Horsepower vs. Torque

[aerobat]

Re: Horsepower vs. Torque

let give me you a alternative explanation- and this even from cars not boats since a combustion engine does not care much if it powers a car or a boat . do you know a little german maybe ? here is one link to the technical data of a nice family sedan , the BMW 3 series :

http://www.bmw.de/de/de/newvehicles/3series/sedan/2011/showroom/technical_data/index.html

you can switch between the models- lets compare the 320 d ( diesel ) and the 320i ( petrol ) . because both have exactly 184 horsepower - but the diesel destroys the petrol when it comes to torque.

now look at the top speed and the accleration. both have the same top speed and the same accleration... its not me , its BMW.

even when the diesel has with the same power output far more torque it is neither able to acclerate faster nor to reach a faster topspeed . why ? why this torque difference at the same power ? what the heck is the torque ?

please try to explain yourself and us in this topic why i am right when i write that engine torque alone means nothing.

try ! we will help you, but let us all keep this topic clean without a torque battle

 

[tomstehr]

Re: Horsepower vs. Torque

So..... I should put a diesel on my boat??

 

[tx1961whaler]

Re: Horsepower vs. Torque

So..... I should put a diesel on my boat??

...and now we start the HP vs. weight discussion ....

 

[aerobat]

Re: Horsepower vs. Torque

a fuel efficent diesel would be a very sweet thing at your boat- especially when you would have to pay european fuel prices but it not the theme .... why is the diesel with a significant higher torque but the same power not able to outperform the lower torque petrol engine ? - due to official BMW technical data ?

 

[tomstehr]

Re: Horsepower vs. Torque

Well you see, it all started one night when we were on a dismounted patrol. I couldnt find my beef jerky, so I figured I would take some cheeto's instead. That was when Senior Cheif came to do PCC's and PCI's. And our truck, which was lighter and faster was surely no match for the CAT. So, in summary the diesel would be 30% more fuel efficient than a comparable gas, or petrol engine.

 

[aerobat]

Re: Horsepower vs. Torque

thats is a superb explanation for torque vs horsepower but the diesel and petrol BMW have the same weight - look at the data. weight is not the answer here ... hehe

 

[Frank Acampora]

Re: Horsepower vs. Torque

Not having actual hard data I can not argue. But I will!

Now, remember: I am comparing 2 cycle against 2 cycle, same engine design, only one more cylinder and more cubic inches. The only difference is the number of cylinders and displacement. With BMW petrol and diesel you are comparing different engines completely. Comparing apples to oranges.

However personal experience with MY engines show that the 4 Cylinder 90 horsepower engine will noticably out-accelerate the three cylinder 90 even though both reach the same top speed. On that point I am not wrong. My interpretation that it is due to the increased torque of the four cylinder engine may be. Note that I am taking the factory at its word for rated horsepower being 90. Indeed, that may not necessarily be the case as horsepower can vary about 5% from rated. Thus I may be in a situation where the 3 cylinder makes only about 86 horsepower and the four cylinder makes 94--an eight horsepower difference that MAY account for different accelerations. BUT, I doubt it, The difference is too noticeable.

Change the torque CURVE and raise (increase) the area under the curve by increasing cubic inch displacement and cylinders, and the engine will make MORE horsepower averaged under the horsepower curve from idle to top RPM even though ultimate horsepower and torque may be the same. That is, the horsepower and torque curves cross at the same RPM. So, yes, comparing a single torque number for a given pair of engines means nothing.


And, I never said that torque alone means something. I do not disagree with you on that point. I said that we could infer some expectations--not actualities. In the BMW ptrtol vs. diesel example, I would expect that the diesel would "pull" better in the higher gears--that is, if it were stick shift it could acheive lower RPM without stuttering or stalling.

 

[aerobat]

Re: Horsepower vs. Torque

Not having actual hard data I can not argue. But I will!

Change the torque CURVE and raise (increase) the area under the curve by increasing cubic inch displacement and cylinders, and the engine will make MORE horsepower averaged under the horsepower curve from idle to top RPM even though ultimate horsepower and torque may be the same. That is, the horsepower and torque curves cross at the same RPM. So, yes, comparing a single torque number for a given pair of engines means nothing.


And, I never said that torque alone means something. I do not disagree with you on that point. I said that we could infer some expectations--not actualities. In the BMW ptrtol vs. diesel example, I would expect that the diesel would "pull" better in the higher gears--that is, if it were stick shift it could acheive lower RPM without stuttering or stalling.

hey frankie ! the torque curve is a point which we should consider at the next lesson. basicly we try to point out what torque basicly is and what not. some points of your posting are interesting for the discussion- but later, it will not be forgotten. the diesel vs petrol comparision is the best demonstration to show two same power output engines but with significantly different torque - and thats what we talk about !

 

[Frank Acampora]

Re: Horsepower vs. Torque

AHH! But we MUST consider the torque curves. Remember that torque published is maximum torque and at a given RPM--not necessarily the same RPM as the competing engine.

In the diesel vs gasoline engine the curves are different even though the area under them is the same. THUS in the same vehicle both engines will have the same acceleration (maybe) and top speed. They both are developing the same AVERAGE horsepower from idle to top speed.

Put both cars on the drag strip though and while they have the same elapsed time and top speed--thus the same acceleration, I would be willing to bet that at any given point except start and finish lines the cars would NOT be neck and neck since at any given point on the drag strip they would be developing different amounts of horsepower thus different instantaneous acceleration.

 

[tomstehr]

Re: Horsepower vs. Torque

Okay.. While I dont plan to drag race my boat, I am planning to use it several ways! Of course, fishing being key of these things. I am not a competition fisher, so holeshot and top speed dont effect my fishing trips that dramatically. I also plan to pull a skier or tube, and just cruise with the family. The one thing that I would want to know the torque for would be tubing, as it requires the motor to work more pulling someone out there and I do not plan on doing it at WOT. If any of you would like to tube behind my boat while I am at WOT, then i guess torque wouldn't seem that important. So, what I have gathered from all of these posts is that torque isnt a factor for boating activities as it is for pulling a load. By which I mean, my truck is rated at over 300hp, but I would never think of trailering a 40000 pound load like a truck tractor rated at over 300 hp. The reason I would think a big truck can pull such a load has something to do with torque. Not just its gearing, but the developed engine torque. I know shifting gears isnt really an option in boats, and I was trying to see if the torque of the engine would help me prop for all activities well, or if I would be swapping blades depending on my chosen venture! I am getting a good lesson from this, but I think the crucial point I was aiming to determine was missed! Can anyone tell me if I need to have three or four props for a week at the lake? Or can I find something that will do all of these things well?? That was why I was curious about torque! Very educational and informational post though.. Thanks guys... OR.. girls!

 

[Philster]

Re: Horsepower vs. Torque

A dream prop for many recreational boaters is one that allows the engine to turn about 100-150 RPM above its engine's upper RPM limit when lightly loaded at low elevation in temperate weather (oh, 70 degrees and low humidity), has four blades and is operated by someone who has the sense to self-limit the RPM should he/she ever find themselves all alone on a light boat in mild weather.

As it gets hot/humid, the engine loses power (and RPM). As you load it up, it gets slower, even more-so if it's not particularly torquey. So, you can change props or get one that falls into the ideal range I described. A huge majority of the time, this prop I describe is only made possible by a prop shop, tuning to very specific specs you provide.

.

 

[Frank Acampora]

Re: Horsepower vs. Torque

For your intended use, you need not know torque (maximum) and torque curves are probably not published. The best you can do is equip the boat with any brand name engine at or near the maximum horsepower the hull is rated at. ( some here on the forums say a minimum of 70% of maximum rated horsepower is required for satisfactory performance.) Then, equip it with two props: A higher pitched prop for cruising, fishing, tubing, and wakeboarding. Carry a lower pitched prop as an extra and for use with waterskiing where holeshot IS important. The quicker you get the skier out of the water, the less strain on him/her.

 

[aerobat]

Re: Horsepower vs. Torque

Change the torque CURVE and raise (increase) the area under the curve by increasing cubic inch displacement and cylinders, and the engine will make MORE horsepower averaged under the horsepower curve from idle to top RPM even though ultimate horsepower and torque may be the same. That is, the horsepower and torque curves cross at the same RPM. So, yes, comparing a single torque number for a given pair of engines means nothing.

hey ! you see we start to agree. you say by yourself that comparing ( or publish like asked here) a single engine torque number means nothing- correct !

further, discussing the torque curve , you start to speak about horsepower in midrange - thats also correct ! its not the torque it pushes your boat- its the power it develops at the prop.

all we have to do is to understand that engine torque is not more not less than the rotational force of the crankshaft. if the the crankshaft starts to rotate a power output is always developed from physics- not just a torque. when power output stays the same torque is 100% tradeable against rpms. you can reach a given power with a lower torque and higher rpms, or vice versa.

so when you have two different engines with same power but different torque the ONLY way to get this are different engine rpm,s . the lower torque engine rev higher and will use a lower gearing for getting the same output at the wheel or prop. when the torque curves of both engines are the same both engines will get the same top speed and accleration ( see again the bmw)

when not- e.g the one engine gives a strong low end torque and keeps a flat torque curve in midrange where the other torque curve is very steep with a summit at high revs the one engine produces in low/midrange simply more power than the other and of course makes your boat acclerate faster in midrange.

the crucial point is : in a combustion engine its finally all about power output since a combustion engine is unable to produce just a force standing still.

when something spins - let it be the crankshaft, the prop or the wheel- we always talk about the final product how strong and with what velocity it spins - and that is nothing other than the poweroutput .

 

[Philster]

Re: Horsepower vs. Torque

aerobat: Your points are valid, but don't use the example of the diesel vs. gas powered car with the same HP and different torque ratings as an example to make your case.

Each is geared for its particular strength, but auto makers have fuel economy standards and, when gearing a car, they don't actually gear it to provide the maximum acceleration or speed possible. So, the the gas engine might have more to offer(not saying it does), but maybe is restrained by gear ratios provide the best around town MPG numbers. Maybe the diesel has more room for MPG error, and relative to that particular vehicle the gearing is slightly better suited to more aggressive gearing while not falling below MPG rating goals the mfgr determined was necessary.

Again, not arguing against your points, but the diesel vs gas car of identical HP can only be used an example if both are geared with absolute top performance in mind regardless of fuel efficiency.

 

[tomstehr]

Re: Horsepower vs. Torque

aerobat, I am starting to see the light! Based on what you posted, it is easier to see why the prop selection for one application wuld be suffiecient for the other. Because of the convergance of the two power curves.

 

[aerobat]

Re: Horsepower vs. Torque

aerobat: Your points are valid, but don't use the example of the diesel vs. gas powered car with the same HP and different torque ratings as an example to make your case.

its a practical way to show up that engine torque alone does not push you any further or quicker . when you take two petrol engines for the same application ( boat engine, road engine ) the higher torque one will usually develop also more power since both rev in a similar region so its useless for the discussion.

here we discussed the question what when comparing two engines with the same power output but significant different torque.

like said - the only way to get such a situation are engines with significant different rpm capability - and the diesel with its limited reving capability but a massive torque advantage compared to the higher reving petrol engine ( the diesel cannot offer high rpm,s so it has a higher torque to equalize the rpm disadvantage to get the same output) is a great thing ro show the final result :

both cars have the same performance .

and this shows that finally its all about power output at the wheel or prop and not the torque at the crankshaft .

cheers !

 

[CaptainKickback]

Re: Horsepower vs. Torque

Reading this thread is just too interesting to not throw not my $.02.

Lets go back to the formula stated by Scott Danforth in Post #2:

Horsepower = torque x RPM / 5252

Now, any torque statement by a manufacturer would be dependent upon the rpm. It appears everyone assumes they publish the torque at WOT. However, since the torque is not constant over the rpm range, if they wanted to talk about, say 2000 rpm, they would give the torque developed by the engine at that rpm, which would change the HP calculation accordingly. So:

1) Any torque number discussion is meaningless without the rpm.

2) Calling an engine a high torque engine is a subjective thing indicating its torque at lower rpms is likely higher than non-high torque engines.

3) At any given rpm, if you look at the formula above, rpm becomes a constant (meaning its the same). You are left with all constants in the formula except torque and hp. Therefore, stating one states the other, its just a different scale. Kind of like Fahrenheit and Celsius.​

 

[QC]

Re: Horsepower vs. Torque

1) Pretty much

2) No, there is a calculation that quantifies this. It is called torque rise, and it is the difference in percent between torque at rated RPM (You're using WOT, but that's not technically correct) and peak torque (rarely at rated RPM). 30% would be a high torque rise engine. I don;t think I have seen this spec in over 15 years. It's misleading as an engine rated at 325 bhp and 1350 lb/ft torque, would have higher torque rise than the same engine rated at 400 bhp and 1550 lb/ft. Kind of stupid. These are fairly realistic HD truck engine examples.

3) Yes, you can't have horsepower without both torque and RPM. So stating "all you need is torque" under any circumstance regarding an engine is ludicrous.

 

[tomstehr]

Re: Horsepower vs. Torque

Ok... awsome technical and scientific information. But, what does it mean to me and my boat? I know it has 150 hp at WOT, but where thetorque curve ocurs and how I can use that info is what I would like to know. Again, at WOT I can get where I am going fast.. but what about pulling a skier? does this torque curve play into how easily I can pull someone(skier) or something(tube) across the water?

 

[MTYPockets]

Re: Horsepower vs. Torque

Diesel vs gas? Volvo Class 8 hybrid truck vs Ferrari
http://www.youtube.com/watch?v=1cVtzt_bbhM