Torque verse Horespower

[JBlock]

Hi guys,



I need your help on this one.



My friend looking at buying his first towing truck.



How do you explain the difference between Torque and power, agasoline verse diesel power.



Here my try:



Torque usual come at lower RPM, it pulling power, ie when you put you foot to the floor and car just take off, ie horsepower comes in at higher rpm and that where the speed come from.



The dodge cummings real shines on torque guys pull over weight loads up to 3o,ooo pound gross vehicle combined weight (GVCW) and get the truck moving.



Are sled pulls torque or horsepower? Both?



Joe

 

[Speedo]

Torque vs. Horsepower

Torque is what does the work horsepower is the rate at which

the work is done. Take two engines of the same horsepower

but different torque the engine with the higher torque does its

work at a slower speed. An extreme example might be a 600hp

Signature Cummins with over 2000 lb. ft . of torque at less than

1,500 rpm and a 600 hp big block Chevy with a little over 600

lb. ft. of torque at around 5,000 rpm. With proper gearing they

will both do the same job but nobody in their right mind would

own a class eight truck with a 600 hp big block Chevy. Then again

nobody in their right mind would drive a Chevelle with a 600 hp

Signature 600 Cummins... ..... that would be a waste of a good

engine.

 

[Oil Ape]

Without sitting your friend down to a long and confusing physics lesson it will still probably go over his head the first time. But here are the basics:



Horsepower is the engines ability to do "Work,"(Work=Force X distance) in a particular amount of time(Power=Work / Time). In laman's terms, the crankshafts ability to rotate with a given amount of force being delivered by the engine in a certain amount of time. Think of this as a vehicles momentum. It allows you to get off of the line and cruise fairly nicely, granted the work being required does not exceed the engines ability to provide that work in that amount of time. If the vehicle is loaded beyond the horsepowers ability to keep up, the vehicle rapidly loses its ability to maintain the momentum and the vehicle slows down. That is why gas engines with large loads quickly decelerate on hills.



Torque on the other hand is the amount of rotation force that is applied and has nothing to do with time. By definition it is the circular force on a rotational body(crank shaft in this case) that is found by taking a specific amount of force(force from the connecting rod) and multipling that number to the distance from the rotational body(crankshaft offset). In laman's terms once the horsepower can't handle the load the torque keeps the crankshaft turning. That is why when you hit a hill with a diesel which far exceeds the torque of a typical gas engine, it decelerates quickly from the lack of horsepower down to the point that the torque takes over and allows you to maintain speed.



Horsepower, in other words, helps you gain speed while torque allows you to keep your speed. Most engines gas or diesel have just enough horsepower to get themselves moving. Under a load the diesels have the advantage of being able to run in a much higher gear thus giving them an efficiency advantage(better gas mileage, ability to tow at higher speeds on hills). Not only that but diesels have that torque at lower speeds which allows them once they have lost the horsepower battle they still may be gaining torque. Gas engines often have the torques at higher rpms and once those are gone you are left putting up the hills in your lowest gears.



This is a lot to digest... and I hope I didn't confuse you more... .

 

[RustyJC]

Real world example - our previous Dodge 3500 V-10, automatic, 3. 54 rear axle ratio was rated at 300 BHP @ 4000 RPM & 450 lb-ft torque @ 2800 RPM. Pulling our 5ver (see signature) on a given 6% grade in the Texas Hill Country, it was in 2nd gear at 3500 RPM and full throttle to hold 50-55 MPH. Our current truck (again, see signature) pulls the same 5ver up the same hill at 65-70 MPH in 6th gear at 2/3 throttle, even though it has less rated BHP, even with the EZ. The difference is the peak torque and where it falls in the engine's operating RPM band.



Rusty

 

[Joe G.]

HP vs torque can sound complicated but it's not. Torque is the measurement of power. A little mathemagic is used to calculate HP from the torque and RPM. Note that below 3300 RPM the torque number for a particular engine is higher than the HP. Above 3300 RPM the HP number is higher. So if you look at the RPM you are normally running at then the important number is torque. That's power to move something.



A big Cat diesel I helped do some work on in a drag boat had a red line of 900 RPM. Not much good on the road, but it really would pull. It was even geared down so the propeller turned really slow. Big propeller moved a lot of water and towed tons of fish in a net. A viper engine would not have moved the boat.



Torque is brute force and HP is how fast you can apply that brute force.

 

[Archive]

Go to DD web site and look at the HP and the Torque cures for the different trucks. . Look at the Dmax cures they are very up and down. . the Dmax has good HP and fair torque at 3200rpms, but it falls fast. . When you are pulling up a hill you want the torque to still be there if the rpms drop. . this is why the Allison in the Dmax shifts so early, because the engine starts falling down its torque curve. . The Cummins curve is very flat, (stock) and holds close to its max torque enough its rpms maybe dropping. (This is called torque rise. ) The powerstroke is better than the Dmax, but still not as good as the Cummins.

Good luck

Bryan

 

[JBlock]

Thanks for all the quick posts

Again thank you



For all the quick posts and good explantions.



JOe

 

[Car_nut57]

Torque is the amount of power available and horse power is the rate the power can be applied. This is a little over simplified, but may make sense when comparing to a gas engine which makes more horse power than the diesel but cannot pull near as much because its torque is much lower and at a higher rpm.

 

[PRout]

Originally posted by Dave - Diesel Power Products

Torque is the amount of power available and horse power is the rate the power can be applied. This is a little over simplified, but may make sense when comparing to a gas engine which makes more horse power than the diesel but cannot pull near as much because its torque is much lower and at a higher rpm.

Another point, is an advantage of gas motors for drag racing is that a gas motor makes its HP at higher rpms, so it can take advantage of the lower gears longer.



The lower gear multiplies the power to the wheels, so its almost like free power. The longer you can stay in the lower gear down the track, the more power you had to the wheels. Thats high RPM, or HP. If your motor ran out of HP (like ours) at high R's you have to shift, therefore using a gear w/ less leverage, or less power multiplier. Less HP to the wheels. Thats why screamer motors w/ low gears turn good drag race times. Thats why HP is related to speed.



That same motor does not work well combined w/ taller gears, because they dont have the torque at lower rpms to pull that taller gear.



Thats where the diesel's low end torque comes in. It will get & keep a load moving easier from lower rpms, and are heavy enough to do it for a half million miles, without screaming itself to death. And, efficiently, since one gallon of diesel fuel has more BTU's than a gallon of gas, meaning more power, better mileage, if you can get it to burn (needs lots of oxygen).



A drag racer wont pull heavy load very long, or very well from a stop at the bottom of a hill compared to the brute force off the bottom that our Beautiful cast-iron inline 6 CUMMINS motors will.



Thats why our motors just keep pulling, no matter what gear its in. Torque, its the REAL number. HP is just a calculation once you measure the torque. Multiply the torque times the RPM, and you have a new number - HP. Its basically "virtual" power, torque is the REAL thing. Yea, like Coke & Cummins. NOT wussymax, that has to rev like a gasser to make its HP (remember, HP is "virtual" power). (Just had to get that shot in)



Added: Thats the theory of the Dmax is that since their motors make power at higher revs, they can stay in the lower gear longer, making more power to the wheels due to the leverage of the lower gear. It does work, at speed, but doesnt help much pulling heavy from a stop on a hill. Thats where the Brute torque shines.



Ok. . Whos next?

 

[Diesel Freak]

Torque = FtxLbf = Energy

Horsepower = (FtxLbf)/sec = Power



Torque is the derivative of Horsepower with respect to time.



here is a very basic article in the subject.



http://www.findarticles.com/cf_offrd/m1185/n11_v31/19833126/p1/article.jhtml

 

[jwilliams3]

Joe G.

I thought the breaking point for a particular engines Horse Power exceeding its torque was 5252, not 3300?

Im confused now!!



Well anyways the equation for calculating horse power.

TorqueXRPM=x/5252=HP



For instance my stock flywheel horsepower from my 99 Cummins is 215HP.

To get this number you multiply 420ft lbs X 2700rpm=1134000

divide this by 5252 and you arrive at 215 Horse power.



Someone tell me if Im wrong here!

 

[Joe G.]

The 5252 funny number constant is derived from having to change measurement units. I don't have the calculation handy so I won't try to show it. Part of the problem is RPM = Revolutions Per Minute and torque is measured in Foot Lbs. One horse power is something like 3300 feet in one minute. Something like that. So in order to have both sides of the equation dealing with comparable units you have to do some unit converson. Thus the funny number 5252. At 3300 RPM the funny business cancels out so HP = torque. Maybe someone will chime in with the derivation of the 5252 number.

 

[jlccc]

where 5252 comes from

The constant 5252 comes from the following unit conversion:

550 ft-lbs/sec  60 sec   1 rev

--------------  X  ------  X  ---------------

 1 hp   1 min   2 * pi radians



550 * 60

--------  = 5252

 2 * pi



So, in the example above:



420*2700*2*pi 420*2700

---------------	= 215hp = --------

   550*60 5252

 

[Ramafid]

I think one horsepower equals 33,000 lbs of work done in one minute, not 3,300.

 

[Speedo]

5252rpm.

Horsepower and torque are equal at 5252 rpm.

 

[rrausch]

I think of it this way:

Torque is force

H. P. is force done over a certain amount of time

 

[jlccc]

Originally posted by rrausch

I think of it this way:

Torque is force

H. P. is force done over a certain amount of time

Gentlemen:

Words have meanings. Torque is NOT synonymous with Force; neither is Power. Pick up a high school physics book and spend 1/2 an hour with it and quit this incessant misuse of terms which does nothing but obscure the issue.



Work is Force X Distance and can be related to Torque as they both can have units of ft-lbs.



Power is the first derivative of Work with respect to Time. For steady state applications, this reduces to Power = Work/Time.

 

[Groover]

I believe the origin of this thread was how best to compare the pulling power of different truck engines. Factory torque and Hp numbers are useless without knowing at what RPM they produce that power.



The best way to compare is to overlay the HP graphs on top of eachother. In my opinion HP is the important # if calculated for both trucks at the same RPM. And look at the RPM range YOU will be pulling in (e. g. 1800-2500 RPM).



Example 99 dodge manual = 235hp/460tq

Duramax = 300 hp/don't list torque

I got a dyno graph of each and overlaid them on eachother. The duramax PEAK hp is impressive BUT it doesn't even catch up to the cummins until around 2500 RPM and then hp quickly rises for the duramax as the cummins begins to fall off.



That tells me that unless I want to scream the RPM the cummins will outpull the Dmax. I personally rarely exceed 2500 RPM.



Just my take on things!