But at a constant rpm more torque = more power. So the torque-meter is really showing the captain total power output.

Quote from skiingman :

An interesting case where the captain cares all about torque and HP is only a figure of academic interest is the PT6 (or probably about any turboshaft/constant speed prop setup) captain pays attention only to torque. Since his prop turns (more or less) at a constant speed, torque is all he needs to know. Its also the only thing that can be easily directly measured, which is probably why they have a torque gauge and not a power gauge.

Absolutely correct. The PT6A-65AR for example uses 3650lbft of torque at max continuous power, 3800lbft for takeoff and will go has high as 4400lbft for emergency power. It will put out more, but every second you are there, you run the risk of throwing a blade off a turbine. This is at a prop RPM of 1700 at takeoff and 1425 for cruise (1300 RPM is the minimum for cruise). The horsepower is somewhat meaningless because we need to factor in the properties and efficiencies of the prop, which produces the thrust.

And Tristian, the Captain does care how and why his engine works the way it does. The way it is measured may obscure the mechanics of it (use % power not actual numbers), but the pilots do know how the power and how much power is being produced.

Quote from Shotglass :

which basically means that you scale the torque curve ... so the highest torque at the wheels for any gear ratio is always when the engine develops the most torque ...
so if your gearing always fits the current wheel speed perfectly youll get the most torque to the wheels at the engines torque peak

Look at the wheel torque/wheel speed graph in GRC again and load up any reasonable gear setup. Now try to imagine a smooth curve which maximises the torque at all wheel RPM, assuming that the transmission is infintely variable. This line will not go through the peak torque points in any gear. I'd imagine that it would go through the peak power point in each case.

Yep, for any gear you get maximum acceleration at the torque peak. But, for any speed (assuming you can pick any gearing), you get maximum acceleration at the rpm of peak power. Say torque peaks at 5000 rpm and power at 8000rpm. And let's also say torque has dropped by 40%. For any certain speed, to reach this speed at power rpm requires 60% more gearing reduction than to reach this speed at the torque peak. So even after the reduced torque from the engine is taken into account, there is still 20% more torque at the wheels. So, as far as the wheels are concerned, you really get the most torque at peak power (for a multi-gear vehicle).

It sounds wrong but it is right.

Good, it seems there is quite a lot of agreement in this thread now. That's a good sign that we do understand the subject properly. Well here are my answers to my questions from above.

Quote :

1) When is the best point to shift up?
a) when the next gear puts the engine at an rpm of higher power
b) when the next gear puts the engine at an rpm of higher torque
c) when the next gear will deliver more torque to the wheels

Obviously c). The interesting thing though is that a) and c) are actually mathematically equivalent.

Quote :

2) To get maximum accelleration out of an engine you can use a continuously variable transmission (CVT) that can keep the engine at a constant rpm. Which rpm should this be?
a) max engine torque rpm
b) max engine power rpm

b). colcob and others have already described why.

Quote :

3) You want to improve the accelleration of your car by getting a new engine. The dealer will only tell you either the max torque or max power value (no rpm value). Which one will give you more useful information?
a) max torque
b) max power
c) both the same

b) will give you an idea. a) is useless without an rpm value to go with it or some other information about the engine.

Quote :

4) You want to improve the accelleration of your car by getting a new engine. The dealer will only tell you either the torque curve or the power curve. Which one will give you more information?
a)torque curve
b)power curve
c)both the same

c) both contain exactly the same information.

Quote :

5) With a given gear ratio you can go through the engine's rpm range by varying the speed of the car. At which rpm will you find the highest acceleration?
a) at max engine torque rpm
b) at max engine power rpm

a) since the torque multiplyer is constant, the highest torque at the engine will give you the highest torque at the wheels.

Quote :

6) At a given vehicle speed you can go through the engine's rpm range by varying the gear ratio between engine and wheels. At which rpm will you find the highest acceleration?
a) at max engine torque rpm
b) at max engine power rpm

b) this question is essentially the same as question 2)

Quote :

7) Which question, 5) or 6) is irrelevant when you are a girl and are trying to determine the overall drag-strip performance of your car? (bash tristan not me )
a) 5)
b) 6)

a) since you can't choose your speed when you are trying to accelerate.

So the prize goes to colcob, who got just about all right. resent: :wow: :chicken: :balloons: :trophy_si

Quote from StewartFisher :

Look at the wheel torque/wheel speed graph in GRC again and load up any reasonable gear setup. Now try to imagine a smooth curve which maximises the torque at all wheel RPM, assuming that the transmission is infintely variable. This line will not go through the peak torque points in any gear. I'd imagine that it would go through the peak power point in each case.

your thinking of it in the wrong way imho ... were talking about a gearing that constantly changes while you acellerate
to help you image this look at the graph while you constantly change one gears setting (the scroll wheel of your mouse will work best for this) and look at what happens the peak torque at the wheel constantly moves towars higher speeds while you change the gear ratio (it should follow some more or less hyperbolic curve) when you think about it its ovious that the highest torque at the wheels and therefore the highest accel when using a ctv happens when youre at the peak torque of the engine and not somewhat to the right of the curve where the engine produces less torque

Quote from J.B. :

b) this question is essentially the same as question 2)

look at bobs last answer in this thread ... your mistaken ... (and you even contradict your answer to question 5)

Quote from skiingman :

:huh:

Perhaps if your gearing is off...

Power limits the acceleration of the car. Not torque. 1500Nm of torque does you little good at 400rpms.

Absolutely correct (MSME here)

Quote from Bob Smith :

The gearing tool I made for LFS, since you missed the link in my sig I'll put one here too: GRC

Doh, I appear to be blind. Thanks! Great tool!

I didn't read any posts below the quiz, or google anything, but i'd have to say:
1c(this was hard)
2a
3b
4c
5a
6a
7a

Quote from FunkedUp :

Absolutely correct (MSME here)

Nice! I'm a year or so into my BSME...quite a ways to go...

Quote from skiingman :

It'd be nice if there were some big conspiracy to that effect....but thats just not the case.

A given acceleration over a given period of time requires a given amount of work to be done. This work cannot be done without power. All the torque in the world is nice, but means absolutely squat if the torque can't be produced far enough to the right of the graph.

I can give a very realistic and very obvious example which proves this point.

" torque/weight is directly analogous to force/mass which equals acceleration."

So I've got this electric motor that produces 2000lb-ft of torque at 1 rpm. It produces 300lb-ft at 1000 rpms. This is not an unrealistic torque curve for an electric motor.

Although it will smoke the tires even in fourth gear from a stop, no amount of gear swapping in the world will make it accelerate faster over any reasonable time period or distance than a 160hp gasoline engine making about 150lb-ft of torque.

By stating this:
Torque/effective wheel radius = delivered linear Force at contact patch.

you are implying that you don't understand the function of a gearbox. Bob did a good job of explaining its torque multiplication duties.

If torque to weight were a better indicator of performance than power to weight (obviously false with a basic look at physics) the izzy Celica would be faster/quicker than the duzzy Celica. Its not. Same goes for ZO6 vettes, which really don't make more torque than stock vettes. They move the torque curve further right, increasing the PRODUCT of torque and RPM......horsepower.

Right on, and thats exactly why the figure given is hp/weight, not torque/weight. Given non-silly gearing you can get pretty much the same result of calculation from hp/weight as you can by plotting torque available at a given engine speed/gear combo over time. Its a lot less labor intensive too, unless you have something cool like the tool Bob is using.

Close, but not quite *grin*. Acceleration from a given speed to a given speed is a quotient of horsepower and weight; that is, acceleration over time. This is determined not by peak power of the engine, but the area that you'd get if you graphed the torque curve of the engine (torque x rpm equaling power, of course) over the used rev range. Acceleration as an instantaneous value, that is at any given moment you're accelerating at Xmetres/sec/sec, is a straightforward torque/mass (and drag) equation. In other words (torsional) force divided by mass equals acceleration at that moment.

So acceleration from point to point IS determined by horsepower, but that's NET hp (over distance) not PEAK hp (instantaneous) which is a fairly meaningless value in this application because you're only at peak hp for a moment and using a work over time derivation for an instantaneous value is just silly. At any given moment best acceleration will be at peak torque.

It's only when you hold a single point or very narrow section of the rev range over a period of time significant to the exercise that you're performing (in this case, pushing a vehicle) that peak power becomes the most significant value. Because you're not accelerating hard, torque over TIME becomes more significant that torque over DISTANCE. So at high power/low acceleration (at or close to top speed) horsepower is more significant. So saying that overall torque is more important or power is more important is a bit simplistic, the emphasis will shift dependant on specific factors.

Or as people who think less and do more would say, torque makes you accelerate, power gives you top speed.

Right... That's some great articulation, thanks for putting the TIME factor into better words than I could previously...

That being said, an vehicle with a high peak POWER output rating is LIKELY to have more area under it's torque curve, and therefore able to perform more work over time and therefore accelerate faster.

Quote from Resound :

This is determined not by peak power of the engine, but the area that you'd get if you graphed the torque curve of the engine (torque x rpm equaling power, of course) over the used rev range.

Thats only true if you don't have the perfect gearbox.

Thus all my statements including "with reasonable gearing" as a precaution.

Quote :

Acceleration as an instantaneous value

Not the way its expressed in automotive publications and the ramblings of enthusiasts. Time to distance or speed is what "acceleration" tends to refer to in these contexts. Not that its right...its obviously not. I've incredibly rarely heard any automotive enthusiast talk about actual accelerations.

Again, all my statements say "acceleration over time" which is what car people talk about.

Time to either speed or distance depends on the power produced. Torque can be used to determine this if you know all of the relevant figures. Since the relevant figures are often difficult to know and consume a good bit of mind-space, relating hp/weight figures is very simple, effective, and accurate.

Quote :

So at high power/low acceleration (at or close to top speed) horsepower is more significant. So saying that overall torque is more important or power is more important is a bit simplistic, the emphasis will shift dependant on specific factors.

This is all pseudojargon. There is no particular end of the scale where one or the other is more effective. The laws of physics are the laws of physics, given certain data you can obtain other data.

Neither is more important...the laws of physics are rather non discriminatory. However, the entire point of the discussion is that a single hp/weight figure is far more meaningful than a single torque/weight figure for determining time to speed or distance...aka "acceleration."

An 800hp/ton car will always go from a to b in less time than the 400hp/ton car, so long as the gearing is appropriate. Even if the 400hp/ton car exhibits a larger value for the integral of the torque curve this is still true.

The 800hp car could perhaps only make power at 6000rpms...a single speed motor. Area under the curve would be zero.

Quote :

Or as people who think less and do more would say, torque makes you accelerate, power gives you top speed.

You can't have one without the other. The torque at the top speed must be sufficient to produce the power.

Its really critical to note here that torque at the wheels does the acceleration as colcob explained.

I thats the fundamental point of misunderstanding that started this whole thing off. Me and bob were thinking of instantaneous acceleration values, and you were thinking of acceleration over time, which granted is how most people probably think of acceleration.

I think also its more accurate to say that the overall acceleration over time is the integral of the tractive effort curve for the speed range, not the torque curve for the rev range.

This obviously includes the gearing effects, so your single speed engine would just have a straight, descending line from its CVT gearbox, with a bit fat area under it

Ahhh, and now it's comes down to torque vs horsepower...pretty soon we'll
go over aerodynamic drag vs downforce, what's this thread about again ?
Hehe. I like how force(torque), acceleration and work(hp) get all mixed up
in a big bowl of 'power'.

I'd just like to clear up some details. It's hard to get good results when you
start out wrong Skiingman, it is wrong to say :

" torque/weight is directly analogous to force/mass which equals acceleration."

Let me start over at the first chapter of physics 101. From reading your
posts, i supposed you are familiar with F=ma, where F=Force(Newton),
m=mass(kg) and acceleration(ms²). Now, 'weight' is a human concept and
it describes what FORCE you exert on an object. Mass is the actual
molecular mass of all the stuff that make up your body. So, what you're
actually saying there is: Force/Force is analogous to Force/Mass. To get
weight, you use mass*acceleration, where acceleration is earth's gravity,
9.8ms².

It's a common error and the fact that some scales give mass while others
give your weight but that all of them express this the same way just adds
to the confusion. Typically, a household scale is a plate with a spring under
it. By measuring the compression of the spring, the scale can tell you what
FORCE you exert on the earth. Since earth's gravity is relatively constant,
it's easy to 'tune' all scales so you only have to step on it to get the correct
weight. Doctors on the other hand have proper scales that measure mass.
The way they do it is very simple. It uses weights to compare. By moving
weights around until the scale evens out, you get the value of the mass your
body displaces. Again, mass and weight are 2 different things. If you start out
mixing those, the rest can only be wrong.

I read detailed explanations on concept barely known yet no one notices
this obvious problem statement. Tell me when we get back to why GTRs
dont reach higher speeds on the straight.

Well it wasnt skiingman who said that, it was me. And I think you'll agree I (and most of the contributors to this thread) can manage without an idiots guide to physics.

If one value equals another value times a constant, the two values can be described as analogous, ie they have a proportional relationship.

So as force*lever arm (a constant) = Torque, Torque is proportional to force. As Weight = Mass*g(a constant), weight is proportional to mass. Hence the statement :

torque/weight is directly analogous to force/mass which equals acceleration.

is entirely correct.

Or if you like, (torque/weight) * K = Force/Mass

where K= 1/(leverArm*g)

Quote from colcob :

If one value equals another value times a constant, the two values can be described as analogous, ie they have a proportional relationship.

So as force*lever arm (a constant) = Torque, Torque is proportional to force. As Weight = Mass*g(a constant), weight is proportional to mass. Hence the statement :

torque/weight is directly analogous to force/mass which equals acceleration.

is entirely correct.

Or if you like, (torque/weight) * K = Force/Mass

where K= 1/(leverArm*g)

There, now i understood, i didn't realise you were comparing torque and force.
It's not about being an idiot, it's about being clear. If you think all people you
don't understand what you say are idiots then when it's everyone who
doesn't seem to understand, maybe it'll be more important to you.

Ok, so now that you guys have talked this over and concluded that
you were saying the same thing, can we move on ?

I hope skiingman doesn't take this personally That statement would make
any physics teacher react. Of course, they might understand faster

Quote from Fonnybone :

Hehe. I like how force(torque), acceleration and work(hp) get all mixed up in a big bowl of 'power'.

uh ... huh ? hp is a dimension of power not of work