you had an engine with 0 horsepower and 700 ftlbs of torque (not possible, but for explanation purposes)....then you'd have no movement. If you had 10 hp and 700ft-lbs of torque...you'd be able to move a lot of stuff but very slowly. Again, horsepower is how fast work is done, torque is how much work is done. Read my posts carefully, it's a lot to grasp in short words.

Again, that 1200 ft-lbs big rig can hall that 80k lbs....pushing out what, 400hp....if you did the opposite, and had 1200 horsepower and 400 ft-lbs...that motor wouldn't pull that 80k lbs....400 ft-lbs wouldn't be enough force. This is the same example as my snowmobile/saturn example.


Another example. Take the 4.6 v-8 in my moms cougar, vs a 4.6L mustang engine.

cougar...205hp / 270ft-lbs....mustang...225hp...300 ft-lbs.

Whats the big difference? The cam...my mom's cougar redlines at 5500 rpm, that mustang will spin to 6200.

Ha I wish I had to, the F-150 doesn't run as of yet. Well, it runs, but badly. lol. In the process of fixing it.

 

Lets not forget how they used to do it in tractors. Load up 2 sleds with a set weight, then hook the tractor to one, and a horse to the other. If it took one horse to do the work of the tractor they said it was "1 horespower".

 

HP is dependant on torque as torque is dependant on HP.

The formula for torque is: T = (5252 * HP) / RPM

So, to answer the question, Yes; as RPM is your divisor, as it goes up your torque will go down.