View Full Version : Duramax compare with the 8.1 litre ??
I'm a little suprised to see you make a statement about the Pontiac with your earlier thoughts on combinations and how important the total package is . By design the Pontiac is limited in head flow and this is really a crippling flaw when it comes to high RPM racing . Mostly they are known for the torque they produce and the lower operating range .
My friend did the Super Stock tricks with a mild stoke and longer rods . A friend of his has a porting business and got the intakes to flow around 275 , which is very good . When you see a car that runs 10.70's and looks like it drove in the gate , that's impressive . He runs it to almost 7000 but would like to limit that to 6600-6700 . So he works on it .
The question about the rears is really about something that ZF said about a semi and a pickup truck and the torque that each one makes at the rear wheels . He feels that a pickup can make the same torque as the semi . What I'm getting at is the shear size of the rears . The theory of torque is force x length . If you are trying to get the torque that is applied to the axles , the length is the diameter of the ring gear / 2 and the force is the torque from the trans applied by the driveshaft to the pinion . In this case the larger ring gear does multiply torque more than the smaller ring gear . The rear axle in a truck , this was a 16 1/2 inch Eaton ring gear that I measured , does make more torque than a 10 inch ring gear in a pickup truck . This observation does not apply to the Dmax vs 8.1 test , as they have the same rears , but when someone makes a statement or calculation that doesn't support their theory ... I start to look real hard at the other thoughts that come along . Some of the earlier calculations were based solely on numbers , not taking into account the actual size of the objects . I don't feel that you can simply multiply torque readings by a gear ratio and get a multiplied torque output .
Look at the statement that you just made , " HP is the goal and torque is what is required to get the HP at the desired RPM " . If you keep going you could say that since you don't have enough torque you can't get to the RPM where you make more HP . What good is 340 HP when you don't have enough torque to get to that RPM to be able to use your maximum HP ? Now then if you had more torque , let's say 520 lb*ft of it , you could get to the RPM where you could gain access to , oh I don't know , let's say 300 HP . That's one way an engine with 300 HP will beat an engine with 340 HP . Notice also the order of your wording , HP is dependent on torque not the reverse .
So now we go back to the blanket statement that ZF made on 1/21/03 : " What gives is that 340hp is more than 300hp . "
Also on 1/21/03 : " What , you think the performance is some kind of combination of the torque and horsepower of the engine ?
It ain't . The horsepower is the performance . "
So you tell me how I should react to statements like that . Is this a " Strawman " theory as you have mentioned or is this a major flaw in the HP theory .
EWC,
Quote: "I'm a little suprised to see you make a statement about the Pontiac with your earlier thoughts on combinations and how important the total package is ."
I guess I need to explain my position better. A 400 CI engine has 50 more cubes than a 350 CI engine, and as such--given the same level of technology--has the potential to make more power. The larger motor is capable of generating more force which translates into more HP at an equivalent RPM, or equivalent HP at a lower rpm. In the case of your friend's car and my car running similar ETs (and assuming a similar weight), it makes sense that the Pontiac would be optimized with a taller (numerical lower) gear than my car.
Interstingly, my heads flowed 175 at 10 inches, which converts close to the same number as your friend's 275 at 28.
Ring Gears:
I think I understand what you are saying, that if you measured power at the outer diameter of the 16.5" ring gear, then you would indeed get a different torque reading than if you took a reading on a 10" gear.
With the pinion driving the ring gear, the torque on the outer edge of the 16.5" gear would be less than it would on the 10" gear, assuming the same torque at the pinion shaft. You have the leverage reducing force in this case. Now if the ring gear were driving the pinion, then applying force at the 16.5" diameter would provide more torque to the pinion than applying force at the 10" diameter.
However, since ring gears drive axles that in turn drive wheels, the power made at the ring gear is not the important factor. The 16.5" ring gear and the 10" ring gear--in this case both 5 to 1 ratios--both turn the axle at the same speed. If the tires have the same loaded rollout, then the power put to the ground--given the same power input to the rear end and not accounting for parastitc losses--would be the same.
Quote:
"What good is 340 HP when you don't have enough torque to get to that RPM to be able to use your maximum HP ?"
At any given torque output, there is an rpm required to get 340 HP. Conversely, at any given rpm, there is a torque value required to get 340 HP. Therefore, if you don't have the torque to get to the required rpm, you don't have a 340 HP motor.
Having said that, I suspect the case of the stock 8.1 is it doesn't wind quickly enough with factory gears to get it's 340 HP to the ground.
HP is dependent on torque and RPM. Without torque, there is no HP. Without RPM--even at exceedingly high levels of torque--there is no HP. Without HP you don't go anywhere. The more HP you have, the faster you can go. The rpm at which you make the HP determines the torque you need to make the HP. However, it's the HP that does the work. Call it "torque in motion".
Quote: "So you tell me how I should react to statements like that ."
I am not suggesting you haven't been polite and rational, but you asked.............I personally strive to keep posts polite and rational, because my conduct on the boards is far more important to me than "winning" an argument. Especially an argument as trivial--though interesting--as this one. I try to give other folk the benefit of the doubt in their posts,and I read my posts critically to see if I am using negative emotion.
Quote: "Is this a " Strawman " theory as you have mentioned or is this a major flaw in the HP theory"
No, I think you are now honestly arguing from your beliefs. I see no more strawmen. However, I do not think we'll ever come to an agreement on what the terms HP and Torque mean when applied to automotive performance.
Blaine
[ 09-23-2003, 05:02 AM: Message edited by: afp ]
The way I see ring gears is : the force at the pinion is pushing on the ring gear . The moment arm , for the 16.5 inch gear , is 8.25 inches and for the 10 inch gear is 5 inches . With 500 lbs of force that equates to 343.75 lbs*ft of torque , for the 16.5 gear , and 208.33 lbs*ft for the 10 inch gear . This goes back to the center line of the axle and from there you take into account the axles and tires . If you follow your line of reasoning , you would get a corresponding change in HP . Notice that this has no RPM , only input force . The RPM's come from the trans or whatever you are using to power the rear . This is an example of what I think the application is , not just grinding numbers on a calculator assuming that with a 5:1 ratio you just multiply for the answer . This is a calculation for torque , not HP as there is no RPM !
I still find it interesting that you keep bringing up that you need torque and RPM's to get HP but don't seem to agree which one comes first . As you have stated , you can have torque without HP , but you can't HP without torque and RPM . In your example of the drag car you saw first hand what increasing the gear ratio did for the combination . Now go the other way , as in the Dmax . Think how much more power you would need to get the same speed , or more , with 3:73 gears compared to 4:10 gears while pulling the same load .
I do agree with you on one thing , we both will disagree what is better for this example . I'll stick with my torque theory and you have your HP theory .
britannic
09-23-2003, 19:13
Interesting link on tq/hp:
Torque & HP Explained (http://www.datsuns.com/torquehp.htm)
Here is an excerpt from the webpage:
Now, it's important to understand that nobody on the planet ever actually measures horsepower from a running engine. What we actually measure (on a dynamometer) is torque, expressed in foot pounds (in the U.S.), and then we *calculate* actual horsepower by converting the twisting force of torque into the work units of horsepower.
Colorado Kid
09-24-2003, 11:29
That's a funny quote, because most chassis dynos measure the rate at which a known mass (the drum) is accelerated, which is a direct measure of horsepower, and then use engine RPM information to caculate torque.
Check out the articule in General Diesel under " World's Largest Diesel " . 14 cylinder inline 2 cycle diesel with 1,556,002 cubes , 108,920 HP and 5,608,312 lb/ft of torque . All at the stunning rate of 102 RPM !
Wonder what's more important in that engine . Torque or HP ?
EWC,
When you are applying force to the "small end" of the moment arm, the further away from the small end you get, the less force that is generated. Try to stop someone from opening a door by putting your foot near the hinges vs putting your foot near the doorknob. It is easier to hold the door at the doorknob location because the force is less than it is at the hinges.
The problem with your calculations have to do with properly multiplying and dividing numbers with values of less than one. The easiest way is to convert everything to inch lbs and avoid the fractional values. Then you divide the force at the point of rotation by the distance. In this case, 500 ft lbs at the point of rotation is reduced to 100 ft lbs at 5" and 60.61 ft lbs at 8.25".
However, this is all meaningless. Not accounting for parasitic losses, the force at the center of rotation of the ring gear is the same as the force at the center of the wheel.
Blaine
Blaine , you have it backwards .
The door will pivot around the hinges and the further out you go , with the same force , the more torque or force you will have at the hinges . If you put someone at the outside of the door and try to keep it from opening , you will have to push a corresponding amount equal to their weight or force in the opposite direction to keep the door from moving . The further in you go , the more force required to keep the door from moving . Think of a breaker bar and socket . A 1/2 inch breaker bar , that is 36 inches long , will have more force than a 3/8 inch breaker bar , that is 18 inches long , with the the same amount of weight or force applied to it . This is the force that is measured at the socket .
The force or torque on the rears is assuming that you already have a known torque value at the wheels . Then you would be correct as we would know the torque and the lenghts . If you have a 500 lb force , that is applied at the ring gear by the pinion , the moment arms are the radius of the ring gears . The center section rotates around the bearings and the force is applied to the ring gear .
How about this :
T = F*ma
T = 500lbs*5inch*1ft/12inch = 208.33lb*ft
T = 500lbs*8.25inch*1ft/12inch = 343.75lb*ft
Then , as you say , the torque will go through the axles , wheels and tires and onto the ground . This is where you are correct in diminishing the torque as the same amount of torque is known and the diameters of the tires come into play . It may help to think of people that put on 40inch tall tires and wonder why the truck can't get out of it's own way . The cure is to put more gear in because the torque output of the engine has not changed .
Notice also the correct labeling for torque that is supported by the equations .
As you have said , the name of the game is torque multiplication . The semi is a good example of this . With upwards of 2200 lb*ft of torque , some of the transmissions have 3 cluster gears that look to be on a 10 radius from the input and output shafts . That is an average gear size of 5 inches . Think about calculating the torque , using T = F * ma , for all 18 speeds and then the rears , over and under drive types , etc and then I think you will see why I don't feel that torque multiplication is as easy as grinding numbers on a calculator . I also don't think that the pickup truck makes as much torque , through gear multiplication , as others have suggested .
Your last statement is obvious , isn't it ?
Just as a final note , when they test a bellhousings specs for SFI approval , the flywheel has holes drilled into it to make it explode at higher RPM's . The RPM's are not what cause this explosion , torque is . This was from an articule that I read years ago and no I don't remember the mag .
EWC,
You got me going down the wrong track with the leverage of the ring gear. After pondering this some more, I don't think the equation for multiplying force via leverage is relevant at all to power output. We can do interesting things with the math for leverage and ratios and torque, but the power output doesn't change. A higher force moving a smaller distance can accomplish the same work as a correspondingly lower force moving a greater distance.
The above negates any lever arm multiplication that occurs in the tranny and rear end. What matters in the the ratio between the input shaft and the output shaft of the tranny and the rear end.
You are correct that torque multiplication isn't just grinding numbers on a calculator, but it's not for the reasons you cite. Parasitic losses are a significant factor, and the more gears and heavier weight of the components, the more power that is lost through the tranny. An old Powerglide takes away something like 28 HP, and out beloved Allison takes away about 60 HP.
I want it to be clear I did not say "the name of the game is torque multiplication". I have maintained from the outset that putting HP to the ground that is "the name of the game". Torque is simply a tool used to do that.
Yes, I know you do not agree with those concepts.
Perhaps we can find one area of agreement. Torque is indeed what stresses trannys, because torque is [i]force[i/] force is what breaks things. I can shear the head off a 1/4" bolt with 100 ft lbs of torque, but I won't hurt it just by spinning it faster at a lower torque.
Maybe there is two. Big tires hurt acceleration and pulling power. Of course, I say it's because they reduce HP and you say it's because they reduce torque. So maybe that's 1.5 areas of agreement............
I am going to sum up my participation in this discussion by making a simple list of how I think these things interrlate. If you do the same, at least folks looking at this thread--if there are any left--will clearly know our respective positions.
Blaine's Position:
- HP is what does the work of moving a vehicle down the road. The faster you go, the more HP you need
- Torque is the force required to generate HP at a given rpm
- You can generate torque without generating rpm. When you do this, you have no HP and accomplish no work
- Optimum gearing for power is determined by the rpm where you can generate the most HP for the longest period of time
- Any leverage generated by the diameter of gears is not significant. What matters is the amount of force transmitted at the center of rotation, and that is determined by the force input, the amount of parasitic losses, and the ratio between the input and output of the tranny and rear end
- While measuring actual power output of the engine, charting power curves, etc gives us a start on what optimum gearing, shift points, etc are; we need to actually test the vehicle on the road/track to find optumum performance.
Blaine
[ 09-27-2003, 01:33 PM: Message edited by: afp ]
Sorry for the reference to torque multiplication . I thought you had said that somewhere in this thread .
I have no comment for the rest .
I to have done some drag races and some roundy round as well.
I am probably older than most that read these posts and have no axe to grind but, I was taught by people much more intelligent than I am the following.
1. Torque provides the power to move the object.
2. HP provides the speed.
If you have ever driven in roundy round races or driven much faster than you should you have have experienced the area when the rate of acceleration continues more slowly than when the vehicle first started moving.
That is the reason the NASCAR cars take several loops before they reach the maximum speed.
Have a great day. :D
britannic
09-29-2003, 20:28
Torque and HP are joined at the hip; one is used to calculate the other as has previously been stated. The following article contains some excellent info on better understanding the arbitrary relationship between the two:
Torque and Horsepower - A Primer (http://www.datsuns.com/torquehp.htm)
From Bruce Augenstein, rba@augenstein.ultranet.com
There's been a certain amount of discussion, in this and other files, about the concepts of horsepower and torque, how they relate to each other, and how they apply in terms of automobile performance. I have observed that, although nearly everyone participating has a passion for automobiles, there is a huge variance in knowledge. It's clear that a bunch of folks have strong opinions (about this topic, and other things), but that has generally led to more heat than light, if you get my drift :). I've posted a subset of this note in another string, but felt it deserved to be dealt with as a separate topic. This is meant to be a primer on the subject, which may lead to serious discussion that fleshes out this and other subtopics that will inevitably need to be addressed.
OK. Here's the deal, in moderately plain English.
Force, Work and Time
If you have a one pound weight bolted to the floor, and try to lift it with one pound of force (or 10, or 50 pounds), you will have applied force and exerted energy, but no work will have been done. If you unbolt the weight, and apply a force sufficient to lift the weight one foot, then one foot pound of work will have been done. If that event takes a minute to accomplish, then you will be doing work at the rate of one foot pound per minute. If it takes one second to accomplish the task, then work will be done at the rate of 60 foot pounds per minute, and so on.
In order to apply these measurements to automobiles and their performance (whether you're speaking of torque, horsepower, newton meters, watts, or any other terms), you need to address the three variables of force, work and time.
Awhile back, a gentleman by the name of Watt (the same gent who did all that neat stuff with steam engines) made some observations, and concluded that the average horse of the time could lift a 550 pound weight one foot in one second, thereby performing work at the rate of 550 foot pounds per second, or 33,000 foot pounds per minute, for an eight hour shift, more or less. He then published those observations, and stated that 33,000 foot pounds per minute of work was equivalent to the power of one horse, or, one horsepower.
Everybody else said OK. :)
For purposes of this discussion, we need to measure units of force from rotating objects such as crankshafts, so we'll use terms which define a *twisting* force, such as foot pounds of torque. A foot pound of torque is the twisting force necessary to support a one pound weight on a weightless horizontal bar, one foot from the fulcrum.
Now, it's important to understand that nobody on the planet ever actually measures horsepower from a running engine. What we actually measure (on a dynamometer) is torque, expressed in foot pounds (in the U.S.), and then we *calculate* actual horsepower by converting the twisting force of torque into the work units of horsepower.
Visualize that one pound weight we mentioned, one foot from the fulcrum on its weightless bar. If we rotate that weight for one full revolution against a one pound resistance, we have moved it a total of 6.2832 feet (Pi * a two foot circle), and, incidentally, we have done 6.2832 foot pounds of work.
OK. Remember Watt? He said that 33,000 foot pounds of work per minute was equivalent to one horsepower. If we divide the 6.2832 foot pounds of work we've done per revolution of that weight into 33,000 foot pounds, we come up with the fact that 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. If we only move that weight at the rate of 2626 rpm, it's the equivalent of 1/2 horsepower (16,500 foot pounds per minute), and so on. Therefore, the following formula applies for calculating horsepower from a torque measurement:
Horsepower = (Torque * RPM)/ 5252
This is not a debatable item. It's the way it's done. Period.
The Case For Torque
Now, what does all this mean in carland?
First of all, from a driver's perspective, torque, to use the vernacular, RULES :). Any given car, in any given gear, will accelerate at a rate that *exactly* matches its torque curve (allowing for increased air and rolling resistance as speeds climb). Another way of saying this is that a car will accelerate hardest at its torque peak in any given gear, and will not accelerate as hard below that peak, or above it. 300 foot pounds of torque will accelerate you just as hard at 2000 rpm as it would if you were making that torque at 4000 rpm in the same gear, yet, per the formula, the horsepower would be *double* at 4000 rpm.
In contrast to a torque curve (and the matching pushback into your seat), horsepower rises rapidly with rpm, especially when torque values are also climbing. Horsepower will continue to climb, however, until well past the torque peak, and will continue to rise as engine speed climbs, until the torque curve really begins to plummet, faster than engine rpm is rising. However, as I said, horsepower has nothing to do with what a driver *feels*.
You don't believe all this?
Fine. Take your non turbo car (turbo lag muddles the results) to its torque peak in first gear, and punch it. Notice the belt in the back? Now take it to the power peak, and punch it. Notice that the belt in the back is a bit weaker? Fine. Can we go on, now? :)
The Case For Horsepower
OK. If torque is so all-fired important, why do we care about horsepower?
Because (to quote a friend), "It is better to make torque at high rpm than at low rpm, because you can take advantage of *gearing*.
For an extreme example of this, I'll leave carland for a moment, and describe a waterwheel I got to watch awhile ago. This was a pretty massive wheel (built a couple of hundred years ago), rotating lazily on a shaft which was connected to the works inside a flour mill. Working some things out from what the people in the mill said, I was able to determine that the wheel typically generated about 2600(!) foot pounds of torque. I had clocked its speed, and determined that it was rotating at about 12 rpm. If we hooked that wheel to, say, the drive wheels of a car, that car would go from zero to twelve rpm in a flash, and the waterwheel would hardly notice :).
On the other hand, twelve rpm of the drive wheels is around one mph for the average car, and, in order to go faster, we'd need to gear it up. To get to 60 mph would require gearing the wheel up enough so that it would be effectively making a little over 43 foot pounds of torque at the output, which is not only a relatively small amount, it's less than what the average car would need in order to actually get to 60. Applying the conversion formula gives us the facts on this. Twelve times twenty six hundred, over five thousand two hundred fifty two gives us:
6 HP.
Oops. Now we see the rest of the story. While it's clearly true that the water wheel can exert a *bunch* of force, its *power* (ability to do work over time) is severely limited.
At The Dragstrip
OK. Back to carland, and some examples of how horsepower makes a major difference in how fast a car can accelerate, in spite of what torque on your backside tells you :).
A very good example would be to compare the current LT1 Corvette with the last of the L98 Vettes, built in 1991. Figures as follows:
Engine Peak HP @ RPM Peak Torque @ RPM
------ ------------- -----------------
L98 250 @ 4000 340 @ 3200
LT1 300 @ 5000 340 @ 3600
The cars are geared identically, and car weights are within a few pounds, so it's a good comparison.
First, each car will push you back in the seat (the fun factor) with the same authority - at least at or near peak torque in each gear. One will tend to *feel* about as fast as the other to the driver, but the LT1 will actually be significantly faster than the L98, even though it won't pull any harder. If we mess about with the formula, we can begin to discover exactly *why* the LT1 is faster. Here's another slice at that formula:
Torque = (Horsepower * 5252) / RPM
If we plug some numbers in, we can see that the L98 is making 328 foot pounds of torque at its power peak (250 hp @ 4000), and we can infer that it cannot be making any more than 263 pound feet of torque at 5000 rpm, or it would be making more than 250 hp at that engine speed, and would be so rated. In actuality, the L98 is probably making no more than around 210 pound feet or so at 5000 rpm, and anybody who owns one would shift it at around 46-4700 rpm, because more torque is available at the drive wheels in the next gear at that point.
On the other hand, the LT1 is fairly happy making 315 pound feet at 5000 rpm, and is happy right up to its mid 5s redline.
So, in a drag race, the cars would launch more or less together. The L98 might have a slight advantage due to its peak torque occurring a little earlier in the rev range, but that is debatable, since the LT1 has a wider, flatter curve (again pretty much by definition, looking at the figures). From somewhere in the mid range and up, however, the LT1 would begin to pull away. Where the L98 has to shift to second (and throw away torque multiplication for speed), the LT1 still has around another 1000 rpm to go in first, and thus begins to widen its lead, more and more as the speeds climb. As long as the revs are high, the LT1, by definition, has an advantage.
Another example would be the LT1 against the ZR-1. Same deal, only in reverse. The ZR-1 actually pulls a little harder than the LT1, although its torque advantage is softened somewhat by its extra weight. The real advantage, however, is that the ZR-1 has another 1500 rpm in hand at the point where the LT1 has to shift.
There are numerous examples of this phenomenon. The Integra GS-R, for instance, is faster than the garden variety Integra, not because it pulls particularly harder (it doesn't), but because it pulls *longer*. It doesn't feel particularly faster, but it is.
A final example of this requires your imagination. Figure that we can tweak an LT1 engine so that it still makes peak torque of 340 foot pounds at 3600 rpm, but, instead of the curve dropping off to 315 pound feet at 5000, we extend the torque curve so much that it doesn't fall off to 315 pound feet until 15000 rpm. OK, so we'd need to have virtually all the moving parts made out of unobtanium :), and some sort of turbocharging on demand that would make enough high-rpm boost to keep the curve from falling, but hey, bear with me.
If you raced a stock LT1 with this car, they would launch together, but, somewhere around the 60 foot point, the stocker would begin to fade, and would have to grab second gear shortly thereafter. Not long after that, you'd see in your mirror that the stocker has grabbed third, and not too long after that, it would get fourth, but you'd wouldn't be able to see that due to the distance between you as you crossed the line, *still in first gear*, and pulling like crazy.
I've got a computer simulation that models an LT1 Vette in a quarter mile pass, and it predicts a 13.38 second ET, at 104.5 mph. That's pretty close (actually a tiny bit conservative) to what a stock LT1 can do at 100% air density at a high traction drag strip, being powershifted. However, our modified car, while belting the driver in the back no harder than the stocker (at peak torque) does an 11.96, at 135.1 mph, all in first gear, of course. It doesn't pull any harder, but it sure as hell pulls longer :). It's also making *900* hp, at 15,000 rpm.
Of course, folks who are knowledgeable about drag racing are now openly snickering, because they've read the preceding paragraph, and it occurs to them that any self respecting car that can get to 135 mph in a quarter mile will just naturally be doing this in less than ten seconds. Of course that's true, but I remind these same folks that any self-respecting engine that propels a Vette into the nines is also making a whole bunch more than 340 foot pounds of torque.
That does bring up another point, though. Essentially, a more "real" Corvette running 135 mph in a quarter mile (maybe a mega big block) might be making 700-800 foot pounds of torque, and thus it would pull a whole bunch harder than my paper tiger would. It would need slicks and other modifications in order to turn that torque into forward motion, but it would also get from here to way over there a bunch quicker.
On the other hand, as long as we're making quarter mile passes with fantasy engines, if we put a 10.35:1 final-drive gear (3.45 is stock) in our fantasy LT1, with slicks and other chassis mods, we'd be in the nines just as easily as the big block would, and thus save face :). The mechanical advantage of such a nonsensical rear gear would allow our combination to pull just as hard as the big block, plus we'd get to do all that gear banging and such that real racers do, and finish in fourth gear, as God intends. :)
The only modification to the preceding paragraph would be the polar moments of inertia (flywheel effect) argument brought about by such a stiff rear gear, and that argument is outside of the scope of this already massive document. Another time, maybe, if you can stand it :).
At The Bonneville Salt Flats
Looking at top speed, horsepower wins again, in the sense that making more torque at high rpm means you can use a stiffer gear for any given car speed, and thus have more effective torque *at the drive wheels*.
Finally, operating at the power peak means you are doing the absolute best you can at any given car speed, measuring torque at the drive wheels. I know I said that acceleration follows the torque curve in any given gear, but if you factor in gearing vs car speed, the power peak is *it*. An example, yet again, of the LT1 Vette will illustrate this. If you take it up to its torque peak (3600 rpm) in a gear, it will generate some level of torque (340 foot pounds times whatever overall gearing) at the drive wheels, which is the best it will do in that gear (meaning, that's where it is pulling hardest in that gear).
However, if you re-gear the car so it is operating at the power peak (5000 rpm) *at the same car speed*, it will deliver more torque to the drive wheels, because you'll need to gear it up by nearly 39% (5000/3600), while engine torque has only dropped by a little over 7% (315/340). You'll net a 29% gain in drive wheel torque at the power peak vs the torque peak, at a given car speed.
Any other rpm (other than the power peak) at a given car speed will net you a lower torque value at the drive wheels. This would be true of any car on the planet, so, theoretical "best" top speed will always occur when a given vehicle is operating at its power peak.
"Modernizing" The 18th Century
OK. For the final-final point (Really. I Promise.), what if we ditched that water wheel, and bolted an LT1 in its place? Now, no LT1 is going to be making over 2600 foot pounds of torque (except possibly for a single, glorious instant, running on nitromethane), but, assuming we needed 12 rpm for an input to the mill, we could run the LT1 at 5000 rpm (where it's making 315 foot pounds of torque), and gear it down to a 12 rpm output. Result? We'd have over *131,000* foot pounds of torque to play with. We could probably twist the whole flour mill around the input shaft, if we needed to :).
The Only Thing You Really Need to Know
Repeat after me. "It is better to make torque at high rpm than at low rpm, because you can take advantage of *gearing*." :)
Thanks for your time.
Bruce
Wapati, your intelligent friends are mistaken ...
"1. Torque provides the power to move the object."
No, torque without rpm has no power. Torque is the rotational force only, i.e. how hard you're pushing. You can have infinite torque and zero power, if what you're pushing isn't moving.
"2. HP provides the speed."
No, rpm provides the speed.
HP is the force times the speed, or in other words, the combination of how hard you're pushing and how fast you're pushing.
That long rambling dissertation on torque and hp is misleading and misses the point. You have to read it really carefully to pick out the important info and it's cluttered with thigs that miss the point.
Sure, a car or truck or whatever will accelerate hardest at it's torque peak in any given gear. But what he fails to point out (at least concisely) is that it'll accelerate hardest at it's horsepower peak at any given SPEED.
To visualize this, just imagine pulling your trailer up a hill. Can you pull the hill best in top gear at your torque peak of 1800rpm? Or can you pull it harder if you downshift and put your motor at it's power peak instead? Anyone who's pulled a trailer knows the answer to that.
The reason is that even though the engine may be making less torque at the power peak, the rear wheels have MORE torque, at that speed. And that's the incredibly important distinction that people tend to overlook. It's not the engine torque that accelerates the truck, it's the rear wheel torque, and the rear wheel torque at any given speed is defined by the horsepower of the engine, not the torque of the engine.
That's not debatable, and it's not a "theory", it's a fact. You really don't even have to undestand the role of gearing to prove this. Just apply the formula to the rear wheels. At any given rear wheel rpm (i.e. speed), if there's more horsepower at those wheels, there's more torque, the formula says so.
I haven't read everything in this thread but I haven't seen mention of the torque and horsepower curves. Usually the HP and TQ are mentioned as the peak of the curve. If you are always able to stay in the peak (eg 300 hp vs 340 hp) you can compare. Aside from the peak, the peak numbers are irrelevant.
One beauty of the diesel engine is its curves are very flat so throughout the RPM range, you get good hp and tq, although not max. Gas engines usually have more of a peak for each which requires you to be in that peak to get good performance.
The rear end is nice because it assists you in being in the range you want, such as peak economy at 67MPH for towing.
If you want to compare the difference between 300hp and 340hp, drive the same hill side by side with the vehicle running at the specified peaks. Acceleration (change in speed) will instead compare power curves.
Rockin: look near the bottom of page 2.
Also, keep in mind that you can't have both a flat torque curve and a flat horsepower curve; the curves are not independent of each other, they are mathematically related per the formula. A flat torque curve, like our Duramax's have, results in a steadily ascending hp curve.
Your comments are correct wrt the importance of area under the curve over the operating range.
ZF , you mean the article at the bottom of page 2 that talks about the Dmax pulling 900 lbs more than the 8.1 and beating it to 60 ?
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