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Understanding Dyno Readings -- is HP "real world" HP?

  1. May 28, 2017 #1
    I did a thread a long while back and I was like really confused because one group was saying it is and another said no. This was bouncing off different sites and I believe I now know why I was having a hard time. I believe the information I am looking for may be rare.

    I understand it much better now and was asking the question in the wrong way because I did not understand completely what I was looking for.

    You have 2 engines, ( this is hypothetical ). - They are the same HP and have the exact same power curve on paper but one needs twice as much RPM to do it so once you cut the gear in half on the engine that needs more RPM to do it's peak HP they are the exact same in real world use or is this wrong.

    One group was saying it is the same and one group says no it is not. One of the groups is saying the engine that does the work earlier with more torque is more powerful in the real world and can drive more load to peak output.

    Thanks for any input.
  2. jcsd
  3. May 28, 2017 #2


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    Since the formula for HP is Torque x Speed x Constant, you are hypothetically right.

    In the real world, the minor corrections would be the efficency of the gearbox and the probable different rotating mass of the two motors. The lower RPM motor likely has a larger bore and therefore a larger rotating and reciprocating mass, but has a higher torque to accelerate them. Don't know if that is significant or not.

    I suppose the difference between the motors could also be that the lower speed one could have better aspiration and a speed limiter.:smile:
  4. May 29, 2017 #3
    Thanks, I was wondering how much of what people are perceiving is the fly wheel affect and the torque rise of the diesel engines makes it to where the average HP throughout the RPM operating range is much higher because it was designed to drive a big load and the MPH - HP is not as important.

    The reason I ask this is because talking to people who have had engines of the same HP rating on a dyno buy a diesel after owning gas and say the diesel just slaughters the gas engines on hills when pulling in a Pick Up even though they are rated similar power or the gas engine is rated even higher HP and they find no matter how you gear the diesel accelerates the load better on hills and maintains MPH much better.

    Then I talked to serious engine builders who said do not be fooled the engine that does the work earlier is a more powerful engine and torque is what current is to electricity and you can only gear around lack of torque to a degree when driving a load.

    The other clan says the math does not lie, in any event the group saying the torque engine is more powerful has never backed it up with physics and math so I am like huh who is right?

    Again thank you.....
  5. May 29, 2017 #4


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    I've owned both gas and diesel cars. Diesels generate their max power at a lower rpm than gas so they feel and sound under much less stress than a gas engine even if the HP is the same. However the numbers cannot lie. If the HP is the same then it's the same.

    There are a lot of other subjective factors. For example the width of the power curve effects how many gears you need to cover the speed range. If the peak is very narrow you need a lot of gears and since each gear change takes time that can be an issue.
  6. May 29, 2017 #5
    Thanks CW, yaa when I say the average HP I am talking how broad the torque curve is. The more low end you have the easier it is to get the load going from a dead stop so diesels have a major advantage here..

    Here was a ? I asked to another poster on this board and he said it was interesting and he would look into it but never got back with me.

    This is complete opposites and the engine builders told me this would teach the people who think HP is HP it isn't in the real world but as stated they never backed it up soooo.

    I found a ship engine that made 2300 HP at 270 RPM and made over 44,000 thousand foot pounds of torque.

    The top engine builders ( Pro Stock n Tractor pull ) were claiming your peak HP is your peak MPH and how much torque you have and how broad it is is your ability to drive a load to that peak.

    So they were saying because this ship engine made the torque of 30 modern day loaded semi's it could drive 40 tons x 30 = 1200 tons to speed on all the axels to a peak MPH of whatever that 2300 HP would get you which I do not know. They said it would do this with no problem with out that much in the way of a lot of gears.

    They told me if you took a 2300 HP V8 9000 RPM car motor and hooked 3 miles of Pro Stock cars which is 1000 in a row which ='s 1200 tons that in no way could it ever accelerate that load, it does not have the nuts coming off the crank to drive that load no matter how you gear. I don't know but I am real curious about it....

    They said the ship engine would just slaughter it in the ability to move big big load....
  7. May 29, 2017 #6


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    There is some merit to that, though it is extreme to the point of being silly; the larger and slower the load, the larger the reduction gear train required to get it moving, and that can add a lot of drivetrain loss.
  8. May 29, 2017 #7
    I am real curious about this because if I can understand this then I can understand how to correctly read a dyno to a T and what it means. I don't mean to be silly but a ship motor in a Pro Stock car might be interesting....

  9. May 29, 2017 #8
    An interesting topic, I guess I go with the maths rather than the other group which seem to be relying on subjective assessment. I've heard too many stories of how changing the oil made the car feel so much faster that I am very weary of these sort of claims.

  10. May 30, 2017 #9
    Ketch 22 said he would figure it out and get back with me but I think I may finally understand it, the motor builders were animate and said most people do not understand HP and if you do not have the torque you need you can only gear around that to a point.

    I was using a ship motor because it is a total opposite.

    I think it goes something like this and I am math stupid so I may be totally full of it but here is a analogy of the problem of why.

    First off the 2300 hundred HP Pro Stock engine does not have the fly wheel mass to get the 1200 hundred tons rolling and here in lies the problem right from the start. It's not that it lacks the power to roll the 1200 ton mass it lacks the mass and nuts coming off the crank shaft to accelerate it no matter what.

    So lets say you ( Hypothetically ) hung a 10 ton fly wheel mass on the crank off the Pro Stock engine and spun it up to 10 grand so you could get the load rolling to 1/2 MPH in first gear then you shift to 2nd AND all the RPM has been taken out Of the Pro Stock engine and it cannot spin up fast enough to 10 grand again to get back at the power to keep driving the load when you shift so it would just crash right there.

    Now the engine builders said it could keep it rolling but never drive it and were saying it may have the work time on paper but real world the torque monster is way more powerful when put in use for this reason I believe?
  11. May 30, 2017 #10
    Torque is merely how much strength the engine is capable of twisting the crankshaft with. Grab a big wrench, put it on a bolt head, and push as hard as you can. The twisting force that is applied to the bolt head is what torque is. Whether the bolt head rotates or not, you are applying torque to it.

    Horsepower is the ability to do work. So it deals with the strength of twisting force being applied (torque), and it also deals with the ability to apply a certain amount of twisting force while the engine is spinning.

    Think of this scenario to compare horsepower and torque:
    There are two anchors on a boat. Each are hanging from ropes that have their own spools. Each anchor has the same weight. Lets say there are two guys cranking them in at the same time. Spool #1 pulls the anchor in 3' per turn of the crank, but each revolution takes 90 seconds. Spool #2 pulls in 1' of rope per revolution, and each revolution takes 30 seconds.

    Both spools have the same horsepower, because they did the same amount of work in the same amount of time. Spool #1 has 3x more torque, but spool #2 got the same work done because it was able to do each of its cycles 3x as quickly as spool #1.

    Horsepower is a function of speed and torque. Here is the basic equation. Horsepower = Torque x Speed / 5252
  12. May 30, 2017 #11
    Applying 20k lbs of load (I assume you mean ft-lbs) to the engine will cause it to stall unless it is capable of generating 20k ft lbs of torque at an RPM at which it will run. However, the engine will stall before 20k ft-lbs is fully applied. It will stall shortly after the amount of torque demanded from the engine exceeds the amount of peak torque that it can apply.

    I don't think that the engine builders and you were talking about the same thing. You almost need to start back at square one on this subject. Forget everything the engine builders told you. I work in engineering for an engine manufacturer. I'll tell you that not all engine builders know what they're talking about on every single subject regarding engines. Some of them know how to bolt on camshafts. Some of them know how to operate a dynamometer. Not many of them understand the physics behind any of it. The ones who do, usually wind up in engineering.

    Read these links:


    Dynamometers apply constant load to the engine by absorbing the torque
  13. May 30, 2017 #12

    jack action

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    What an engine builder is saying - maybe without realizing it - is that the Pro Stock engine has very low HP at low rpm. Knowing the maximum HP engine does not say anything about the power at low rpm. This is why they often rely on maximum torque figure (which often happens at low rpm) which is an indication of how much power is produced at lower rpm. The problem is that torque will compare only if the rpm is exactly the same.

    I can assure you that the ship and pro stock engines do not have comparable power curves at lower engine speed when their rpm are matched by proper gearing. The pro stock engine will have terrible power below 4000 rpm (about half of 9000 rpm) as the ship engine will probably still deliver good power when as low as 60 rpm (about 20% of the 270 rpm). The power will always decrease with lower rpm, but it will happen much faster with the pro stock engine (that is why they don't have a good idle).

    The major difference between the ship engine and the pro stock engine is the mean piston speed at which they produce their maximum HP. The pro stock engine is producing max HP at mean piston speeds of 25-30 m/s (and is very bad below 15 m/s), while the ship engine is probably at 7-10 m/s when at max HP and is still making good power at 2 m/s (idling). The reason is that the mean piston speed range that a single camshaft design can cover is about 10 m/s-ish. So if you tune your engine for 20 m/s, it will be good for 15-25 m/s, if you tune it for 10 m/s, it will be good for 5-15 m/s. Outside this range, the valves will close too fast (choking at high speed) or too slow (flow reversal at low speed). If you drive often at low speed (stop & go), you will need an engine tuned at low mean piston speed and in that case - if you want power - you have no other choice than increasing engine size to increase the torque. Race cars start the race once and then try to keep their rpm high for the rest of the race, so no regular stop & go. Having high-speed engines allows them to have smaller engines, thus smaller weight, thus higher acceleration.

    If you want to compare engines, use their mean piston speeds instead of their rpm's.
  14. May 30, 2017 #13


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    Well, what you are saying here is that the "torque" people are starting by saying the HP is the same and then presenting a scenario where it isn't.

    A drag racer might use the inertia of the flywheel for part of the starting torque, but in typical operation of a normal vehicle you should not. That's what a clutch or torque converter is for.

    The bottom line is that one isn't "better" than the other in general; it depends on the needs of the vehicle....which is why you see such a wide range.
  15. May 31, 2017 #14
    Thanks for all the replies I will read the links.

    What the engine builders are saying is you loose something ( POWER WISE ) in real physics when you are required to turn more RPM to make your HP. They claim the peak MPH is the same but the foundation of the power is not, hills or wind blows will affect the lower torque engine more even if the power curve is matched equal on paper it will loose more drive and MPH.

    Essentially saying in a drag application all things being TOTALLY completely equal on paper including rev gain the engine that does the work earlier will loose less drive shifting and getting off the line because the extra foundational back support to the HP that the higher torque engine provides.

    They said it would run a slightly faster time and that a ship engine would show you in real world moving massive load just how big a difference it can be when really pushed to the limit.

    Ketch 22 said he will chime in on this, again thanks for the help I....
    Last edited: May 31, 2017
  16. May 31, 2017 #15


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    So many of these statements contradict each other or are gibberish (what is "foundational back support"?). But the last takes the cake: the top speed of cargo ships is only about 30 mph!*

    Look, you said you want to know if the numbers lie, but you aren't looking at numbers. Please go get some numbers! Google for the specs of a real pickup truck with both gas and diesel models and calculate what speed they operate at in their top gear at peak (assume rated) horsepower.

    *Counterpoint: For a while my other car was an Oliver Hazard Perry frigate with a top speed of about 30mph, powered by a gas turbine engine at 3,600rpm. So there's that...
    Last edited: May 31, 2017
  17. May 31, 2017 #16

    jack action

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    In your first paragraph, you compare 2 engines at the same power output but different rpm and state that one gives «better» power than the other, which is wrong.

    But then - to prove your point - you say: «the engine that does the work earlier will loose less drive shifting and getting off the line [...]». Shifting being the operative keyword. When you are shifting, you are not at the same rpm anymore. You are at a lower rpm. We already established that the lower rpm of the high-rev engine has less power than the low-rev engine. That's where you are loosing momentum.

    The real difference is that the high-rev engine power curve is more «peaky» than the low-rev engine power curve. If you wanted to keep them at equal power (i.e. maximum power), you would need a lot more gear ratios with the high-rev engine compared to the low-rev engine. That's because the rpm range where the power is at its maximum for the high-rev engine is narrower. If you can have 10 or 20 gear ratios with a low-rev engine in a truck, imagine how many you would need for a high-rev engine. In real life, when you are shifting, you are also loosing power because you usually need to disconnect the engine from the driveline somehow. No matter how fast you do it, it will affect your power output in a negative way.
  18. May 31, 2017 #17
    Let me go back and read all this Real Closely so I do not sound so stupid and make a mess and they lock the thread like last time.

    When I am saying back support they are saying the amount of twisting force coming off the crank, I know about the #'s and how to read it I think? and how broad the Torque curve is and average HP. These were engine builders who test on a dyno's all the time and I am not saying they are right but they were just saying a lot of people do not understand HP to a T and they are engineers.

    Take a semi, when you buy one to pull in the mountains you need a higher HP package. A dealer I was talking to said all you need is 50 HP more on the average for this condition going from 435 HP to 485. They give you all torque for the HP not more RPM, yes a tractor trailer motor has lots of torque rise so it makes a good average HP over the entire operating range.

    It only needs 50 more HP of pure additional torque to move 40 plus tons up mountains effectively which is real impressive. In a higher RPM turning engine an extra 50 HP ( Lets say 500 HP at 8000 RPM ) would not do much for being able to move 40 tons up mountains no matter how broad the torque curve I believe I may be wrong.

    They also said a modern tractor trailer is a real good example of why HP is not HP in real world work application.

    Thanks, l"ll read more on the subject.
  19. May 31, 2017 #18


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    That's not called "back support", it is called "crankshaft torque". But you don't want crankshaft torque, do you? You want torque applied to the wheels.
    I'm trying to decide if I should lock the thread, keep prodding you to be more rigorous or do it for you. I don't care what "they" say or what vague scenario you remember from a conversation with "them". I want to see *real* numbers comparing *real* vehicles and engines to each other and suggested a specific scenario that should be pretty easy to run. Can you do that?
  20. May 31, 2017 #19
    Forget all of your conversations with them.

    Torque is twisting force.
    Horsepower is the amount of work that the engine can do in a given amount of time.

    To do lift a certain weight to a certain height within a certain amount of time requires a certain amount of horsepower.
    To do the same lifting job with an engine that is spinning at at lower RPM would require that the engine makes a lot of torque at that RPM. It would also require a gearbox so that the engine can move the weight quickly while not changing the engine's actual speed.

    Again, forget everything that those guys told you and start fresh. Your conversation with them was confusing.

    Look at this equation. Pay attention to it. This equation is how dynamometers calculate horsepower. Dynamometers measure Torque, and they measure speed. When you combine torque and speed, you get horsepower.
    More torque at a certain RPM = more horsepower.
    Same torque, but at a higher RPM = more horsepower.


    Where T = Torque (lbft)
    N = Speed (rpm)
  21. May 31, 2017 #20
    I once had a full time job running an engine dynamometer. I met plenty of dyno technicians who knew how to push buttons and make the machines run, but they did not understand what the numbers meant or how they would translate in the real world. They didn't need to know that stuff. Their job was to turn the wrenches and execute the test, just like an engine builder who tests on a dyno in his shop.

    Anybody who does not thoroughly understand the concept horsepower or torque is automatically disqualified from referring to themselves as an engineer of any sort. You cannot make it past freshman year in college if you don't understand horsepower and torque thoroughly.
  22. May 31, 2017 #21


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    [sigh] Ultimately my impatience (and wanting to see the answer) makes the decision for me:

    Let's look at the Ford SD (2011-15) and try to compare similar horsepower options:


    Gas: 6.2L V8, 385 HP @ 5500 RPM and 405 ft-lb @ 4500 RPM
    Diesel: 6.7L V8 (turbo), 390 hp @ 2800 RPM and 735 ft-lb @ 1600 RPM

    If I'm reading the wiki correctly, they have the same transmission, which makes the math so easy you almost don't even have to do it:

    The gas version is geared for twice the speed as the diesel. Comparing torque is tougher, but even if near flat the gas version would generate about half the torque of the diesel at a given rpm.

    What this means in the real world is not complicated: if you want to drive fast but not haul much, get the gas version. If you want to haul much but not go fast, get the diesel. And if you want to compare gas to diesel, make sure the comparison is fair and criteria clear!
  23. May 31, 2017 #22
    I understand all that and how to read a dyno, I am essentially wondering how much of it is broadness of the torque curve or the fact there is more torque coming off the crank. I understand the broader your power spread the fewer gears you need.

    Something does not seem right to me, like I was saying about the Semi Tractor it only needs 50 more HP to run 40 tons in the mountains.

    So lets just say for comparison on a smaller scale.

    I own a CR 500 motorcycle that makes 55 RWHP at 5500 RPM, so lets say it makes 1/10 of the HP of a semi tractor for doing mountain work and it does it over a kinda broad area for a dirt bike.

    So lets say you take a 80,000 pound rig and divide that by 10 you get 8000 pounds. The bike weighs 400 pounds with rider then you tie 7600 pounds behind it with wheels.

    In no way in hell could this bike drag that weight up a hill of any grade with any type of MPH no matter what the set up. It would have a hard time with just 2 people aboard let alone dragging 7600 pounds even with a arrow package.

    Please do not lock the thread and I will study in detail...... Thanks
  24. May 31, 2017 #23
    If the gear ratios accommodated it, the bike could apply 55hp of work to the system. You would need to gear it very low, but it could do it. The bike would be at 5500RPM, under full load, total system speed could be low, and the bike would be applying 55hp worth of work to the system. The 8,000lb system would move accordingly.

    The gear ratio change allows there to be a lot of twisting force at the rear wheel, while the twisting force and speed of the engine stay the same.

    Read this article

    By the way, a CR500 will not struggle with two people on it! That bike will flip over backwards before any amount of rider weight can cause it to struggle.
  25. May 31, 2017 #24
  26. May 31, 2017 #25


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    These numbers don't mean anything because the scenario is not clearly defined. So we can't offer any help with that question.
    Why not?
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