torque

[MajBach]

Re: torque

Tee-boy:<br />

Are you referring to a car engine with a multi speed transmission? It seems that if I put my car in 1st gear and smoothly pressed on the accelerator, the needle on my tach would continuously move to the right

To answer your question: it doesn't matter. An increase in rpms in simply a sign that the power being produced by the engine is GREATER than the load being placed on it. It doesn't have anything to do with the total power. If you go down a hill the rpms will go up even if you take your foot off the gas. In neutral only a slight amount of pedal pressure will take a car to redline - this does not mean it is even close to max power though. I think the 'confusion' might be becasue you are thinking of engines with no variable transmissions -such as an OB - where rpm is steadily increses with throttle. If you were to suddenly throw all the people out of your boat, it would go faster right? You didnt change the throttle nor did you change the power being produced.

 

[tee-boy]

Re: torque

Majbac,<br /><br />If you take your equation that you stated earlier: HP=Torque*RPM and take first partial derivative with respect to RPM you will find that anytime you increase RPM, you increase power. As you stated yourself, HP is a funtion of rpm and torque. So, how can you say that an increase in RPM has nothing to do with power?<br /><br />Not to be picky, but there is no such thing as RPMS only RPM.

 

[Forktail]

Re: torque

Or how much work something can do in a given amount of time.

Ahh, someone finnaly heads in the right direction. <br /><br />Study the equation...<br /><br />HP = (Torque X RPM)/5252<br /><br />An engine turning 2000 RPM with 200 ftlbs of Torque can have 75 HP. And an engine turning 4000 RPM with 100 ftlbs of Torque can also have 75 HP. Both engines do the same amount of work. Potentially both will get your load from point A to point B in the same amount of time.<br /><br />An engine making 200 ftlbs of Torque between 2000 and 3000 RPM will produce the same amount of work as an engine making 100 ftlbs of Torque between 1000 and 3000 RPM. And an engine making only 50 ftlbs of Torque between 1000 and 5550 RPM will do more work than either.<br /><br />On the other hand if both engines produce 200 ftlbs of Torque, but one does it at 2000 RPM and one does it at 3000 rpm, then the engine at 3000 rpm will produce more HP, and thus do more work. <br /><br />Torque is nothing without RPM. RPM is what turns torque into HP, and without RPM the Torque just sits there. The object is to get as much Torque as you can over as much RPM band as you can, at as high an RPM as you can, and gear for it. That's what gets the work done.<br /><br />As far as connecting rod length, stroke, crank throw, etc....there's really too many factors to consider. Some shorter stroke engines will produce more Torque than longer stroke engines. And some longer stroke engines will produce more HP and RPM than shorter stroke engines. See, it's all in the engine's own unique design and operating characteristics. Things like compression, intake and exhaust valving, internal forces, rotating masses, and an engine's durability all come into play.<br /><br />To answer the original question...<br /><br />

Given two engines with same HP; what is the advantage from a boating perspective of the one with more torque? It seems that the 4strokes have more torque than the equvivalent 2strokes.

If both are producing the same HP, and one has more Torque, then the one with more Torque must operate at a lower RPM. <br /><br />In outboards (which don't have variable transmissions) things are a little different, because the HP is rated at the prop, not the engine. And to make matters more confusing that HP is rated at engine RPM. So the engine's Torque and RPM have gone through a transformation at the gear case by the time the prop is turning in the water. <br /><br />The increased engine Torque and drop in engine RPM is usually made up for with a smaller gear ratio (closer to 1), and a bigger, higher-pitched prop. A similar HP outboard operating at a lower Torque and higher RPM will have a bigger gear ratio (further from 1), and smaller prop. In both cases, the object is to maximize the work the outboard can do through propping and gearing, according to the way it will be used (pulling a skier, speed boat, high-thrust trolling, etc)<br /><br />To boaters, engine Torque will always be an advantage when pushing heavy boats, pulling skiers, high-thrust trolling, or any instance where speed isn't a big concern. Remember, HP for HP, higher engine torque means lower RPM. Lower Torque, higher RPM outboards are better suited for performance and speed. But also remember that through the gear case and prop, we can get the HP to produce just about any combination of Torque, thrust, and RPM. <br /><br />Yes, some 4-strokes will have more Torque than some 2-strokes, and the reverse is true as well. Again, the main key is maximum torque over the highest and widest RPM range. If you only operate at maximum RPM, then you want the most Torque at that rpm...because that will give you the most HP (work).<br /><br />I suppose you could discuss an outboard's efficiency, durability, and longevity in relation to RPM, but that discussion would be too ambiguous, and difficult for most to understand here.

 

[Forktail]

Re: torque

By tee-boy - If you take your equation that you stated earlier: HP=Torque*RPM and take first partial derivative with respect to RPM you will find that anytime you increase RPM, you increase power.

Only if the Torque remains constant. In the case of your original post, you are saying HP remains constant. So with a constant HP, you could have both an increase in RPM and a drop in Torque to get the same HP.

 

[Forktail]

Re: torque

Something to ponder that's closely related...<br /><br />Ever notice that a dragster can get the top speed in the 1/4 mile, but lose the race due to a slower elapsed time?<br /><br />Kinda the same thing. That dragster might have made the most HP at one point, but throughout the entire RPM band (start to finish) it did not do the most work.

 

[phatmanmike]

Re: torque

bla bla bla... geez guys, lets all agree that torque is better than good... its COOL

 

[1986mariner150]

Re: torque

Tourqe is what pushes you back in the seat. Horsepower is what blows your eyes shake around in your head. I have resigned from the debating socioty but I will relate a personal experience. I have a s-10 hotrod, 6/71 blower on a build for boost smallblock chevy, never dynoed but somewhere over 600 hp and goobs of tourqe. I take pride in the fact that I beat a lot of the cars that ride in on trailers and almost all the cars driven to the track, then I lined up with this supra He's got 3 liter displacment stockblock 1 big turbo. He has 1/2 the displacment I do. I left on him and had him smoked to about 100 ft then he hit his power band and put a 6.22@122mph with a 2.3 60 ft time on the otherside of my time slip. I ran a 7.56@95mph with a 1.76 60 ft time. He smoked me from 1/2 track on. Someone more famous and smarter than me once said "Horespower sells cars, tourqe wins races", I guess he said that before them dang supras came along. Cause that time will beat about 95% of the fullbody race cars at my track and he drives it to the track, puts the slicks on, and removes a headlight, kicks everybodys a$$ and then drives it back home. If that car had a trans brake and a nine inch it would be unbeatable.

 

[Forktail]

Re: torque

bla bla bla... geez guys, lets all agree that torque is better than good... its COOL

Sorry we're boring you phatmanmike...or maybe all this stuff is over your head?<br /><br />Yep Torque is "COOL", but that description doesn't hold much traction in this discussion. So thanks anyway.

 

[tee-boy]

Re: torque

by fork tail: Only if the Torque remains constant. In the case of your original post, you are saying HP remains constant. So with a constant HP, you could have both an increase in RPM and a drop in Torque to get the same HP<br /><br />Read my post you are referring to not the original (that one is outdated). Anytime you take a partial derivative all other variables must remain constant (in this case Torque is constant b/c the partial is respect to the dependent variable RPM). And that is the definition of a partial derivative (excluding the part about the change in the dependent with repect to a change in one of the independents-only over miniscule moments). And we can only take a partial derivative b/c the aforementioned equation has more than one independent. Fork Tail: So far, I've got confidence in you.

 

[tee-boy]

Re: torque

And to the other guy who posted about torque in one of my other posts:<br /><br />Torque and HP do not cross at 5500 RPM or anyother RPM. These two are measured in different units; therefore, they can never cross. In fact you can't even graph the two on the same axis.

 

[Dhadley]

Re: torque

You gragh it as torque in ft lbs per rpm and horsepower per rpm. The two lines will cross at about 5250 on stock carb'd two stroke outboards. <br /><br />Dynos like a Super Flow with the computer readout will graph it automatically on each pull.

 

[MajBach]

Re: torque

HP=Torque*RPM and take first partial derivative with respect to RPM you will find that anytime you increase RPM, you increase power. As you stated yourself, HP is a funtion of rpm and torque. So, how can you say that an increase in RPM has nothing to do with power?

orktail has more or less answered this bt to paraphrase consider this:<br />Torque is a function of the fuel consumed.<br />You can agree that an engine can operate at 3000 rpm at 5% throttle and 3000 rpm at 100% throttle. Different loads on the engine is what allows for this scenario The difference is, you'r consuming a different amount of fuel - whether is be electricy, lead plates in a battery or gasoline - thus a different amount of torque is produced. <br /><br />HP is a function of torque BUT torque is a the product of the fuel being converted into energy. More fuel, more torque assuming all other factors remain the same like internal resistance.

 

[Forktail]

Re: torque

Yep, that's why a dynomometer applies a load. This produces Torque, at some RPM. From there we calculate HP.<br /><br />HP and Torque will always cross at 5252 RPM on a dyno chart. If it doesn't, something is wrong.

 

[kenimpzoom]

Re: torque

The fastest car at the track (all things being equal) is the one with the most area under the Torque vs RPM curve.<br /><br />I.E. you have an engine that has 300 ft/lbs torque from 1000 to its rev limit of 5252 rpm. It will have 300 peak HP.<br /><br />The other engine has 10 ft lbs of torque from 1000 rpm to 5000 rpm, then 500 ft lbs of torque from 5000 to 5252 rpm. It would have 500 peak HP.<br /><br />Plot those two torque curves and see which one has the most area under the curve.<br /><br />The first engine is the winning engine, with these circumstances: both start in first gear and they start from 1000 rpm, dump the clutch, and race till the both get to 5252 rpm.<br /><br />The next variable is the transmission. This could make the second engine the winning engine if you have a transmission that can keep the second engine in between 5000 and 5252 rpm.<br /><br />(whew... this post started out simple and grew!!!)<br /><br />Ken

 

[Forktail]

Re: torque

The fastest car at the track (all things being equal) is the one with the most area under the Torque vs RPM curve.

...or in other words, the one that produces the most work. Work = HP. And HP = Torque X RPM/5252.

 

[jim dozier]

Re: torque

I absolutely agree with Forktail. Most of my discussions on this topic and screwy examples were a roundabout way of saying that the area under the torque curve is what counts not the peak.

 

[tee-boy]

Re: torque

Now that helps. So the constant 1/5252 in the equation is the inverse of the max torque which also equals inverse of the max of torque/one revolution per minute. Therefore, the proportion of the instantaneous RPM to the 1/5252 times the torque is approximately equal to your instantaneous HP neglecting any power transfer.

 

[Forktail]

Re: torque

I think we're going to lose everyone here by getting into a calculus discussion, although it and some other discussion would eventually be necessary in order to determine the total work done throughout the RPM band. <br /><br />At this point, all we need to know is that HP is calculated from measuring Torque and RPM. We must understand that this will give you the HP at one point, and that Torque and RPM will vary over the entire power band. <br /><br />5252 is not derived from Torque, and in fact it is not a derived constant at all. 5252 is simply a unit conversion. One foot pound of torque at 5252 rpm is equal to 33,000 foot pounds per minute of work, and is the equivalent of one horsepower. Thank you Mr. Watt.

 

[walleyehed]

Re: torque

but again, the one with the most displacement (and thus more torque) will be faster. <br />

Gotta laugh at this one, sorry!<br />Dhadley was puttin' a serious hurtin' on the 149ci. V6's with his 99ci. V4's.....1/3 smaller, but faster...... <br /><br />Forktail, good to see ya still at it!<br />I have a Yamaha dealer about 2 miles up the road from me that is selling out as another firm has bought the buis., and has offered an F150 for 10,200 installed, with a SS prop. I've had the oppertunity to run several of the Yammys now, in Tx, and man are they smooth......I hate to say it, but I'm considering this 150....I know it's a new model, but any insite????<br />ktsanderson@st-tel.net<br />Thanks!<br />K