Which is more important on the track and why?

Thanks.

 

best to have both...torque gets you going (assume traction) and hp gets the mph...

 

All things being equal, how many hp do you need to hit the traps at 120 mph? How much would a 3500 stall convertor effect hp on a dyno?

 

For the 1/4 I like torque................I want to get from point a to b ASAP and moving 3680#s with HP ain't gonna cut it. I don't care about MPH....all though I usually pick up 26mph between the 1/8 & 1/4.
With my treaded street tires I run a 11.67@120+ @ 1.75 60's.............60' is key to 10s and I do it with drs
You can see in my sig the mods to get to 120+ in the 1/4.

 

Torque is higher at lower rpm, horsepower is greater at higher rpm. So if you drag race and you leave the line at 5000 rpm you'll never drop much below this range when you shift. This is when the HP is to your advantage.

 

"Which is more important on the track and why?"

HP is more important because the maximum amount of force applied by the tire patch at any given speed is when a gear(s) are selected that puts the engine at its peak HP RPM. The relationship between HP and TQ is mathematical so if someone says that the have more TQ, they are also stating the they have more HP at any given RPM. Whether it is a car excellerating or a truck pulling a heavy load up a steep hill, knowing the HP curve and selecting a gear that averages the most of it will get the job done quicker.

EJ

 

Torque is force through a distance.
(Work is force actually moved through a distance)
Power is Work over time. (RPM).

Torque is often used to infer low-end power and is a marketing ploy that has worked well, (Hemi any one).
A torque curve is redundant compared to the HP and visa versa. Torque is actually calculated from the HP. Torque is static and power is dynamic, a dyno is a dynamic device.

As said before, maximizing the HP through the gears to the ground "gets the job done quicker."

 

no no, sorry, HP is derived from Torque. Torque is all that matters. HP is just another calculation from tourque.

 

Interesting,
Which engine would you like in your car if they were the same weight and size;
the 30' water wheel that develops 8000 lb-ft of torque at 10rpm
or the LS1 5.7L that only develops 400 lb-ft of torque at 5200 rpm.

This should point out that torque is meaningless unless it is associated with velocity (RPM) which is Power (HP).

If I was to ask the same question this way

the 30' water wheel that develops 8000 HP at 10rpm, (4,000,000 Lb-Ft Torque)
or the LS1 5.7L that only develops 400 HP-T at 5200 rpm.

You'd have a hard time finding a clutch to handle 8000 HP.

 

No, HP is calculated from Torque and RPMs.

HP = (Torque * RPM) / 5252

The best acceleration is obtained by cars with the best torque area, regardless of peak HP. This is average torque multiplied by RPM range.

Assuming perfect traction and instant shifts, this is the entire rev range from time of hook-up until RPM-of-shift. For all subsequent gears this is the torque area from starting RPM in the new gear until RPM-of-shift.

For example, you can have two engines that both have 400 HP at 5,600 RPM and redline at 6,000 RPM. But, all else being equal, the larger displacement engine will have the superior acceleration due to its superior torque area.

 

Same weight and size? I'd take the LS1 due to its possible superior torque area. (I'm assuming, since you didn't give the red line nor the torque curve of the water wheel, which are both needed to make a logical decision as to the superiority of one or the other.)

 

Sorry, often TQ is calculated from HP, such as on an enertia dyno like DynoJets. They use one of Sir Isaac Newton's laws, I think 2nd, that goes something like "moving a certain mass (the approx 2500 LB enertia of the roller) in a given distance (revolutions) within a certain time = HP. From that an enertia dyno takes the RPM of the engine to compute TQ.

EJ

 

Great stuff guys. It is appearing there is not a consistant opinion on the matter..

 

 

There's a difference between calculating one from the other and one being based on the other.

HP comes from Torque, not the other way around. You need Torque to have HP, but you do not need HP to have Torque. Thus, HP is based on Torque and RPMs.

 

Yea, but you can have X amount of TQ but zero HP. HP is the measurement of work actually or capable of being done, which I'm most interested in. You drag race a car and base your shift points to average the most TQ of the engine's TQ curve and then you race the same car and shift so that you average the most of the HP curve and you will go much quicker. This same principle works on a truck pulling a heavy load up a steep hill, he will select a gear to put the engine closest to its peak HP, not TQ. Why is this law of physics so hard to get across?

EJ



EJ

 

Torque measures how hard the ENGINE is twisting.

Power measures how hard the WHEELS can be twisted (at a given wheel speed).

Even if an engine has less torque, it will twist the wheels just as hard if it is rotating fast enough to have equal power, since it will take advantage of greater multiplication through gearing (at the same wheelspeed)

Ignoring rotational mass and traction etc., acceleration is proportional to rearwheelpower, inversely proportional to vehicle mass, and inversely proportional to wheel speed. This is why cars accelerate harder from 20-40mph than from 120-140mph (It's not just drag).

Ignoring rolling restistance etc., top speed is proportional to the cube root of rearwheelpower, inversely proportional to the cube root of drag coefficient, and inversely proportional to the cube root of frontal area. In other words it takes 8 times the power to go twice as fast.

 

And now time for some more "FACTS"........

There's been a certain amount of discussion, in this thread and elsewhere 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 dynomometer) 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, incidently, 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:


Torque * RPM

Horsepower = ------------

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. Torque is the only thing that a driver feels, and horsepower is just sort of an esoteric measurement in that context. 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. Therefore, horsepower isn't particularly meaningful from a driver's perspective, and the two numbers only get friendly at 5252 rpm, where horsepower and torque always come out the same.

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 drivewheels 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 drivewheels 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 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:


Horsepower * 5252

Torque = -----------------

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 occuring 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 preceeding 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 preceeding 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*." :-)

 

That seat of the pants, push in the back feeling is greatest when acceleration is greatest.

It is true, the greatest acceleration IN A CERTAIN GEAR occurs at peak torque.

However the greatest acceleration AT A CERTAIN SPEED occurs at peak POWER.

It is no accomplishment that IN A CERTAIN GEAR you accelerate harder at peak torque. Acceleration as I said before is inversely proportional to speed. You are going slower at peak torque than at peak power since peak torque is ALWAYS at lower engine speed than peak power.

In the real world, you don't race in one gear. To accelerate the hardest from point A to point B, you must accelerate as hard as possible (on average) at every point from A to B. This means accelerating as hard as possible at every SPEED from 0 to X. Ideally this would mean staying at peak power for the duration and continuously decreasing gear ratio (mechanical advantage). Although this is only possible with CVTs, 5-6 gear transmission upshifted at redline keep average power very close to peak power.

Torque doesn't matter any more than engine speed. Both are equally responsible for POWER which is the important factor here.

 

ALLTHROTTLE&NOBOTTLE, A very eloquent and loquacious redress.

Consider this:
A torque wrench measures torque.

When do we know how much torque is on the bolt? When it Stops twisting, that is the measured Torque.

Your first comment agrees, “exerted energy, but no work will have been done”.

By definition torque (Length x Force) is not dynamic (moving). Torque or force is not a requirement for motion.

Power is the dynamic application of Torque or force; torque is a static (at rest) component. As with the water wheel, having 8000 lb-ft of torque means absolutely nothing until we have the dynamic component of RPM,

Question? In a rocket car would you measure the Torque or the HP to determine the acceleration? I can express thrust in terms of Power (the ability to accelerate Mass) but not as Torque.

This probably sounds like semantics, it does to me.
Again: Power moves the car, and torque is a linearly dependant variable.

But then again I could be wrong.