HP-vs-Torque Ratings

Hp is "work over time" while torque is measured 'instantaneously', as in RIGHT NOW.

How hard can the PTO twist before it will be forced to stop turning is torque.

How many bales of hay will it lift to the mow in ten minutes ... is HP.

You know how a mower will slow down when you get in tall grass? That is 'work over time', a continuous load. The engine is producing enough 'twist' or torque at that point in time to keep the blade turning. When it stalls, and kills the engine, you have maxed out on torque.
Most engines have a different torque and horsepower curve over their rpm range. The hp will peak at one rpm while the torque will peak at another. It is only 'luck of cam timing, cylinder bore and crankshaft throw' that could make them peak at the same time, but in that case, you'd have an engine that would fall on its face when out of that 'peak' spot. The throw of the crankshaft increases stroke, and also the 'lever arm' that is used to twist the crankshaft. The bore of the cylinder, and diameter of the piston, vary the effective area that the expanding fuel/air push against. A smaller diameter gives less area, while a larger diameter, of course, gives more, but remember, the fuel/air expansion has to fill a larger volume, so the pressure will be reduced given specified cam profile.
It's all a mix of compromises. Ask Red, and he'll tell you what to do to make a Honda {and others} scream... but you'd not want to use that engine on everyday mowing or pulling, I'd think.
tom

Horsepower is how fast you can move weight in a certain time. The more the weight, or shorter the time = more horspower. 550 lbs per ft per sec = 1hp. So if you could move 1100 lbs per ft per sec, then it would be 2hp. Or if you could move 550 lbs per ft in .5 sec, that would equal 2hp also.

Torque is how strong something is. How many lbs of force to move an object.

Picture a race horse and an elephant having a race around the track. Now we all know who will win, if by chance the elphant won, it's time to get a new horse. Now, have each one pull 10,000 lbs and the elephant will make it to the finish line first. Hope this clears things up a little.

I remember being told it's torque that's gets you going and HP that keeps you going. The formula is as indicated. Apparently the engine makers switched to quoting TQ instead of HP due to a frivolous but successful court case. Of course since HP is just calculated what difference did it really make? Apparently it had to do with following the SAE standard. But if the HP was misleading then it meant that the TQ had to be misleading in the first place, so I don't understand how just giving torque instead of HP solved the issue.

Its the fact that Engine manufacturers were advertising power ratings achieved without a muffler, without an air filter, at engine speeds that exceeded the governed top speed of the engine.

FOr instance...a 31ci single cylinder engine, at 4100 might make 21hp, but at 3600 it makes 18hp, so they advertise the 21hp, but it was limited to 18hp, ad an air filter, muffler, intake tract...16.5hp or less...but still advertised @ 21hp.

RED-85-Z51 said:

Its the fact that Engine manufacturers were advertising power ratings achieved without a muffler, without an air filter, at engine speeds that exceeded the governed top speed of the engine.

FOr instance...a 31ci single cylinder engine, at 4100 might make 21hp, but at 3600 it makes 18hp, so they advertise the 21hp, but it was limited to 18hp, ad an air filter, muffler, intake tract...16.5hp or less...but still advertised @ 21hp.

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and changed to TQ with muffler off etc LOL.

Harley Jeff said:

HP can only be calculated mathmatically. TQ is the number that is found on a dyno, and HP is calculated from that. I believe the formula for calculating HP is: TQ x RPM divided by 5252. I could be wrong, but that's how I remember it.

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If the formula is just that simple, How is it that though two engines have the same torque at the same RPM, one of them is discribed as "torquey"?

RED-85-Z51 said:

Its the fact that Engine manufacturers were advertising power ratings achieved without a muffler, without an air filter, at engine speeds that exceeded the governed top speed of the engine.

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And now they will do the same thing with torque. Then, they will be limited to only using c.i. discriptions.

HP and Torque are separate entities and it's not fair to directly compare them. The importance of each is application specific.

For example, If you are towing something heavy you probably want higher torque values at low rpm's (unless you tow with your motor at the red line).

Now suppose you have a boat and you want the best top speed. This is a pure HP situation because the outdrive has fixed gearing. Output power is directly related to rpm's here. Prop the boat so that the engine hits peak HP rpm's at wide open throttle and you are there. There is likely higher peak torque at lower rpm's. It is not going to make the boat faster to prop it for peak rpm's to match peak torque.

I've seen Torque vs HP debates go south on other boards so I hope I'm not opening up a can of worms here.

To use a small engine example (since thats the thread we are in) If I was to buy a snowblower I would definately focus on torque. When you hit the end of the driveway and the engine sees a dramatic increase in load torque is what dictates to what degree the engine will bog down when the rpm's drop from the load.

For a leaf blower (the kind on wheels, not the handheld) that does not have to recover from a load--> HP. Again, here you want the maximum output power, not reserve power at lower rpm's.

scipper77 said:

For example, If you are towing something heavy you probably want higher torque values at low rpm's

Then if the formula is valid, you have higher HP at lower RPM.

Prop the boat so that the engine hits peak HP rpm's at wide open throttle and you are there. There is likely higher peak torque at lower rpm's. It is not going to make the boat faster to prop it for peak rpm's to match peak torque.

??? So at higher RPMs the torque goes down, but the RPMs go up, so how does this affect the formula? I understand what torque is and I understand what HP is, I just have trouble understanding how they are interconnected by a formula.

I've seen Torque vs HP debates go south on other boards so I hope I'm not opening up a can of worms here.

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No worms on this board!

Think of it this way. If torque stays the exact same fro say 4000-5000 rpms. HP will go up because HP = torque * RPM's / constant number. so in this case as RPM's increase so does HP.

Now if there is a slight loss of torque as rpm's increase then depending on the rate of torque loss HP can increase. This is exactly what happens. as Torque goes past it's peak the rate of loss is gradually increasing (slope for engineering types). There will be a region where torque is decreasing and HP is increasing immediately following peak torque rpm's

Look at any dyno sheet (google dyno). I'll bet that peak HP will always be at higher rpm's than peak torque.

edkedk said:

SO! Still confused! In the horse/elephant example, which has more horse power, (or elephant power), the horse or the elephant?

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That example is not a real world application, just an alliterative device to illustrate that torque is where it's at if you want to pull a load slowly, and HP is where it's at if you want to go fast. It's a generalization and can easily be picked apart.

Also do not confuse top speed with drag racing. Higher HP does not always win a drag race. I was careful in my example to use a boat and only discuss top speed. This discussion gets very complicated in drag racing where gearing and area under the curve between shift are brought into the mix. Here Highest HP might lose.

All I can say here is it's important to understand that HP and torque are separate entities and should not be interchanged directly. Understand how they work and apply them based on the situation.

Hp is defined as 33,000 foot pounds per minute. James Watt observered that the "average horse" could pull 330 pounds 100 feet in a minute. To add rotation to the equation you must multiply by 2pi or 6.28 times the torque in foot pounds times the rpm and then divide the total by 33,000.

The 6.28 converts the radius in feet to the circumference,

so HP = 2pi (T) (RPM)/ 33000 Since 2pi =6.2832 then 6.2832/33000 = 5252

then HP = (T) (RPM) / 5252 when T is in Foot Pounds

So if we run our equipment at 3550 RPM then the HP will be 2/3 of the torque

3550/5252=67.6%

steve

LilysDad said:

If the formula is just that simple, How is it that though two engines have the same torque at the same RPM, one of them is discribed as "torquey"?



And now they will do the same thing with torque. Then, they will be limited to only using c.i. discriptions.

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-----they make have same torque at one particular rpm only. I would think an engine whose torque is higher than the other at low rpms would be considered torqueree .Generally on any engine the torque increases with rpm but gradually increases less and less until it peaks and then drops for further increase in RPM . that is called the torque curve-------

scipper77 said:

Think of it this way. If torque stays the exact same fro say 4000-5000 rpms. HP will go up because HP = torque * RPM's / constant number. so in this case as RPM's increase so does HP.

Now if there is a slight loss of torque as rpm's increase then depending on the rate of torque loss HP can increase. This is exactly what happens. as Torque goes past it's peak the rate of loss is gradually increasing (slope for engineering types). There will be a region where torque is decreasing and HP is increasing immediately following peak torque rpm's

Look at any dyno sheet (google dyno). I'll bet that peak HP will always be at higher rpm's than peak torque.

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I think I see now. Good example.