Sam Elassar

Hi there
i was just wondering what is the difference betweent the two. for example if we have 2 cars one has got 250bhp and 280lb/ft and the other has 280bhp and 250 lb/ft which one will be faster ?

the other thing is what is the limiting factor for BHP. is it the turbo size or the restrictor? or am i talking rubbish again? i mean rally cars have a restricted 300php due to the restricor size limitation but the run a lot of torque !!!

Neil Smalley

Check out the Torque Vs BHP section(under tuning) on Lee's website

AndyMc

Sam you might already know know this but the restrictor fitted to our cars is different to the one fitted to the rally cars.

The restrictor fitted to ours is a small piece of brass with a small hole drilled in it similar to the jet in a carburettor,it is fitted in one of the small bore boost control pipes and it can be used to adjust the control characteristics of the boost.

The restrictor the rally cars use is a metal plate with quite a big hole eg 40mm through which all the air that the engine consumes must pass,this limits the 'max power' the engine can produce. I think!

Sam Elassar

thanks neil

andymc
i know that bit but isn't the restrictor used to reduce the air to the turbo ( i am talking about rally cars here) hence the cars only produce 300bhp max. while there is no limit to the amount of torque you can have and that make a big difference.

Denz

so to have a faster car what do we need mor BHP or torque??? or both

thanks

Denz

MorayMackenzie

More torque will make for better acceleration. More BHP should increase V-Max. As stated before, it's really the same thing being expressed differently.

AndyMc

Just got back from watching the X-MEN,cool film.

Sam that is exactly what the restrictors on the rally cars do.

The rally cars do make a lot of extra torque when compared to our road cars but it is not unlimited.

If you look at the article on Lee's site you can see that torque and bhp are related to one another by the speed of the engine for example if the rally engine can only make 300 bhp then at

4000rpm max torque = 394lb/ft
6000rpm " = 263lb/ft
8000rpm " = 197lb/ft


The above is always true. It cannot make more torque than this otherwise it would be making more than 300 bhp.

Its hard to believe but at high revs a road Impreza probably makes more torque than the rally car(due to the restrictor),lower down the revs it is a different story and as Sam says ,the much higher figures for the rally car makes a big difference and means it would pull much harder and murder a road Impreza so thats where it gets the extra performance/speed from.Its also what allows Burnsy to spin all four wheels on slick tyres exiting hairpins etc

Going back to the first question normally the car with the highest bhp figure will be the quicker.Thats why we all want 600 bhp .

If two cars have the same bhp but one has more torque ,it could be slightly faster or slightly slower!, this depends on other things such as where in the rev range the extra torque is produced ,or how close the gears are spaced etc ,complicated isn't it

I do hope somebody understands what I'm on about

DavidRB

BHP sells cars, torque wins races.

JohnS

iTrader: (1)

An engine only produces one sort of turning force - torque. BHP is just a different way of measuring it.

A car will accelerate fastest at peak torque in any given gear.

WRC cars produce huge amounts of torque at low rpm. I've heard that some of them are producing 500+ lb/ft of torque at 3,000rpm and 3 bar of boost! Combined with short gearing, this gives them staggering level of performance, despite only having "300 bhp". The engine can't get enough air to maintain performance at high revs due to the inlet restrictor, so most of the times the drivers change up gear between 5k and 6k rpm.

I have hear a rumour that without the inlet restrictor (and a bit of re-work of course), some WRC engines would rev safely to 9,000 rpm and produce in excess of 650 bhp!

robski

Dont forget to take account of gearing tho.
If you best torque is produced from say 2500-5000, then the chances are you will hold till approx 5000 revs when accelerating (probably over this when pushing hard)

So when you change up, you would ideally still be within this band, hence having similar amounts of torque available (albeit gearing reduced).

This is really the key point, you want smoothish bands of power within the used rev range. Having very high BHP figures for a narrow rev range will give good accceleration whilst within this band, but bad outside.

Case study. Ask any maxy P boy whether he would be happy to gain 20BHP between 6000-6500 revs, whilst loosing 10BHP from 3000-5000 and he would say yes. This is the problem, most tuning companies are restricted to this type of customer. BHP certainly sells mods.

Take a look at some of the STi dyno runs, a lot are not producing a lot more torque than UK cars, but they have a bigger range of useable torque (ie dont drop off like UK spec cars). Hence a lot higher BHP figure, because there is still torque at higher revs, and they are using a wide band whilst accelerating.

The Vtec hondas are a good example, whilst in the vtec band, they are stormers, but you have to keep them there. Drop outside and it feels like someone attached a trailer full of bricks to the back whilst you changed gear!

robski

Hoppy

Somebody's confused here. Is it me? I'd appreciate it if one of the techies could put us straight. Calling John F?

Torque is what's measured on a dyno, and from this bhp is calculated. The formula is simple: torque x time = bhp.

It is not true that torque is what makes you go. It cannot because torque can't be applied to the road (you need a transmission). It's bhp that makes you go (although bhp is of course a direct function of torque). The more bhp you put on the road, the quicker you'll go, even if you're producing less torque at any given engine speed.

Quoting peak torque and bhp figures is great for grabbing headlines, but says very little about how quick the car is to actually drive. Type R Hondas, which produce high bhp figures but only at high revs, are a case in point. Unless you can keep them on the boil all the time they are gutless whereas an Impreza (using a turbo which drastically distorts the power output graph of a normally aspirated engine to give dollops of mid-range bhp) will pull the headlights off it in real world driving.

A useful saying: "The power of an engine is the area beneath the curve on the graph." In a single sentence, that explains why we all love Scoobies!

Hoppy

Hoppy

I've just re-read my post above and the bit about measuring torque and bhp doesn't come over right. Whatever!

I still say that the more bhp you've got at your wheels the faster you'll go, regardless of torque output. (Anyone want to throw in bmep at this point?)

And if you're not confused enough by now, just try and read the (lots of) bo11ox on the Scoobmania link above! Jeez. Some of it sounds right, then he goes and contradicts himself, then he gets it wrong.

No wonder people get confused.

Hoppy

simes

Well, for my tuppence bit...

My ZX6R makes bugger all torque and 100bhp ish.

A Harley makes shed loads of torque and bugger all bhp.

I know which one is faster...

The difference I guess is that mine revs to 14000 rpm, and feels like it's barely ticking over at say 6000rpm, whereas a Harley probably red lines at 6000 rpm.

I would guess that if you tied the two together and did a tug of war the harley would win though.

Cheers

Simon

AndyMc

Hoppy

The formula is torque x speed = bhp not torque x time

you could have 1000lb/ft of torque applied to a shaft for 1 minute or 1 hour and no work will have been done and no bhp will have been made

David RB I know that is a popular saying but it is not always true.If two cars have the same bhp the one with the most torque may not be faster ,it may just pull better lower down the rev range(the bit that is not used when racing)

Denz to make a car go faster you need more torque,where you make this extra torque will have a different effect on overall performance.And may or may not increase max power.

If you make more torque lower down the rev range but not at the top of the rev range the car will be faster lower down and make more bhp lower down but the 'max power' will be the same.The 0-60 time may improve slightly, although in gear times will probably be much better.The engine is more useable.

If you make more torque at the top of the rev range the max torque may be the same but the max bhp will increase.The car will not push you back in the seat any harder than it did before but it pulls for longer.This will improve the 0-60 time much more than in the above example.The car may not feel any faster.
This is why bhp figures tell you much more about a cars ultimate performance than the max torque figure does.It is also why we buy low torque petrol engines instead of high torque diesels when we want a fast car.

Hoppy you said "it is not torque that makes you go"

That is completly wrong ,it is torque that makes you go.

Everybody knows that torque is a twisting force but people don't seem to be able to relate this to cars.

If you put a spanner on the wheel nut of a car and pulled on it the car would start to roll forward ie it would accelerate,the torque you are supplying is causing this.If you pull on the spanner harder you apply more torque and the car will accelerate faster.Apply no torque and the car will not move.

From this you can see that not only is it torque that makes a car move but the acceleration is directly related to the amount of torque,clearly if we want to go faster we need to make and apply more torque.

People also think that as the power rises the car will pull harder and harder.This is not true .The car can pull just as hard when making 100 bhp as it does when making 200bhp
eg An sti pulls the hardest(pushes you back in the seat the most) when making about 180 bhp by the time it is making 280 bhp the pull is much less.

Also not many people realise that the gearbox multiplies the torque that the engine produces.

eg in 1st gear an Impreza engine may produce 240 lb/ft of torque but the gearbox and final drive multiplies this to around 2000lb/ft at the wheels ,yes that is correct 2000lb/ft or 500lb/ft per wheel.

in 5th gear this will have reduced to around 700lb/ft (or 175 per wheel)this is why cars wheelspin mainly in the lower gears.It is also why cars are not run in the lower gears on rolling roads,you would need some pretty
hefty slings to hold it down and you would never maintain traction.

Hoppy are you talking about the Torque and horsepower article posted above by John because if you are I cannot see anything at all wrong with it,it seems 100% correct to me ,care to explain?

Simes if you tied the two together your bike would easily win. This is because although you engine makes less torque it does so at high speed so you can gear it lower .This means the effective torque at the wheels (which is what counts) is much higher.

Bugger,where did John F's post come from ? I've just repeated half of what it says

Anyway I'm off for a paracetamol

PS John I hope you didn't type that in by hand,If you did it must be in the running for the longest post ever!

Andy

[This message has been edited by AndyMc (edited 22 August 2000).]

[This message has been edited by AndyMc (edited 22 August 2000).]

ian/555

Errr so would i be right in saying that to apply it in laymans (insert my name here) terms,
of say a body builder who can lift 100kg once in a minute has high bhp but low torque, and if another body builder could lift the same 100kg 60 times in a minute would have the same bhp but more(60 times more?) torque?
(if 100kg was the max that they could lift)

I always thought that torque was the speed at which you could produce bhp!
if i have got it wrong i had better shut up

SDB

John...

you crease me up!

Hoppy

Oh my! Now I really am really confused. Who started this anyway.

I've got some work to do. Back later.

Hoppy

johnfelstead

OK, first a few facts on turbo restricted rally cars.

The WRC and group A cars run with a 34mm restrictor on the inlet port to the turbocharger. This oficially limits the engines to 300BHP, in reality its nearer 340BHP.

To improve throttle response and power output the engines run fairly high compresion ratio's, in the 9.1:1 range.

The turbo's use a fairly small exhaust and compressor sizing to again aid response, no point running something like a T4 if the restrictor only flows what a t3 can supply.

ALS helps big time on WRC cars, WI is very important as the small turbos run pretty high boost, about 2 bar, and this makes the charge temp high, also the intercooler size is restricted both in the rules and also to improve response once more.

I have no figures on what the works engines are producing but our mountune spec clubman GroupA engine using 9.1:1 is putting about 450Lb/ft of torque out. We rev this to 6500rpm on the stages as the restrictor just kills the engine above that, the high compresion helps here, on 8:1 it ran out of steam at 6000rpm. Most of the time however the engine is kept within the 3500rpm to 6000rpm range, only using 6500rpm plus on very fast long straights, prety unusual on UK rallies.

Our gearing limits the car to 120MPH, however that is seen more often than you would think.

We know of some groupN cars that have higher gearing hitting 150MPH on some of the long straights on rallies like the dukeries in mansfield, however overall they loose out big time as most of the stages require 120MPH as an absolute top speed.

now to an explanation of torque v power.

Here is an article written by Bruce Augenstein which explains all this far more eloquently than I ever could:

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:

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. 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 *doubled* 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, and especially so 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. This is a key point. If you mess about with the formula, you can see that, as long as torque values aren't dropping at a rate that is as great or greater than the rise in rpm, horsepower will climb.

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. In fact, gearing up (so as to increase the speed of the output), means that you lose torque at the output in a proportional manner. That is, if you gear up the output for twice the speed, you lose half the torque at the output, and so on.

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 (one sixtieth of the direct torque), 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 262 pound feet of torque at 5000 rpm, or it would be making 250 hp or more at that engine speed, and would be so rated (262 foot pounds times 5000, over 5252 = 249 hp). If it were making 263 or more foot pounds of torque at 5000 rpm, it would be making 250 or more hp, and Chevrolet would likely publish that peak figure and engine speed. 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.

Note: This is a side point, but the optimum shift point for best acceleration occurs at a time when the torque at the drive wheels in the next gear just equals the torque at the drive wheels in the current gear. You shift well above the power peak (and obviously way past the torque peak), because the next gear gives you less mechanical advantage (less torque multiplication) than the gear you're in. As an example, with a 3.00:1 first gear and a 2.00:1 second gear, you wouldn't want to shift until the torque curve dropped by at least 33% from peak - and even then, that would only be true assuming that you'd be *at* the torque peak in the next gear. Otherwise, you'd shift even later. As a practical matter, this usually means shifting at an engine speed of 10 - 15% above the power peak with two-valve engines, and at the redline in four-valve engines, or maybe even the rev limiter . If you know your torque curve and gearing, you can plot this out yourself. If you do this, drop your one-two shift point 2-4% from the calculated optimum, and by lesser amounts in subsequent shifts, to account for flywheel effect. More on that later.

OK. Back to the hp vs torque comparison.

As we've said, the L98 has dropped way off on torque by 5000 rpm, but on the other hand, the LT1 is fairly happy making 315 pound feet at 5000 rpm (300 hp times 5252, over 5000), 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 Vette. Same deal, only in reverse. The ZR-1 actually pulls a little harder than the LT1, although its torque advantage (385 foot pounds at 5200 rpm) 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 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. It doesn't pull any harder, but it sure as hell pulls longer. Per the formula, it's also making *900* hp, at 15,000 rpm (315 foot pounds times 15000, over 5252).

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 600 or more 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 difficulty with such aggressive gearing would be that it would introduce really massive polar moments of inertia (flywheel effect), and that rather complex topic is best addressed through a document of its own, though I'll take an abbreviated poke at it in the next several paragraphs.

Suffice it to say that rotating objects tend to resist either acceleration or deceleration, and engine components are no exception. Gearing up (by either selecting first gear, or in fact tripling the final drive ratio, as we've done with the Vette) means that the engine and other rotating components have to speed up by a greater amount for every mph the vehicle gains, so more energy is expended in accelerating these items to gain a given amount of speed, and thus less energy is available to actually belt you in the back.

As an example of how flywheel effect dampens performance, my old '85 Vette would pull .50 Gs at peak torque in its 1.91 second gear (measured with a Vericom). With a 2.88 first gear, one would expect it to pull around .75 Gs (2.88 over 1.91 = 1.51, times .50 Gs = .75 Gs). It would actually pull a peak of .66 Gs in first gear. The difference can be attributed to a tad more tire slip (maybe sucking up .01 G at most) and the fact that first gear is marginally less efficient than second in most transmissions, thereby sucking up another .01 G (or less), but the main reason that first won't pull as hard as you'd expect (in *any* car) is that the engine uses more energy accelerating itself in first than in second (to gain the same amount of speed), so you get less energy at the drive wheels. This is why you adjust calculated shift points downward, since the actual torque available at the drive wheels is always reduced a bit from what you would calculate it to be, compared to the next higher gear. Flywheel effect goes up as the square of the gearing, which is one reason why the one-two shift point is affected the most.

In the example I used of the 900 hp LT1 using 10.35 gears, the car would drop into the nines for a quarter mile, but in so doing, the trap speed would climb to about 148 mph, because the car is essentially putting more average power to the track with the stiffer gearing. However, drag race nuts are snickering again, because any self-respecting car that can get to 148 mph in a quarter mile ought to be able to do this somewhere in the mid eight second bracket.

The reason this fantasy car doesn't get into the eights is that, in order to get it to effectively use its power, we had to gear it so stiffly that flywheel effect took a major toll from its relatively paltry 340 foot pounds of torque, and since flywheel effect is most pronounced in the lower gears, elapsed times suffer, while trap speeds are affected less.

You can see why drag racers think torque is what wins races. It isn't strictly true, but high rpm, low torque (as opposed to lower rpm, high torque) cars are at a disadvantage in a drag race as long as overall power to weight is similar, because they either only start getting effective somewhere down track (thus crippling elapsed times), or they suffer greater flywheel effect if you gear them aggressively enough to create high torque at the drive wheels (thus crippling elapsed times).

What's really needed in a drag race is high torque (for that massive belt in the back) *and* high horsepower (extending the torque curve), so you can take advantage of gearing.

Of course, looking for top speeds, it's a simpler story......

At The Bonneville Salt Flats
Looking at top speed, horsepower absolutely wins, 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*. Remember, there isn't any flywheel effect at top speed because you're not accelerating.

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. (This is another reason why you *must* be at least at the power peak (or higher in most cases) before you shift to the next gear.)

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*." For any given level of torque, making it at a higher rpm means you increase horsepower - and now we all know just exactly what that means, don't we .


sorry about the arsey american language but it makes interesting reading.

[This message has been edited by johnfelstead (edited 22 August 2000).]

Martin Cook

If this is a competition of who can write the longest reply ever, I think you have just won it John

I think i'll print this one off and read it later.

Martin

Jay m A

iTrader: (2)

JohnF

I swear I was only reading your post for a few minutes, but now I'm the only one left at work, I'm an hour late and my dinner is in the bin (I haven't got a dog). If it wasn't for the fact that I'm online the missus would be giving me an earbashing down the phone!
I've always wondered what travelling at the speed of light was like - now I've got a fairly good idea
Thanks a bunch

Justin (still none the wiser)

RichS

Christ!

Now I remember why Physics wasn't my favourite subject at school!

Thanks guys for the detailed explainations, but simpletons like me might want to stick to those layman's terms...




Rich

Hoppy

For my sins, I've made myself read the piece by Bruce Augenstein above, as posted by John F. I am forced to bow to superior knowledge but am still perplexed by Augenstein's argument. These quotes, for example:

"Torque is the only thing a driver feels and horsepower is just an esoteric measurement."
"Horsepower isn't particularly meaningful from a driver's perspective"

So I take it that bhp doesn't matter, then? Err, no. Here's some more:

"At top speed horsepower absolutely wins."
"I know I said that acceleration follows the torque curve in any given gear, but if you factor in gearing vs car speed, the horsepower peak is IT."
"For best acceleration you change up well above the power peak and obviously way past the torque peak."
"As a practical matter (for best acceleration) this usually means shifting at the redline."

Maybe it's the bit about gearing that's confusing me. But in the real world, the effects of transmission, gearing, time and speed cannot be avoided.

AndyMc, if torque is all-conquering, how come the Kawasaki 600 beats the Harley? As you say it "the effective torque at the wheels (which is what counts) is much higher."

So this is where I'm going wrong. What's the difference between torque and 'effective torque'. Is that just another term for bhp? I dunno.

Here's the final quote from Augenstein's piece, headed 'The Only Thing You Really Need To Know'. Here goes: "It is better to make torque at high rpm than low rpm, because you can take advantage of gearing. For any given level of torque, making it at a higher rpm means you increase horsepower - and now we all know just exactly what that means, don't we."

Well, no actually. You have to admit that Augenstein's argument is confusing and appears contradictory. Any further help welcomed.

Hoppy

simes

AndyMc,

When you say "you can gear it lower" do you mean it would be possible to gear the ZX6R to outpull a Harley in a tug of war, or that it would win in it's current state?

It seems to me for example, that machines capable of pulling tree stumps out of the ground for example, are low powered, low revving, high torque. Whereas as performance vehicles tend to be low torque, high revving, high power.

You could of course gear the high performance engines to pull out tree stumps but you'd need to give them an almost unusable gear ratio to do that, whereas the true stump pullers remain relatively practical in the gearing they need. (Apart from the fact they don't need an engine rebuild every time they pull a tree stump!)

Anyway, to summarise my experience, and ignoring theory and race tracks i.e. real world driving...

The more BHP you have the faster you go.

The more torque you have, especially at low revs, the easier it is to drive.

For example, my last car made 140 bhp but not a lot of torque. You had to rev the nuts off it to see any performance. Consequently, in practical terms it was hard to drive. However at a race track that wouldn't be a problem cos you'd always keep it in the power band.

The scoob makes lots of power obviously, but also shed loads of torque at quite low RPM, so it shifts from quite low revs, easy peasy.

Which is nice...

Cheers

Simon

AndyMc

Hi

I'll have a go at explaining some of your points Hoppy

you have probably noticed i'm obsessed with torque at the wheels,and for a very good reason.Before we can go any further you need to understand why this is.

If you read the bit in my previous post about the spanner on the wheel nut you should be able to see that the performance of the car depends entirly on the torque at the wheels.simply the more we have the faster we go.If you don't understand this you will not understand whats below.

In a uk Impreza the torque produced is as follows(James Neill's car used as an example)

3000rpm = 200lb/ft
3500rpm = 220 "
4000rpm = 220 "
4500rpm = 220 "
5000rpm = 220 "
5500rpm = 218 "
6000rpm = 205 "
6500rpm = 185 "

The bit about "acceleration following the torque curve"and "torque is the only thing a driver feels" should now make sense.
As the torque rises the car will accelerate faster and as the torque reduces near the redline the acceleration will reduce.

Next we need to understand what the gearbox does.In a uk Impreza the gear ratio's are as follows

1st 3.4
2nd 1.9
3rd 1.3
4th 0.97
5th 0.73

The final drive ratio is 3.9.This makes the overall gearing

1st 13.26
2nd 7.41
3rd 5.07
4th 3.78
5th 2.85

This means that in 1st the engine rotates 13.26 times everytime the wheels rotate once and in 5th the engine turns 2.85 times for every turn of the wheels.

It also means that whatever torque the engine is producing is multiplied by the same amount (This is the same principle as using a long pole to lift a big rock).

This means that at 3500rpm in 1st the engine is producing 220 lb/ft and the ratio is 13.28:1 so multipling the two together we get the torque at the wheels which is 2922 lb/ft if we do the same for 6500 rpm the figure is 2457 lb/ft .

As you can see these are big numbers.If we now change to 2nd gear the engine speed will drop to 3643rpm (take it from me it does)at the same road speed.The gearbox ratio in 2nd is now 7.41:1. If we work out the torque at the wheels again we get

At 3643revs wheel torque is 1630 lb/ft and at 6500revs it is 1371lb/ft as you can see these numbers are much lower and do not provide nearly as much acceleration(push in the back)

If you changed to 2nd at say 5000rpm the torque at the wheels will drop from around 2900 lb/ft to about 1600 lb/ft and acceleration will reduce by quite a bit. If you had stayed in 1st you would still have had 2900 lb/ft to use for another 1000rpm's
This is why we use high revs when we want to accelerate as fast as possible.

The bits of the article below should now make sense,hopefully.

"For best acceleration you change up above the power peak and way past the torque peak"
"As a practical matter (for best acceleration ) this usually means shifting at the redline"

I deliberately haven't mentioned bhp but hopefully by know you should have a pretty good idea about torque.

I will post again when the keyboard cools down

Andy

AndyMc

Simes in its current state your ZX6R would out pull the Harly because it's gearing is lower, so the all important torque at the wheel is higher.

If they both had the same gearing then the Harly would easily win because now it would have more torque at the wheel.

I think we were saying the same thing in a different way .

The rest of your post is a very good and simple way to understand the difference between torque and bhp

My mate has a ZX6R, if I do a good start I can keep up with him for about half of first gear .Throw some corners in and its about even

Andy

[This message has been edited by AndyMc (edited 23 August 2000).]

Hoppy

AndyMc, I appreciate the time you've taken to explain things and yes, it all begins to make sense.

The difference between 'torque' and
'effective torque' is clear and I think this is what I was confusing with bhp.

What is the relationship between effective torque and bhp?

Thanks in advance.

Hoppy

AndyMc

Hoppy

There is no difference between 'torque' and 'effective torque'
I have just been calling torque at the wheels 'effective torque'as a way of separating it from torque at the engine.

I'm going out now, but will try to think of an easier way to explain bhp. All the info to understand it is already in this thread though!

Andy

SDB

I Still don't understand it...

Mind you, I thought it was sheer magic that made cars go forward until I looked under the bonnet, last week.

I was happy to live with this, until 1/2 hour ago when I had a conversation with a *girl*!!! who totally understood it! (You know who you are!!)

This leaves two options...

1) I'm a total bellend!
2) She's the answer to my dreams? (this one would explain the horn )

Regards

Simon de Wish I wasn't totally crap at this stuff

AndyMc

Hoppy

Having gone on for long enough about how important torque is I will now try to explain why the torque figure alone is not very meaningful.

Say we have an engine with a flat torque curve which makes 200 lb/ft between 3000 and 6000 rpm we know that the acceleration(push in the back) at 3000 rpm is the same as at 6000 rpm (remember "acceleration follows the torque curve")

However if we now work out the bhp using the formula above we get

3000 rpm = 114 bhp
6000 rpm = 228 bhp

This seems strange because the car is accelerating just as hard at 3000rpm as it is at 6000rpm yet is making only half the bhp

the bit of the article that says"horsepower is not particularly meaningful from a driver perspective" should now make sense.

now you need to use your imagination for the next bit.Imagine a UK Impreza in second gear,at 3000 rpm it will be doing about 30mph and at 6000rpm it will be doing 60mph

Say we now wanted the car to do 30mph at 6000rpm instead of 60mph we would have to use the gearbox to halve the speed of the wheels, but if you remember the gearbox bit from above this will cause the torque at the wheels to double. If the torque is now double the car will accelerate twice as fast as it did to start with.

This is where it starts to get confusing if we talk about Torque alone.
In the first example the car is doing 30mph and the engine is producing 200 lb/ft of torque.
In the second example the car is still doing 30mph and the engine is producing the exact same 200 lb/ft of torque yet the car will be accelerating twice as fast(because the torque at the wheels is twice as high).

The only difference between the two examples is the speed of the engine.From this you should be able to see that to get a true picture of how fast the car will accelerate we need to now the speed of the engine as well as the torque produced.

Bhp is just the two combined to give us a figure that is more meaningful and representative of the performance that will result.

The bhp figure actually tells you how much energy the engine can supply and therefore how much work it can do in a given time.

In theory if one car has twice the bhp of another it should be twice as fast.(If there was no such thing as air resistance and friction that is)

here is a simple analogy to finish with.

Imagine two blokes,one is strong and can lift three 100kg bags onto a table in one minute.The other cannot lift 100kg but can manage 50kg ok and can lift six onto the table in the same time.

In this analogy the torque is represented by the weight of the bags, the first bloke has more torque than the second.

The bhp is represented by the weight on the table.Both blokes have put 300kg onto it during the minute and have done the same amount of work so their bhp is the same even though the second bloke is not as strong

As you can see knowing how strong an engine is does not tell you how much power it can make.That is why we have both torque and bhp.

The second gear example above should also make the following bit of John F's article make sense "the only thing you need to know, it is better to make torque at higher rpm because you can take advantage of gearing"

I hope some of this makes sense,I could't think of an easier way to explain it.

Andy



[This message has been edited by AndyMc (edited 24 August 2000).]

Hoppy

Andy, excellent. I'm there. Thanks very much for that. I hope I'm not the only one following this thread!
Hoppy