I've been researching turbo upgrades for awhile now and am confused about HP and torque curves. If 5200rpm is always where torque and HP meet, and then torque drops off after that, how do some turbos pull strongly to redline while others poop out at the top? My stock K16's drop off at higher rpm, while I've heard K24's do not. If torque equals acceleration, how do any turbos pull strongly after 5200rpm? I track my car and need the power up top or am I completely out in left field. Some say the sooner a turbo spools to boost the quicker it will get down the straight, but shifting from close to redline will put the next gear's rpm level higher to begin with, thus needing power in the higher rpm range. Help a brother understand so I can make a proper choice for track work.

 

I know nothing but I'll give a stab at it in very simple terms that may be wrong or not help.

The K16 run out of wheel at higher RPMs, the K24 does not. The K24 takes a little longer to spool but it last longer because it's bigger.
The loss of torque is due to the car needing less effort to go faster; as it reaches its top end the car isn't working as hard it is just running.

Small turbo will get you boost faster, but won't last as long.
Big turbo takes longer to spool but last longer. The bigger turbo can be spooled faster w/ better flowing headers that increase the exhaust flow.

Depending on the style of track, short and curvy or 1 that has lazy turns and long straights will create the turbo you need.

They are both good and have a place.

Experts how did I do?

 

I think this is incorrect because the faster a car is going the more wind resistance it has to work against thus it needs to work harder to go faster still.

I understand about the wheel sizes but that doesn't explain away the drop in torque after 5200 rpm's. Isn't it torque that makes you feel the increased pulling to redline that some turbos exhibit, and that is above 5200rpm's?

 

I said it may be wrong.
The wind is a factor but at the same time you aren't talking about a flat wall either.
I see it the other way around, just look at the amount of gas a car uses at speed as opposed to getting started? At lease on the cars that I have had the little computer option it always shows the MPGs being way low at a standing start as to the cruise speed.
The resistance the engine has at a start trying to move the car is a lot, I would think as it builds up inertia moving the car would be easier.

you may be right but it can't be by much.

But Honestly, I gave my opinion to see how much I have learned about these cars. Like I have said in the past, they are nothing like the HP I am use to w/ American and diesel motors.

 

The formula for horsepower is HP = TQ*RPM over 5252

Unless the torque dives like hell, the RPM would still keep the HP climbing.

 

Hog, remember I'm asking about track driving not starting from a standstill. No disrespect meant.

I know the formula for finding HP, what I'm trying to understand is the relationship between HP and torque. My car pulls hard in the torque curve but drops off above it. If 5200 rpm's is the torque curve limit, how are the other turbos pulling hard above this? If you say the bigger turbos continue to pull above 5200 because of HP, how are HP and torque different?

 

None taken. I was giving that as an example to show why I thought the engine created more torque at lower speeds. Here I hope this helps.

http://www.straightdope.com/columns/...wer-and-torque

http://wiki.answers.com/Q/What_is_th...in_automobiles

 

Torque to the wheels equals acceleration. So, with fixed gear ratios and tire sizes, the more TQ at the crank, the faster the car accelerates.

"pulling harder" up top is just for comparison between turbos.

The highest acceleration value (the G's you feel) comes where the engine produces peack torque. When tq begins to fall, so does the acceleration.

An engine that has a smaller "torque falling rate" after the peack, will pull harder at high rpm.

 

Does this "falling rate after peak" relate to the size of the turbine wheel?

 

Because, due to gearing, power is all that matters.

 

Landjet,



The other part of it is how many CFMs the turbos flow. As you no doubt know an engine is an air pump and the turbos increase volumetric efficiency by forcing more air into the cylinders than they could otherwise hold. A bigger turbo can push more air into it.

As RPM rise, the air needs of the engine increase if you want to keep increasing (or even hold contant) the amount of work (torque) the engine can do. What you are experiencing is that the amount of air your turbos can push has been maximized without exceeding their efficiency. When you hit that limit, if your rpms keep going up, your power is going to go down because the engine is calling for more air than the turbos can give it. Without going into pressure ratios and the sweet spot of the turbo, the simple explanation is that a bigger turbo, which can push more air, can keep pushing air up to a higher rpm.

Does that explain it?

Yes, this clarified it a lot along with some of the other posts. Thanks.

 

HP can climb, even if torque is falling...

 

That's true, and I haven't said anything to the contrary.

 

I can't answer precisely to this question...I think it's both turbine and compressor wheels (which are somewhat sized accordingly each other) that determine how the turbo will perform at high rpm and therefore the falling rate of the tq curve.

As GTgears said, a big turbo will be able to push more air into the engine than a smaller one, and so it will allow the engine to produce more work (torque). But torque will always fall a bit after the peak.


Anyway take my words with benefit of the doubt...

It would be great if some tuner would chime in and discuss a bit more in depth this subject...

 

Sorry ...... I do not have the author but it isn't me.

Horsepower Vs. Torque - What the Heck is the difference?
"Horsepower sells cars, but torque wins races." - Carroll Shelby

This article is meant to be a primer only, as we by no means claim to be an absolute authority on the subject. That being said, we have done our best to sum up the knowledge we have, present it in a nice little package, and then leave the facts for you to talk about amongst yourselves to decide what you feel is the more important of the two :Horsepower or Torque.

(We certainly give credit where credit is due and wanted to take a moment to mention that a lot of our information for this article has been taken from a website article by Bruce Augenstein. In fact, the latter half in all bold italics are his exact words. I was unable to reach Bruce after several attempts, but without his work, none of this would have been possible… so, if you are reading this, thanks Bruce!)

I’ll start with the basic definitions, then explain each them in further detail.

Horsepower Definition: How much work is done over what period of time.

Many years before Mercedes Benz was producing engines with over 450 horsepower, a man named James Watt made some observations, and concluded that an average horse 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. Watt published those observations, and stated that 33,000 foot pounds per minute of work was equivalent to the power of one horse, or, to cite on of our topic of the day, one horsepower.

To further explain, let us look at the following scenario. 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 (hence, you have zero horsepower.) 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.

Torque Definition: The measure of the force applied to an object to produce rotational motion, usually measured in foot-pounds.

Torque literally refers to the turning or twisting force of an engine An engine may be very powerful, but if it has little torque then that power may only be available over a very high and limited rev range, making it of limited use to the driver. An engine with more torque - even if it has less power – often proves to be much quicker on the track, as the power is available over a far wider rev range and hence more accessible. To give a slightly more technical explanation, one 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. Imagine, if you will, this one pound weight, 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. 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. 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

(…from Bruce Augenstein’s article on horsepower vs. torque)

Now, what does all this mean in when it comes to cars?

It basically comes down to this. Torque is the only force actually measured when the car is on a dyno. It is what you FEEL when driving. It is the capability of an engine to TWIST the camshaft, thereby thrusting the car forward. 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 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:

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*."