Horsepower and torque question

[Moretorque]

Thanks for taking the time, because they are long stroke don't they take a little more time to wind up to make power. Can they be a touch slower in this regard ?

So not being able to turn the RPM limits them in power, I take it the compression is why they do not spin a lot ?

 

[Moretorque]

I am realizing seat of the pants is just that seat of the pants and the real #'s do not lie, before I got into this I actually understood it pretty good but went by what people were saying in the real world and thought I was not seeing something in the math that was not there.

Here is the next ? , is it possible the backstop of all this is in maintaining the HP under heavy load once it is built up is the fact on a bench when you take a low RPM motor vs a higher RPM motor of same HP value with no transmission and put a brake on it to stop it from spinning it will be harder to stop the motor that has more torque over RPM to make the HP ?

 

[jack action]

Thanks for taking the time, because they are long stroke don't they take a little more time to wind up to make power. Can they be a touch slower in this regard ?

It's a good point that you are bringing the stroke. The reality is that the piston goes at the exact same speed in the low-rpm engine or the high-rpm engine. The longer stroke gives a mechanical advantage that gives a higher torque, but the rpm is reduced. So increasing the stroke of an engine doesn't increase its power, it acts more like an «internal» gear set. Read about mean piston speed to learn more.

So not being able to turn the RPM limits them in power, I take it the compression is why they do not spin a lot ?

If you want to create a high compression engine, you will have to lower the deck clearance (see figure below). A lower deck clearance creates thermal losses and the fuel mixture is harder to fully burn. But if you increase the stroke (or reduce the bore) while increasing the compression ratio, you will regain your deck clearance (it's a simple geometry exercise). If you reduce the bore, you will need to increase the number of pistons to keep the same bore area such that the power output is the same, which is very complicated. The preferred method is to increase the stroke, but it will lead to lower the rpm if you want to keep the same mean piston speed (which is more of a limiting factor than the engine rpm). Since the mean piston speed and bore area are the same, the power output is also - theoretically - the same (there are other effects to take into account, but those are very important and fundamental).

Here is the next ? , is it possible the backstop of all this is in maintaining the HP under heavy load once it is built up is the fact on a bench when you take a low RPM motor vs a higher RPM motor of same HP value with no transmission and put a brake on it to stop it from spinning it will be harder to stop the motor that has more torque over RPM to make the HP ?

First, there is no «backstop of all this is in maintaining the HP under heavy load»: Once you have the HP, you have it. Maybe you had less HP in lower rpms and that made it more difficult to reach the rpm you are actually in, but once you are there, there's no quality grading in the HP you have. If you have enough HP to hold, it will hold. Naturally, assuming the power is properly adapted for the given load, i.e. set to the appropriate torque and rpm.

The second part of your question is also of interest. «Will it be harder to stop?» It all depends what you mean by «harder». This is where the difference between the concept of «torque» and «power» is important. Assume you are stopping your motors with a friction brake, i.e. by converting the mechanical energy into heat.

The friction brake must be able to handle the torque applied to it. If one motor has a greater torque, higher stresses will be applied to the part and they could failed if they are not strong enough. In that sense, the higher torque engine will be harder to stop.

The friction brake must be able to remove the energy from the rotating motor, either by absorbing it (its temperature increases) or by transferring it (to the surrounding air or a coolant). The amount of energy going out of the engine in a given period of time must be the same that is going in the friction brake in that same period of time. So the engine power must be equal to the braking power. Since the power is the same for both motor, the brake power required will be the same in both cases.

Here is an example to help visualize. Two workers have a job to do: moving one stack of brick a certain distance within a certain time. The first worker is not very strong but is very fast and takes one brick at a time, running the distance many times to move all the bricks in the given time. The second worker cannot run fast but is very strong. So he takes the whole stack of bricks and travel the entire distance once, at a very slow pace such that it takes him the same time as the first worker. Both workers have done the same work, in the same period of time, thus have the same power. But one was stronger and the other was faster and because of the way they are «build», they couldn't exchange places as they would of failed to do the job if they've tried the other worker's method.

 

[Moretorque]

So basically it's all in set up but for pulling heavy the diesel because how efficient it is by not turning much RPM is the clear choice and you do not need a lot of MPH but pulling efficiency.

Thanks..

 

[cjl]

So basically it's all in set up but for pulling heavy the diesel because how efficient it is by not turning much RPM is the clear choice and you do not need a lot of MPH but pulling efficiency.

Thanks..

The diesel is the clear choice for pulling heavy because large diesel engines are very reliable, last a long time, and are more efficient than high power gas engines. In terms of actual pulling capability (if set up correctly), all that really matters is the horsepower and powerband.

 

[cjl]

@cjl

I tried to read between the lines with @Moretorque to understand what he means and what he's referring to, rather than analyze every single word he said.

What I think he's referring to is the small highly-boosted engines vs large displacement engines. It's not really about diesel vs gas. Your Corvette engine is not a good reference in this case, as it can be considered as a truck engine fitted within a car (I mean 7.0L ! C'mon, that is huge!).

It's a sports car engine, through and through (look at the redline and power peak), definitely not a truck engine. It is definitely a big engine though, designed for smooth power throughout the rev range.

Here's a dyno sheet I retrieved from the web that is claimed to be from a 1999 Honda Civic with a turbocharged 1.8 L:

​

If you look at the run with 503 hp and 334 lb.ft, the 75% of peak torque rpm range is 5200-8500 rpm (a ratio of 1.6).

That's a highly tuned modified engine though, clearly made for bragging rights and high peak power output (not for useful power). No car manufacturer would deliver an engine like that from the factory, since the usable power band is too small.

So from that point of view, he is right, even with your criteria (I personally prefer qualifying useful rpm range by the power curve rather than the torque curve, since engine torque is irrelevant to acceleration as you and I stated). With two engines with the same peak power, the one with the highest power in the lower rpms should accelerate faster (Even though he used the terminology «diesel» and «gas», I considered he meant a generalization for a «wide power band» vs «narrow power band»).

He was comparing diesel trucks to gas trucks earlier though, and a gas truck would often use a large displacement V8 or V10. Also, as I said, in many (I would even say most) cases, gas engines have a wider power band than diesel. Looking at a modified 1.8L turbo engine isn't necessarily a great example either - for comparison, look at this 360hp factory turbocharged 2.0L engine from Mercedes:

This shows the difference between a tuned engine and a factory engine - a lot of people who modify their engine just put a giant turbo on and look for high peak horsepower, with little concern for powerband, while an engine designed for a particular power from the factory will have a much broader and more usable range (look at the peak torque range on that engine - nearly full torque from just over 2000RPM all the way up to 6000 or so). As for the rest of your post, it's hard to say what anyone's intention is at any time online, so I won't bother trying to do a detailed response. I just wanted to respond to your claim here that small engines (and gas engines in general) don't have wide powerbands and that the LS7 is a truck engine.

 

[Moretorque]

So you are saying a Indy Car motor can power a Semi but what would the gear set be like to keep it chugging happily without falling off the pipe ?

 

[cjl]

So you are saying a Indy Car motor can power a Semi but what would the gear set be like to keep it chugging happily without falling off the pipe ?

An indy car motor could easily power a semi faster than its normal speed. It could probably pull it at 85-90mph, given the horsepower (550-700, compared to 200-400 for a normal semi). I can't find a torque curve online, but assuming that it's making 600hp at 12000RPM (the rev limit), it would probably chug happily along at 55-65mph at 8 or 9 thousand rpm or so, and it should climb shallow hills nicely at 60+mph at 12krpm. Assuming that 60mph on flat ground takes 200hp for an 80klb semi (which seems pessimistic to me - I'd expect the actual power requirement to be lower), it should be able to climb a 3% grade at 60mph at 12krpm.

(Of course, there's no guarantee how long it would last spinning at 8000-12000RPM all day long under a heavy load, since they're only really designed to run a few hours at a time, unlike truck engines which run for months between services. It'd also need a large amount of cooling, since it is less efficient than a truck engine so it makes more heat for the same power output).

 

[Moretorque]

Thanks CJL but what would the gear set be like, I mean would it need more than the standard setup, would a air shifter help a lot under that kind of load ? I know going 240 MPH in a Indy car is like pulling 80 grand at 80 for a semi load wise ?

 

[cjl]

You wouldn't need an air shifter or anything (what is an air shifter anyways?). You'd just set it up so that you would wind it up to 10k or so in first (going all of 5mph or something like that), then shift into second which would be turning something like 7krpm at that same 5mph, wind that up to 10 or 12k at 8 or 9mph, then shift into third (which should be at 7k or so at 8 or 9mph), wind that up to 10 or 12k at 13mph or something like that, and keep repeating that sequence until you're at 55 or 60 in 10th gear or so. You could have gears set up such that the top speed in each gear went something like this:

1st: 4mph
2nd: 6mph
3rd: 9mph
4th: 13mph
5th: 19mph
6th: 28mph
7th: 35mph
8th: 48mph
9th: 68mph
10th: 85mphThat would probably work pretty well, and wouldn't require a really broad powerband either, since each gear only covers about a factor of 1.5 in speed. You could do it with fewer gears, but fewer gears means less overlap in usable speed between gears, which means you'd pretty much need to wind it all the way to redline in every gear.

(It'd also sound pretty ridiculous when accelerating, since you'd hear a racecar engine screaming to 12krpm in a big truck as it slowly lumbered up to highway speed, shifting 5 or 6 times before 20mph)

 

[SteamKing]

You wouldn't need an air shifter or anything (what is an air shifter anyways?).

Air shifters use pneumatics to actually shift the gears in the transmission, without using a manual shift mechanism. The gear changes can be done much quicker, and the driver doesn't have to take his hands off the steering wheel to shift (it's like using paddle shifters).

http://www.racegadgets.com/airshifter.htm

 

[Moretorque]

Thanks,

 

[Moretorque]

This is actually an interesting thing here, motors builders are telling me what is on paper is wrong and it is not like this in the real world. Like I posted about the Mack with only a 5 speed and 237 HP moving a 70000 pound load with no problem and that is a real setup. You can look it up with drive reviews. The real world consensus is you will never be able to maintain the RPM on a car motor under such a load to keep the 237 HP pulling that kind of weight.

I was told the torque is what allows you to maintain and build the HP under such a load and that the HP is your peak MPH in real world use, I will research this further. Thanks for the education on the #'s and math on this subject and on my end it all adds up but top engine builders have told me no.

 

[insightful]

The real world consensus is you will never be able to maintain the RPM on a car motor under such a load to keep the 237 HP pulling that kind of weight.

What if the "car motor" had 15 speeds to work with?

 

[Moretorque]

Then yaa but what do I know and what the people say here is right and I am not arguing that but the latest guy to tell me no was Roland Stuart face to face and he is crew chief on the Spider mans top fuel bike. Not only him but other builders as well who build unlimited tractors in both diesel and gas multi configurations.

Until one of these engine builders take the time to break it down and explain why in like real math the math guy's are right because the #'s do not lie. Thanks for everybody's time here to explain it and just trying to get the engine builders on the same page.

 

[cjl]

This is actually an interesting thing here, motors builders are telling me what is on paper is wrong and it is not like this in the real world. Like I posted about the Mack with only a 5 speed and 237 HP moving a 70000 pound load with no problem and that is a real setup. You can look it up with drive reviews. The real world consensus is you will never be able to maintain the RPM on a car motor under such a load to keep the 237 HP pulling that kind of weight.

If the motor is putting out 237 horsepower, it's putting out 237 horsepower. There's nothing particularly special about whether it's doing that at 1600RPM (like a truck engine) or 6500RPM (like a car engine), both will be able to do the same amount of work in the same period of time (since that's the definition of power). If it takes 237hp to move the truck at 70mph, then the car engine will maintain 6500RPM just as easily as the truck engine will maintain 1600, assuming they're both geared to hit those RPMs at 70mph.

Also, I'd be skeptical about 237hp and a 5 speed moving a 70klb load "no problem" - sure, it'll move it on flat ground OK, but it'll be awfully slow up hills, and the gearing will probably leave something to be desired compared to a more modern truck with a 10, 13, or 18 speed.

I was told the torque is what allows you to maintain and build the HP under such a load and that the HP is your peak MPH in real world use, I will research this further. Thanks for the education on the #'s and math on this subject and on my end it all adds up but top engine builders have told me no.

Horsepower tells you rate of acceleration under a given load, and also determines top speed. Torque on the other hand doesn't tell you much about the performance of the vehicle, just about what kind of gearing you'll need.

 

[Ketch22]

Thanks for sharing Rolands comments. I would caution just a little about the anonymous character of the interwebs, There is so far nothing to say that you are not already talking to real world engine builders. I personally am against playing that card in either direction.

Bear in mind that Roland is partially correct. If you had a fictional engine that was a diesel. For round numbers let's look at 1000 FtLbs of torque at 2000 RPM This delivers approximately 380 Hp. Carried to the road via a transmission with a 4-1 ratio this would then be 500 RPM with 4,000 FtLbs. This same engine makes 1000 FTLbs of torque at it's limit of 2500 RPM or roughly 476 Hp.

Compare this to a Petrol engine also fictional. This one delivers 280 FtLbs of torque at 7000 RPM approximately 373 Hp. In the same vehicle this now requires a 14-1 ratio to achieve 500 shaft RPM. Bringing the delivered torque to 3920 FtLbs. The same engine makes 280 FtLbs of torque at it's limit of 10,000 RPM, roughly 533 Hp.

The true side is that if it takes 3,400 FtLbs of torque to maintain equilibrium, greater than that number will accelerate the vehicle. Given the Diesel producing 4,000 and the Petrol producing 3,920 the Diesel will in fact provide more acceleration than the Petrol.

However, The Diesel will soon be overcome. As it jumps forward (for a bit) the endpoint of that engine is at 2,500 RPM which has seen a decline in Hp delivered and the actual shaft power of now 625 RPM and 4,000 FtLbs. or roughly flat acceleration for 20% of the range. Followed by falling of for ( name your own reason for redline)
The Petrol Engine will be slightly slower but at the endpoint of 10,000 RPM there will still be an increase in Hp (due to the revolution count) The actual shaft power will show at 710 RPM and 3,920 FtLbs. Translated as roughly flat acceleration but covering 30% of the range. The Petrol engine will by virtue of it's increased RPM (think total power) catch and overtake the Diesel. This will happen at each transmission equivalent ratio and actuality the added 10% will accumulate due to shifting time and other gains.

The instant acceleration of the diesel does not overcome the ideal that greater power happens at a place where the diesel is starting to run out of lungs while the Petrol is still gaining.

 

[Ketch22]

Oops, I just realized that I got distracted while I was composing yesterday. The missing paragraph in my reply regarding the two engines is the end of where that benefit from Petrol engines raises it's head.

With the additional 10% of RPM brought into the design of the drivetrain. Let's shift our Petrol engine to maintain a 20% range so as to match the Diesel. This now brings the Engine speed to 8000 RPM. Adjusting reduction ratio to maintain 500 RPM shaft speed now shifts from 14-1 to 16-1. The resultant power delivered to the shaft would be 4,480 FtLbs at the same RPM. We have shifted from the Petrol engine (measured at the tailshaft) being at an 80 FtLb deficit to it being at a 480 FtLb advantage.

It is as you can see all in the application engineering not so directly with the engine itself. If as in this example the effort required is lower than what can be produced [3,400 FtLbs required and both engines producing more] the petrol engine can catch up and pass the Diesel. If the parameter falls into a range between the engines [ for this example let's say 3,950 FtLbs required] the diesel would continue to accelerate for a little bit more while the Petrol engine slowly fell on it's face.

 

[Moretorque]

Thanks for keeping this thread alive and all the help. I am still studying the subject. It seems as though twisting force at the crank beats spinning more RPM in being more powerful in the real world when making more power and maintaining it under load. The dyno does lie ?

 

[cjl]

Thanks for keeping this thread alive and all the help. I am still studying the subject. It seems as though twisting force at the crank beats spinning more RPM in being more powerful in the real world when making more power and maintaining it under load. The dyno does lie ?

Nope. Both are important, and the dyno doesn't lie (assuming you read it correctly). Of course, that's been said many times throughout this thread...

 

[Moretorque]

cJl, thanks. will read over everything real closely. I have been spending time talking too drag racers and motor builders.

 

[Ketch22]

Moretorque, I would continue to applaud your search. This subject is ruled by a lot of good old boys that have been told stuff (and some if it close to correct) but not directly studied it. There is also a growing bunch of engine builders that are trying to change the popular conception to the accurate one. As you look I think that you would be well served to check out this article from a well backed engine build magazine.

http://dsportmag.com/the-tech/learning-curves-recognizing-a-race-friendly-dyno-graph/

 

[Moretorque]

What if the "car motor" had 15 speeds to work with?

Sorry I missed this, it would need a air shifter so the RPM would not drop off.

 

[Moretorque]

Moretorque, I would continue to applaud your search. This subject is ruled by a lot of good old boys that have been told stuff (and some if it close to correct) but not directly studied it. There is also a growing bunch of engine builders that are trying to change the popular conception to the accurate one. As you look I think that you would be well served to check out this article from a well backed engine build magazine.

http://dsportmag.com/the-tech/learning-curves-recognizing-a-race-friendly-dyno-graph/

Thanks.

 

[cjl]

Sorry I missed this, it would need a air shifter so the RPM would not drop off.

No it wouldn't. The only time shift speed would be important would be something like climbing a hill from a dead stop, and that's because the truck itself will slow down during the shift (and this is just as true with diesel as gas). On a level surface, shift speed really isn't that important for getting a load going, since the truck will continue rolling while the shift is happening.

 

[Moretorque]

If the motor is putting out 237 horsepower, it's putting out 237 horsepower. There's nothing particularly special about whether it's doing that at 1600RPM (like a truck engine) or 6500RPM (like a car engine), both will be able to do the same amount of work in the same period of time (since that's the definition of power). If it takes 237hp to move the truck at 70mph, then the car engine will maintain 6500RPM just as easily as the truck engine will maintain 1600, assuming they're both geared to hit those RPMs at 70mph.

Also, I'd be skeptical about 237hp and a 5 speed moving a 70klb load "no problem" - sure, it'll move it on flat ground OK, but it'll be awfully slow up hills, and the gearing will probably leave something to be desired compared to a more modern truck with a 10, 13, or 18 speed.



Horsepower tells you rate of acceleration under a given load, and also determines top speed. Torque on the other hand doesn't tell you much about the performance of the vehicle, just about what kind of gearing you'll need.

Thee Maxidyne 237 diesel engine is real and yes it is slow but can move 35 tons from 35 to 60 in 5th gear and that is no easy task. So you are saying a 237 HP 6000 or 7000 RPM Gas engine with a broad torque curve can do the same ? cjl, what is your experience in this ? I have been at a drag site learning a few things on this. Thanks for the input.

 

[cjl]

Thee Maxidyne 237 diesel engine is real and yes it is slow but can move 35 tons from 35 to 60 in 5th gear and that is no easy task. So you are saying a 237 HP 6000 or 7000 RPM Gas engine with a broad torque curve can do the same ? cjl, what is your experience in this ? I have been at a drag site learning a few things on this. Thanks for the input.

My experience is that I'm an engineer who has been interested in cars for a long time. I also work on my own cars (mostly, though I'll pay for someone else to do the really annoying jobs), and have also done some endurance racing (admittedly with a very low budget amateur racing team).

As for the diesel, I don't doubt that it's real, and as I said, I don't doubt that it'll go 60mph on flat ground (though I'd imagine it only barely can - I very much doubt it would do 70-80 though). I doubted its ability to climb hills at speed though, and the acceleration will be pretty slow (some back of the envelope calculations indicate that it would take more than a minute and a half to get to 60mph). A gas engine could move the same load with similar performance, though the fuel economy would be rather appalling. The advantages diesels have for heavy hauling lie largely in their substantially better fuel efficiency (and overall thermal efficiency) and their durability, not in their physical ability to pull the load.

 

[Moretorque]

Your saying a 237 Hp gas engine which spun a good degree of RPM could do it with a 5 speed ? like a 350 CI or something ?

 

[cjl]

If it has a similarly broad power curve as a percentage of peak RPM to the diesel, then yes it could.

(Specifically, the breadth of the power curve determines how many gears the engine will need, and the peak power determines its actual ability to haul the load, given an ideal gearbox)

 

[Moretorque]

I am just trying to understand this, Your HP is just the amount of work being done at a given RPM. It is not how you got there and how quickly you got there under a load the amount of torque { ability to spin up and build power under a load } you have at the operating RPM determines this. When you drop the clutch with a load of 35 tons and the RPM drops way down your torque out of the crank shaft determines how well you spin up and recover your RPM HP MPH back. You can clutch it but ultimately if you have more torque the motor will move up the RPM range much better and faster under load.

The 237 HP Mack can move 35 tons like it does because it has the torque of a healthy Pro Stock drag car at the crank. When I tell motor buiulders there are people out there saying you can move 40 tons down the highway at a good clip with a small block car motor HP being equal they practically laugh. They are the ones saying it would have to have a tranny with many many gears and be electronically or air shifted if it had any chance of working and they still say it would not work well at all. You cannot get 40 plus tons rolling from a dead stop and moving up the RPM range unless you have the nuts coming out of the crank shaft to do it. The drag boys know their stuff when it comes to HP and I applaud them for there input in setting me straight. They do not care what the HP # is for the most part they want to know how much torque and at what RPM, when they look for more power they look for more torque to get it and not more RPM spin. Trading torque for HP will not get them down the track faster in most instances.

Torque is to HP what AMPs are to electricity, a 350 small block V8 is a 15 pound 200 watt Kraco car amp and a Semi engine is a 200 pound 200 watt Krell you can weld with. I have come to the conclusion in my book HP is not HP and a motor that does it's work at lower RPM is more powerful HP for HP..

Thanks for all the help.

 

[jack action]

OMG!

They are the ones saying it would have to have a tranny with many many gears

You don't need many gears to get the torque, you need many gears to extend your speed range.

They do not care what the HP # is for the most part they want to know how much torque and at what RPM

If you need to know at what RPM is your torque, that means that you are looking for HP; Because Power = Torque X RPM. If they didn't knew the RPM, torque alone is a useless number to determine performance (and vice versa). HP is a unit that was created just for that: Evaluating the effect of the torque combined with RPM.

when they look for more power they look for more torque to get it and not more RPM spin.

Tell that to a professional Top Fuel dragster or a Formula One engine builder; Then see who will laugh ...

Torque is to HP what AMPs are to electricity,

Actually, torque is to HP what VOLTs are to electricity. AMPs is related to RPM. And - look at that - in mechanics:

Power = Torque X RPM

and in electricity:

Power = VOLT X AMP

And in fluid mechanics:

Power = Pressure X Flow

and in mechanics (linear):

Power = Force X Velocity

See - in any case - power is proportional to a force applied (Torque, VOLT, Pressure, Force) AND the rate at which that force is delivered (RPM, AMP, Flow, Velocity). And 1 mechanical HP = 1 electrical HP; Because power is a unit of measure that represents the same thing, no matter where you apply it.

I have come to the conclusion in my book HP is not HP and a motor that does it's work at lower RPM is more powerful HP for HP..

Again, HP is unit of measure, it's a definition. Let me show you how silly the sentence you wrote sounds like:

I have come to the conclusion in my book a pound is not a pound and a motor that have more volume is heavier pound for pound..

If you keep the same density, yes, a larger motor will be heavier. But if we assume that they have the same mass (in pounds), then two motors can have different volumes if they have different densities (for example, one is made of aluminum and the other one made of cast iron). This is true because:

Mass = Volume X Density

which is the same mathematical relationship as

Power = Torque X RPM

1 lb of aluminum is 1 lb of cast iron is 1 lb of water is 1 lb of air is 1 lb of ... anything! The volume occupied by this mass depends of the density of the material.

Similarly, 1 hp is 1 hp, no matter where it comes from. The torque needed to produce that 1 hp depends on the RPM at which it is produced.

This very fundamental mathematical concept holds everywhere, in every field. Hydroelectric power stations, satellites sent in space, ballistics, name it: They are all parts of science based on that principle. Do you really think engine building is the black sheep of science that do not follow the proven laws of physics?

When I tell motor buiulders there are people out there saying you can move 40 tons down the highway at a good clip with a small block car motor HP being equal they practically laugh.

If they do laugh, it is because they don't understand physics. They usually can build only one type of engine (V8, I4, Chevy, Honda, etc.), because they mostly monkey what others around them have done more than they understand what they are doing.

It doesn't mean they don't know how to build good and powerful engines, but they cannot explain why and how in terms of physics. And people used to build a lot of great stuff before understanding what we know now. But we were able to improve ourselves once we begun understanding what we were doing.

Don't fear the knowledge, it will open your mind to a wonderful world.

Read (and re-read) carefully the link by @Ketch22 in post #92. Make it your bible, because that is engine building based on an understanding of physics.

 

[cjl]

I am just trying to understand this, Your HP is just the amount of work being done at a given RPM. It is not how you got there and how quickly you got there under a load the amount of torque { ability to spin up and build power under a load } you have at the operating RPM determines this. When you drop the clutch with a load of 35 tons and the RPM drops way down your torque out of the crank shaft determines how well you spin up and recover your RPM HP MPH back. You can clutch it but ultimately if you have more torque the motor will move up the RPM range much better and faster under load.

Nope. If you just dump the clutch and the RPMs drop, what determines how fast it will accelerate is still the horsepower. Again, the gearing will need to be dramatically different to make this equal, but the acceleration of the vehicle is directly related to the horsepower the engine is producing at all times. I suspect you keep thinking that both drop down to near idle when the clutch is dropped (although I don't know why you would "drop" the clutch - you get much better results by feathering it at a desired RPM), but there's no reason that needs to be true.

The 237 HP Mack can move 35 tons like it does because it has the torque of a healthy Pro Stock drag car at the crank. When I tell motor buiulders there are people out there saying you can move 40 tons down the highway at a good clip with a small block car motor HP being equal they practically laugh. They are the ones saying it would have to have a tranny with many many gears and be electronically or air shifted if it had any chance of working and they still say it would not work well at all.

And they're wrong. They don't have practical experience with this because nobody actually does this (for a number of good reasons). It's not necessarily intuitive, but horsepower really is all you care about.

Let's look at it this way. Suppose I handed you two black boxes with a throttle. One of them contains a diesel engine (let's suppose it's a 10L turbodiesel I6), with 300hp at 1800rpm (which means it makes 875 lb-ft at 1800rpm) and a torque peak of 1100lb-ft at 1000-1300rpm. The other one contains a small gas racing engine (say, a 1.5L V8) with 300hp at 18,000rpm (thus making 87.5 lb-ft at this speed) and a torque peak of 110lb-ft at 10k-13krpm. This second box also contains a 10:1 reduction gearbox, so the output shaft spins at 1/10 the speed of the engine. This means that the output shaft will spin 1800rpm with 875 lb-ft when the racing engine is at its power peak, and 1000-1300rpm with 1100 lb-ft when the engine is at its torque peak. For the purposes of this thought experiment, let's say that they both weigh the same (because I've added a bunch of lead or something to the second box), and you have no way of telling which is which. Why would you think that box 1 would power a truck any better than box 2? How would you even be able to determine which is the gas and which is the diesel?

You cannot get 40 plus tons rolling from a dead stop and moving up the RPM range unless you have the nuts coming out of the crank shaft to do it. The drag boys know their stuff when it comes to HP and I applaud them for there input in setting me straight. They do not care what the HP # is for the most part they want to know how much torque and at what RPM, when they look for more power they look for more torque to get it and not more RPM spin. Trading torque for HP will not get them down the track faster in most instances.

If drag cars look for more torque, not more RPM, why do the fastest drag cars spin 500ci V8s at almost 9000RPM? Horsepower is what gets you down the track faster, and I'm rather skeptical that you're actually talking to "drag boys" at all, since drag racers should really know this.

Torque is to HP what AMPs are to electricity, a 350 small block V8 is a 15 pound 200 watt Kraco car amp and a Semi engine is a 200 pound 200 watt Krell you can weld with. I have come to the conclusion in my book HP is not HP and a motor that does it's work at lower RPM is more powerful HP for HP..

Thanks for all the help.

Actually, this is another example where power (watts) really is all that matters. A 200 watt car amp will drive speakers just as well as a 200 watt Krell, as long as the following conditions are met:
1) They are both rated with similar methods. A lot of car amps are somewhat dishonest, and promise ludicrously high "peak" power levels that they could never deliver continuously. It also matters what the distortion threshold is for the rating - an amp rated 200W @ 1% THD is not the same as one rated 200W @ 0.01% THD.
2) They are both rated to drive the load. If the car amp is rated 200W into 4 ohms, and the Krell is 200W into 8 ohms, obviously the Krell will be better if you're trying to drive an 8 ohm speaker.

At the end of the day though, watts are watts. If both amps are actually delivering 200W to the speaker (with low distortion and no other flaws), the speaker will perform the same, just as a truck with 200hp will accelerate the same so long as the engine is actually delivering 200hp.

 

[Ketch22]

Moretorque, i appreciate your search. It is a little confusing to me who you are talking with. I deal a lot with racers and truck drivers and they seem to get what we are talking about. I do also see where you are listening to other inputs than the ones here.

Please also consider what you just see in normal life. Two points of reference, a relatively plain family sedan of approximatley 4000 lbs will require somewhere between 16 and 20 hp. to maintain 65 mph on level ground. A typical non-aero tractor trailer fully loaded will run between 200 and 240 hp. to maintain 65 on level ground. It is really interesting that many of the car drivers (that pay attention) and all of the truck drivers know that when you need more power to go uphill you need to downshift. The action of downshifting increases rpm and thus increases power. It most of the time reduces torque as it moves the engine past its peak. If torque is as you say more important how does it play into your observations that even drivers who are not engineers know to do the predictable thing when they need it?

 

[Moretorque]

Sorry about not getting back and I appreciate everybody's time and knowledge on the posts. I understand all the math and what everybody is saying but I guess I am getting bad info but I have talked to top engine builders and they have told me the engine that makes HP earlier is more powerful at building and maintaining it under load in real world app. Top Pro Stock engine builders have told me this and they live and die by the dyno. Most of what I may have been missing is the " power under the curve " which allows you to throw more gear at a motor than one that is more peaky but makes more HP on top.

You guy's back it with math and none of the engine builders have done this so you all are probably right.

Here is what a sled operator told me who operates the sled at a tractor pull. He said one class will be rated at less HP more Torque and will be say 700 HP semi engines then another class will be 1000 plus HP car engines with way less torque and he says the higher HP engine cannot pull close to the load the higher torque semi engine can with less HP. He says the sled is setup for the box to move up slower on the 1000 HP motored car engines. He claims the 1000 HP engine class will get out of the gate faster and run more MPH at first but then it gets choked down way faster. Is this more than likely the power under the curve ? the diesel engine makes way more average HP throughout it's narrow operating RPM range ? Thanks...

 

[Ketch22]

[QUOTE=" Is this more than likely the power under the curve ? the diesel engine makes way more average HP throughout it's narrow operating RPM range ? Thanks...[/QUOTE]

This is more realistically a case of "invalid test." In all classes of tractor pulling the track is 100 meters long. The Sled is also the same including the weight. The operator is correct in that the rate of transfer and positioning of the weight is different. The engineers consulted by the regulating committee decided on a transfer protocol that can use approximately the tractive force developed by each class limiting them to the same "full pull" distance. When a person says I ran two different engines on the same track with entirely different protocols I say you can't compare apples and oranges. My experience in the tractor pulls has me seeing more consistent full length pulls with the modified fuel motors than diesels. I know how closely the operators adjust their weight to meet class regulations. These two things only tell me that the motors in the class are pretty close and drivers are consistent with each other. The short and missed pulls are all obviously a result of loss of traction or steering causing loss of distance. It is the same as going to the drag races. It is almost certain that when I see a racer smoke his tires at any point before the finish I know who is going to loose.