Just got home from my 250 mile round trip out into the hill country of Ne, and brought home some cows.

Believe me, This has been bugging the hell out of me.

First of all, I decided to call on the big gun.

My Dad was an mechanical engineer, and I had quite an upbringing as a kid. a lot of principles were hammered home.. Almost Nazi style.

Now, 45 years later, some of these principles are a little hard to bring out of the archives! My career path took me out of the loop for studying and practicing the sciences.

Dad passed about 10 years ago, I sure miss being able to call him up and quiz him about how to build this, or what materials to use here..

So, I did the next best thing! I called my Uncle John. He is a tenured physics professor and rabid car builder. perfect.

I had him view my miserable attempts at explaining how the HP= T*RPM/5252 applies here.

He said: We all had some valid points, but more interestingly was some points were correct, but for the totally wrong reasons.

First off:

My analogy of the 2 electric motors with identical torque, but double the speed. motor #1 would indeed have 1 horsepower, and would lift the 550 lb weight in 1 second. Motor #2 would then indeed have 2 hp. only because it's turning 2x the speed. It would indeed lift the weight at 2x the speed as the 1 horse motor. I was wrong there.

He said I need to forget the initial acceleration of this weight.. and concentrate ONLY on the performance of the weight going up the rope.

he said both motors will struggle momentarily as they spin up to speed.. Motor 2 will struggle just a tad longer ONLY BECAUSE IT HAS HIGHER TO GO.. AND spin up time will be longer.

This is no consequence. It's the performance of the speed of the weight After the initial acceleration.

What this boils down to, is torque is what moves the weight at the certain speed. John pointed out that it doesn't take any more energy to move the weight upward regardless of speed (well until air resistance becomes a factor!)

since both motors are equal torque, they are able to lift the exact same amount of weight. Motor #2 will do it 2x faster, not because of the HP, but because it's wired to run that rpm in the first place.

John said we need to look at HP as an 'after effect'. It's the PROOF of work being done, or the potential of work to be done.

again, this is after both motors have accelerated the weight to it' full potential.. then the test begins.

Now, look at it this way: after the test, we can now measure the performance of both motors. #1 did a 1 foot per second, #2 did it in half the time. both with identical torque. Because of this fact, the #2 motor did in fact move the weight in half the time, it should be considered 2 hp.

HP again should be looked at as the 'after effect'.

Why? John told me to visualize both motors doing their finest with the weights attached.. and pulling the weight up a 100 foot cliff. (for better visualization here). Halfway up both runs, I take an additional 55 lbs and put on each as they buzz by. Both motors will suffer proportionally, actually this would be 10%. means Motor #1 is now hoisting the weight 10.8 inches per second, and motor #2 would be hoisting the weight 21.3 inches per second.

Now, lets apply this to 2 identical cummins powered vechicles. both are bone stock. but one will have a governor that stops ALL rpm progression beyond 3400.. and the other having this stop @ 1700. Both will have identical fueling properties.

Engine 'a' will be exactly double the HP of Engine 'b'. This is because of the doubling of the rpm. Both here have exactly the same torque. (same fueling, same air)

Both are also geared identically too.

What's the difference? MAX SPEED. Both will accelerate side by side.. and when the governor kicks in on the slow motor.. that truck will march along @ what, 55 miles per hour... while the other truck continues up to 110. Why? not because of the HP. it's because the governor allowed it to.

This is where we need to look at HP as the AFTER EFFECT. Yep, that motor propelling the truck down the road at 110 mph was indeed making double the horsepower... only because it was allowed to spool up to double the rpm.. But no more fuel for the acceleration, than the first motor was added. The acceleration and maintenance level of fueling is next on the list;

John told me about the 'butt feel' of why our trucks seem to 'hold better' at 2500 rather than 16-1700 starting point..

Look at where we are at on the torque curve. Even though 1700 and 2500 are pretty even for torque, the guy that is @ 1700 is right on the brink of falling off the cliff.. The guy already @ 2600 has 900 rpm of 'play' before the truck falls off. John said momentum helps.. for a split second! not what I'd call a real helper!

So, lets now apply this to the 2 cummins powered trucks from the last paragraph.

Lets send them both down the road @ 55 mph. (remember, the one truck is governed for 1700, and this is all it has) Both are turning the same rpm.. both are burning identical fuel. Both are producing identical torque... they have to be, otherwise one would be traveling faster than the other..

Since both are fueled identically, the outputs will be identical right up to the point the slower governor stops all upward rpm change. this is where the faster engine begins to pull away from the slower one horsepower wise.. and we are now looking at the electric motor example from waaay above!

Both motors are identical in torque, but because of the rpm difference, the faster one is double the horsepower.

What really needs to be said here is.. no diesel 'makes' torque, (above what is required to maintain a rpm at a no load situation) unless it's needed.

If I keep my foot on the throttle, and have a no load situation on the engine, only a very little amount of fuel is required to maintain the set rpm. It takes very little fuel to keep a non loaded engine at a set rpm. If we could measure the amount of this torque, it'd be minimal to say the least.

So, back to our 2 identical trucks. traveling side by side. Both are CAPABLE of maxing out the torque at say 450 ft. lbs. In order to maintain 55 mph, let say they need enough fuel to produce 120 ft. lbs at the crank to maintain this speed. Or about 40 horsepower.

Lets now introduce them to a hill. still side by side. who thinks the double hp truck will walk away from the other one?

It's a tie. sorry. both will lug to death at the exact same rate. Both are putting down the exact same torque to the rear tires, both are injecting the same max fuel. Why doesn't the double hp engine pull away? because it's not turning the rpms to PRODUCE the double HP. Since it didn't make the rpms, it never reached it's potential.. this is where we need to look at HP as an after effect. You must have torque, AND rpm to produce HP.

John basically told me this as the exact same way he has answered a lot of gear heads in his classes. He also kicked my *** for a couple of knuckle headed answers I gave as well.

"bench racers" will conjecture.. and myths are rampant.. but physics are physics.

I offer my humble apology for any bad thoughts I might've had over the past days posts.

Robert, rancherman

Bringing it back.... I was doing a little studying last night and came across a section that is relevant. 

 

a.        Power

 

     (1)       Power is defined as the rate of doing work.

 

          (a)       Assuming a book weighs 1 pound and is lifted 3 feet off the table, 3 foot-pounds of work has been done.

 

          (b)      It does not matter how fast (1 second) or how slow (1 year) the book is lifted, the same amount of work is always done.

 

          ©       It does, however, take more power to lift the book in a lesser amount of time.

 

          (d)      Consequently, the units of power are defined as the amount of work (ft-pounds) per unit of time (seconds).

 

     (2)       The most common expression of power measurement is known as horsepower.

 

          (a)       Horsepower is defined as the amount of weight (pounds) a horse could move 1 foot in 1 second.

 

          (b)      By experiment, it was found that the average horse could move 550 pounds a distance of 1 foot in 1 second.

 

 

 

That is a very clear and concise way to explain the interaction and importance of both hp and torque. Since horsepower is not effected by gearing it is very important to have enough hp to do the job, while torque can be multiplied thru gearing.  

http://www.epi-eng.com/piston_engine_technology/power_and_torque.htm

 

I`m a little late reading this thread, but this Link has some interesting facts about the relationship between the two

Nice article... Not all dyno's work that way thou, as dyno's that cannot vary the load only measure power based on a known load. But all in all a very good read.

True, engine dynos are configured to match an engines torque output. I think in Ranchermans original post where load exceeds the engines torque , HP becomes irrelevant.

When we are measuring a vehicles performance there are so many variables to consider, particularly gear ratio which actually multiplies torque.

 

Measuring speed or RPM as a performance parameter in a towing situation is kind of apples and oranges imo..

Why don't you think speed or rpms are important for towing as a performance parameter?

I am not sure hp is irrelevant if tq exceeds load. Assuming the load needs 500 ft/lbs to move it a 1 hp 1000 ft/lb motor isn't going to do a lot of work anytime soon, but a 500 hp 500 ft/lb motor is going to.

Remember there is no time associated with torque, it takes horsepower to get it done. More hp the work gets done faster.

As you mentioned gears multiply torque, but hp doesn't change with gearing. That means the hp has to be available for the desired rate of work. You can always gear down for more work, but the rate is constant for a power output.

Well, when you look at any dyno sheet you will see the torque curve peek and then begin to fall as the rpm increases. then you see a HP curve that continues to rise to its own peek value. There is no better place to study this than in a camshaft comparison chart where they use a test mule and track the perfomance of different grinds.

Torque and HP will be both change with RPM, so I do think speed and RPM are important.

 

I just think Rancherman was describing a real world scenario where the load he was pulling up a hill has overcome the torque output (Twisting force) of his engine so he begins to lose speed, and RPM

 

On my day job, I am a electrician at Newell Coach Corp. Which is a up scale luxury Coach manufacturer.  They are powered by Cummins ISX engines. Part of my job is programing, clearing codes, and checking for the latest calibration updates.  The ISX we use is roughly 600hp at 2300 RPM but the torque output is 1950 lb/ft at 1200 rpm.  These are 60,000 lb motorhomes and they will get up and haul !

Edited April 25, 20159 yr by angus

Well, when you look at any dyno sheet you will see the torque curve peek and then begin to fall as the rpm increases. then you see a HP curve that continues to rise to its own peek value. There is no better place to study this than in a camshaft comparison chart where they use a test mule and track the perfomance of different grinds.

Torque and HP will be both change with RPM, so I do think speed and RPM are important.

 

 

That's just the nature of the engine and relationship of hp and tq. Any engine that has a peak power rating above 5252 rpms will have more hp than tq, and any engine with peak power below 5252 will have more tq than hp. 

 

The more rpms the bigger the delta. The advantage is that gearing can bring the torque back and the hp is still there, so those enignes can do a LOT of work at a good rate. Helicopters are a great example. 355 lb/ft of torque at 21109 rpms is 1428hp, gear that down to rotor speed (292) and  you have 25663 lb/ft available. So rpms are great, 1428 hp and 25663 lb/ft... basically you can do a LOT of work at a quick rate.

 

On the flip side a 1 hp 1 rpm engine can do a LOT of work, VERY slow and there is no way to increase the rate of work.

 

A 300hp ISX and a 300hp 1.4L 4cyl can do the same work at the same speed as long as they are both geared to be making their respective 300hp. That fact alone tells me the importance of hp.

 

But I still drive a diesel because the low rpm power means better acceleration, easier to drive, less shifting, quieter engine, and less wear and tear from rpms.

 

 

I just think Rancherman was describing a real world scenario where the load he was pulling up a hill has overcome the torque output (Twisting force) of his engine so he begins to lose speed, and RPM

 

 

 

Without further testing it's hard to say if he ran out of hp or torque, because both would have the same effect, but I would guess he was out of hp as it's the rate at which work is done and the nature of a diesel power curve). With a diesel we generally run out of hp, simply because the torque curve is generally flat. 1800 rpms and 2500 rpms can have the same torque, but there is almost a 40% increase in hp. As I pointed out in the thread that started this discussion a rig with  flat torque curve will pull a grade at 2500 better than 1800, assuming same gear and equal torque despite the additional power requirement from the increased speed.

 

 

On my day job, I am a electrician at Newell Coach Corp. Which is a up scale luxury Coach manufacturer.  They are powered by Cummins ISX engines. Part of my job is programing, clearing codes, and checking for the latest calibration updates.  The ISX we use is roughly 600hp at 2300 RPM but the torque output is 1950 lb/ft at 1200 rpm.  These are 60,000 lb motorhomes and they will get up and haul !

 

I'll bet they do!! That's a LOT of power. 1950 lb/ft at 1200 is 445 hp, and that is why they scoot!! Lots of work can be done at a fast rate. That is where big displacement diesels really shine, lots of torque and hp at a low rpm... it's just takes a BIG drive line to not tear apart.

When I was a teenager, I used to hangout at a local machine shop with all the drag racers. One day we were discussing a guy who set a new national record in NHRA super stock class.  I remember asking how in the hell can a argueably "stock" Camaro run a mid 10 second quarter mile?? 

The guy was running a 68 Camaro SS rectangular port 396, otherwise known as a 375hp.  He was running a 6.00/1 rearend gear and turning the engine 8,800 RPM !!

One of the Guru`s explained it like this; To increse top end HP you have to shift the engines peak torque value higher in the rpm range which obviously sacrafices the low end.  and thats where the incredibly low gear ratio compensates.

 

This primarily applies only to naturally asperated engines, because the miricle of boost can give us the best of both worlds.

 

Drag racing and towing are about as polar opposite as you can get, in that you want a big fat torque curve in the lower RPM for driveability.

SOB... had a whole response written up and it's gone!!!

 

The Guru's explanation is only correct if the engine was modified. I read your post to mean that the only change to the car was the gears, which doesn't have any effect on engine output only rear wheel torque. Am I reading that wrong?

 

Increase rear wheel torque and have a decent amount of horsepower on tap and you will accelearte like a SOB. I'd be willing to  bet he had lower trap speeds than similar weighted rigs with 200 more hp running the same ET with "normal" gears.

SOB... had a whole response written up and it's gone!!!

 

The Guru's explanation is only correct if the engine was modified. I read your post to mean that the only change to the car was the gears, which doesn't have any effect on engine output only rear wheel torque. Am I reading that wrong?

 

Increase rear wheel torque and have a decent amount of horsepower on tap and you will accelearte like a SOB. I'd be willing to  bet he had lower trap speeds than similar weighted rigs with 200 more hp running the same ET with "normal" gears.

 

No, the old "super stock" NHRA class racers engines were definitely modified! They all used some very exotic camshaft grinds to get them to turn that kind of RPM.. The old Big block Camaros were very popular back then because of their massive port designs which could really breath at high RPM.

These types of cars would be miserable to drive on the street, and Could you imagine trying to tow something with one! LOL!

Ok, that makes a lot more sense!!!

 

Higher horsepower at higher rpms means you can take advantage of more gearing reduction, similar to the helicopter scenario above. Torque is down, but as you stated the gearing compensates.

The OEM 375hp cam might have made 420lb/ft of peak torque @ 3000 RPM and 375hp @ 5800 RPM. By contrast the exotic cam grind might make 550lb/ft peak torque @ 5800 RPM  and 600hp at 8,800 RPM.

 

I`m just making up those numbers out of thin air, but thats how the relationship works when you are aiming for high end HP.

 

Small cc Dirt bikes are a great example.. anybody thats ever rode one knows how they bog at low RPM then take off like a rocket once they hit their powerband, or torque curve.

I`m just curious about the helicopters.. is there alot of difference between the engine on an Apache, and thosed used in something like a (Chinook) sp? sorry about the spelling.

I`m just curious about the helicopters.. is there alot of difference between the engine on an Apache, and thosed used in something like a (Chinook) sp? sorry about the spelling.

 

Same basic operation, but the engine on the Chinook is MASSIVE compared to the Apache. IIRC one Chinook engine makes more shp than both Apache engines.

 

Apaches and Blackhawks use the same engine.