torque vs horsepower

[mooleycow]

what is the correlation of torque and horsepower?  how much torque equals 1 hp. on a gasoline engine ie for instance a lawnmower?

[brdmkr]

You know, I would be interested in seeing someone address this too.  I don't think there is a standard relationship.  Some engines don't really have that much HP, but may have tons of torque (thinking about old jeeps with a straight 6).  Others have plent of HP, but not so much torque.  I am sure there is a relationship, but I'll bet it is not straightforward. 

[pineywoods]

There is a direct relationship between hp and torque. HP=torque X rpm with a few fudge factors thrown in.  Look at it this way. A 2 to 1 speed reduction (gears, belts etc) will double the torque but cut the rpm in half = same horsepower. The torque and horsepower ratings on all engine are MAXIMUM ratings. Real world is usually considerably less.

[shelbycharger400]

this is a very difficult question to answer,  im not shure what application you are trying to get at.
as with any engine, their is peaks and valleys. also, like my 2.4 in my plymouth breeze, (really noticed it when i put a 2.4 in a neon) their is 2 power bands, with the gear ratios and tire sizes in play, i had a powerband from 40 to 55, then another noticeable one from 65 to 80 something.

some advertise HP, some torque.   I have driven cars and trucks that say $$$ HP, and it was like a rubber band, NO TORQUE.    small bore long stroke engines dont make Usable torque  at NORMAL rpm.  torque is where its at. now best way to "read" and engine is to obtain a dyno sheet, their will be 2 peaks that cross paths at mid range point  this is NOT My dyno sheet http://www.dragtimes.com/images_dyno/8832-2004-Dodge-Neon-SRT-4-Dyno.jpg

MOst usable "power" will be optained at or within a few hundered rpm of where the torque and hp cross.

Link
this one shows around 3k  peak (  read small engine, this is where they set high rpm govenor)

[Brucer]

Pineywoods has it right, except it's not a fudge factor, it's a constant to convert from one unit of measure to another.

The relationship between Torque, Power, and Speed is ...

  Power = Torque x  Rotating Speed

Using our common everyday units of measure this works out to ...

    Power [HP] = Speed [RPM] x Torque [foot-lbs] / 5252

Where 5252 (or 5252.113122 if you want to get carried away) converts between the different units of measure.

Torque is a measure of rotating force -- it takes a certain amount of torque to rotate the drive shaft on a bandsaw. That in turn will depend on the radius of the drive wheel and the amount of force it takes to pull the blade through the log. The force further depends on the width of the log, the sharpness and angle of the tooth, the type of wood, moisture content, and how quickly you're trying to push the saw through the log.

What complicates the issue is when you are using an internal combustion engine to supply your power. At maxium throttle setting an IC engine will produce different amounts of Horsepower at different speeds. Just because the label says 5 HP doesn't mean it always produces 5 HP. That's what shelbycharger was alluding to.

Electric motors are different. They produce approximately the same amount of horsepower at any speed. External combustion engines (e.g., steam engines) are similar.

[HuskyFan1977]

Not sure on how to calculate that however, here is a post MORE HP AND TORQUE I recently added to the forum that will give any saw more HP and more Torque throughout the entire RPM range. Hope this helps the OP or someone!

[Gary_C]

A word of caution about understanding torque and horsepower. There is a big difference between actual measured torque and HP at a specific RPM and good at only that one RPM vs. horsepower and torque ratings which are applied to power sources. The horsepower and torque ratings are only guides as to what any motor will produce. The guides do not tell you what the power source is actually producing.

[Al_Smith]

Actual horse power and advertised horse power often times can vary by a lot .It's an old game used to sell things .Small engines,cheap electric motors and often wood splitters fall into this category .The later they just plain lie about it because to get the advertised tonnage ratings the hydraulic systems would need  to have a few thousand pounds of pressure above what the system could ever produce .It sells things though which is what the intent is in the first place .

[moodnacreek]

Questions about horse power are very confusing. Steam engine H.P. comes to mind . Think of a dragster doing the quarter mile, that's horsepower. A bulldozer pushing a full blade of dirt, that's torque.  If you pushed that dirt with the dragster it would destroy itself. The KW rating used today is a better guide to the power of an engine or electric motor.

[Al_Smith]

They figured it out on electric motors years ago .It's 746 watts per HP and people keep arguing about it like they have changed the laws of physics all of the sudden .I just shake my head .
It's word usage that causes the problem .If it is stated "develops" such and such it only means the electric motor is capable of running in an overloaded condition for a short while before it burns itself out .Once all the smoke escapes it will never run again and is only good for a door stop if it doesn't stink real bad . 

[SawyerTed]

Just compare the diesels in current 3/4 and one ton trucks if you want to see how terminology makes a difference.  Manufacturer claims aren't apples to apples comparisons.  Getting real unbiased specs on these engines' hp and torque values is difficult.  

An example of the horsepower and torque of a large displacement six cylinder is the Cummins 6.7 liter.  The Duramax and Powerstroke are 8 cylinders of similar +/- displacement.  Similar displacements, different configurations, different torque and horsepower.

I am not trying to create a which is better discussion here but am reiterating that sorting through the marketing claims can be difficult.  That's even true looking at small gasoline engines, tractors, skid steers, trucks, generators etc.

[Al_Smith]

A prime example of diesels could be my parents  (RIP ) motor home .Inline 6 Cummins ,turbo ,inter cooler  .Rated at 300 HP but it also produced 1680 Ft pounds of torque .Compare that to a large gasoline engine . I do know from experience it will pull Jellico pass on I-75 both directions at over 90 MPH because I've done it .--Probably not the smartest thing I've ever done .Danged semi's were tail gating me so I showed them where the bear went wee wee in the woods .Didn't see or hear them again until I got out of the mountains at a fuel stop .

[Gary_C]

Questions about horse power are very confusing.

That's a gross understatement and I read a lot of misunderstanding in the comments here.

For example, assuming the engines were similar the difference between the dozer and dragster would be more about gear ratios and traction than about torque vs HP. In other words it takes the same amount of power to move a given amount of dirt a fixed distance regardless if it is done with a screaming 10,000 rpm motor or a slow speed high torque motor. Theoretically you could push the same dirt with a one ft lb torque motor if you could get it spinning fast enough and had a big gear reduction and enough traction.

The second part of the confusion arises from the HP ratings of a given motor. Ratings are subjective and yes, not always consistent with similiar engines. Plus engines do not always run at rated HP. They will produce just enough torque to maintain desired speed until they can't do it any longer. The "can't do it any longer" point should be well above the "smoke comes out point" or when the RPM's drop too much to continue running. 

So yes, this is not a subject that is easily understood and explained with simple examples.

[Al_Smith]

To add to the above post it also has to take into account where the HP rating is taken from .In the case of my antique 1954 Oliver crawler it uses draw bar HP which is 34.5 .The same size engine,bore and stroke is a 1954 Chevy which is upwards of 90 HP .However that's at 3500-4000 RPMS and the dozer is governed at 1600 rpm .It's made with a large cooling system,larger bearings etc .Made to run all day every day for years between an overhaul .The Chevy back in the day would need some attention at 40-50,000 miles if it made it that long .This was in the day auto engines were not as robust as they are today but industrial engine by and large were .

[Al_Smith]

Rambling on .Electric motors come in an array of frame sizes .Little bitty ones and very large units rated at the same HP and RPMs .It would only make sense the larger frame sizes would in effect produce torque easier because of the rotor size related to the smaller ,more windings etc .better cooling .
I can't remember the code size is for same but in the auto industry it was usually the large frame motors were used and they might  run 7 and 24 for decades before they might need a bearing change .

[moodnacreek]

If the word was that you need 100 hp. to run that mill and you took an automobile or truck engine to do the job, It would have to be 200 brake hp. to get the 100 needed. This is for gas. Diesel might be 150 to get 100 on the belt.  Here's another thing; If your diesel is used in a farm tractor, pick up/farm tractor, look up the pto hp. This rating is the same as electric motor hp. and is about half of the vehicle brake hp. I know this from rigging up cheap engines to sawmills. Hard to explain this to others. And then there is the rpm at which the most torque is developed that needs to match the hammered speed of the saw.

[reride82]

Horsepower is how fast you hit the wall, torque is how far you move the wall once you've hit :D

[Gary_C]

If you really want to be confused, take a look at this brochure about the Cummins ISX engines.   Cummins ISX

This quote is taken from that brochure.

There are no hardware changes from  485 hp/1850 lb ft to 600 hp/2050 lb ft, and  all ratings deliver best-in-industry engine brake performance of 600 retarding horsepower.

Now scroll down to the HP and Torque charts and see how different they are for different versions of the same engine and for the same hardware. Note how flat the torque charts are before they drop at higher RPM's and how the torque values affect the HP values. 

So yes, HP and torque for internal combustion engines are values not easily understood. And then electric motors are a whole different ball game.