Is Horsepower the whole story?

[magicheater]

I've seen this formula many times, and in theory it is probably correct, but I have a problem understanding how a "fixed" formula results in different curves for HP and torque when run on a dyno. Typically both curves are very different and not linearaly related.

Torque is measured directly and H.P. is calculated.

 

[carpenter383]

I wouldn't be suprized if the box store lawn mower engines are over rated, just like air compressor motors being over rated.

 

[CharlieR]

I believe in the old saying - "there is no replacement for displacement".
Or - "big and slow makes it go".
HP is a calculated number with RPM being a very big variable often over utilized for advertising purposes.

I'd look for the slower running, higher displacement engine any day!:thumbsup:

 

[RoyJackson]

I'd look for the slower running, higher displacement engine any day!

Me too, but that's not what you're going to see in most CUTs and subCUTs.

 

[SPYDERLK]

The formula for HP is: HP = (( Tq x RPM) / 5252 ) => Measured Tq times the RPM at that Tq divided by 5252 = HP produced at the measured RPM.

For example, two engines, (G) = Gas, (D) = Diesel with the same HP rating....

G: HP = 20 @ 3600 rpm, so what's the TQ? Tq = (( HP x 5252) / 3600 ) = 29.177 lb-ft @ 3500 rpm.

D: HP = 20 @ 2400 rpm, so the Tq is... Tq = (( HP x 5252) / 2400 ) = 43.766 lb-ft @ 2400 rpm.

Now, if you wanted to move something heavy, would you have an esier time using 30 lb-ft of of Tq or 44 lb-ft?

Well, for ease it really only depends on the spread of rpm over which a flat torque curve exists. [Id call it tractability actually.] The rest is gearing. See below and last Quote.

I've seen this formula many times, and in theory it is probably correct, but I have a problem understanding how a "fixed" formula results in different curves for HP and torque when run on a dyno. Typically both curves are very different and not linearaly related.

Above comment and last Quote.

Here's how I keep Torque and HP in perspective:
Bear with me on this ...

Torque is a unit of Work: It's a force acting through a distance.
I like to think of this as carrying a 100 pound box up a 10 ft ladder.
100 pounds = force (force of gravity is pulling it down)
10 ft = distance (distance you have to work against gravity)
If you dont have enough torque, you will stall out before you reach the top of the ladder and not get anything done.

Horsepower is a unit of Power : Power is the time rate of doing work.
HP tells you how many times you can carry that 100 pound weight up that 10 ft ladder in an hour. So power is important, too, if you want to finish before sundown.

So you need enough torque to get the job done at any speed you choose. As was mentioned by others, most Diesels have lots of torque over a wide range of speed, so they can usually get the job done over a range of lower speeds. Gas engines (at least most big box lawnmower engines) usually develop their claimed HP at some high rev speed, and only at that speed. Through the rest of their lower speed range they can be very weak. So you have to keep them pegged to get any heavy work done. They usually don't last very long when run that way - something breaks.
-Jim

Careful ... you are describing energy not torque. The do not relate. This confusion is what has led standards makers to start stating the twisting force, torque, as lb-ft ... so it wouldnt get confused with the ft-lb unit of energy. A ftlb will raise a pound a foot above ground. A ftlb of torque will exert [no motion] a pound of force at a distance of 1foot from the pivot point.

Torque is important (more so then engine HP, IMHO), but don't forget gearing. It's that low gearing (combined with the torque) that allows ShenandoahJoe's 16 HP engine to pull that 4' cutter up a steep slope.

HP ratings are pretty deceptive. Most are gross HP measured with no load on the engine. Some are "estimated" and are what the engine should develop (in theory). For tractors, drawbar and PTO HP ratings are a much better indicator of what the machine can do.

Lotsa good information here and Ive seen some that seem to have it perfect. Stiil its hard to discuss with full clarity without everybody understanding all the key issues. I know that can seem real tedious until the light flashes on.:thumbsup:
The best answer I can give to the thread title is, It depends how immediately responsive the transmission is...:confused2:
larry

 

[ModMech]

No real disagreement to what's been posted thus far.

Remeber, "gearing" is by definition, Tq MULTIPLICATION.

For example, let's take a 24 HP tractor rated at 19 PTOHP at 2400 engine RPM.

At 540 RPM and 19 HP you have: ( 19 x 5252)/540 ) = 184.8 lb-ft @ PTO

The engine is not capeable of 184.8 lb-ft so where did that come from? Gearing! The engine makes ( 24 x 5252)/2400 ) = 52.52 lb-ft @ 2400 rpm (rated power).

2400/540 = 4.44:1 (theoretical tq multiplication assuming no frictional losses)
184.8/52.52 = 3.52:1 (measured tq multiplication after frictional losses)

HP cannot be measured, no matter the method, HP is a calculation based on Tq and RPM. If Tq = 0, HP = zero.

Those that mentioned a HP or Tq "curve" are completely correct. Neither Tq nor HP is constant, but varies over the engine RPM band (at full power). None the less, HP is calculated from the measured Tq at any given point in the "curve" (specific engine RPM).

 

[CrownHawg]

I just bought a new Husquvarna rear tine tiller. It states "9 lb pounds torque", but I could not find anywhere the HP rating. I guess B&S is now listing torque, not HP. When I bought this thing, I thought it was a 9HP, thinking that is plenty for a tiller. Still not sure, but it seems to do ok.

 

[Flatheadyoungin]

When I used to work at a outdoor power equipment store/hardware store, we used to always say, "HP sells, torque preforms."

 

[dodge man]

Think of a world class power lifter and a world class sprinter. Have them each carry a 10 pound bag of sand on there shoulders and have them run the 100 meter dash. The sprinter would finish first, and since they both carried 10 pounds, but the sprinter moved it faster, he had more horsepower. Give them each a 80 pound bag of sand and have them run 100 meters. Say they finish at the same time. They both did the same amount of work in the same time, same horsepower. Give them both a 200 pound bag of sand, the sprinter can't pick it up and move it, the power lifter can pick it up and move it, he can run the entire 100 meters if he wanted to. The power lifter has more torque than the sprinter, he has the ability to pick up the 200 pound bag of sand and move it.

A diesel engine is kind of the power lifter and a high rpm gas engine is the sprinter. Probably a bad example, since torque is truely a twisting motion, but you get the idea.

 

[orezok]

Given the formula posted, it then means that torque and HP are always equal at 5252 RPM. Interesting.