Power vs Torque Questionnaire -RESULTS

[hollus]

You can still run in a different gear to stay close to optimum power, the only thing that matters.

[Manoah2u]

meanwhile, the comparison is flawed because we don't have a similar engine.

one is a DIESEL engine, the other is a PETROL engine. so we can't compare!

we can compare a diesel engine which has 200lbft of torque and 300hp
and compare it to a diesel engine which has 300lbft of torque and 300hp
and compare it to a diesel engine which has 100lbft of torque and 400hp

we can compare a petrol engine which has 200lbft of torque and 300hp
and compare it to a petrol engine which has 300lbft of torque and 300hp
and compare it to a petrol engine which has 100lbft of torque and 400hp

comparing a diesel to petrol does not work

[hollus]

Power is all that ever matters. Power=force*distance/time. At any given speed both cars have the same distance/time and you are left with more power = more force. It is up to you to use your gears to keep each engine where it produces about max power, which is just over 200hp in both examples. Same power and same speed -> same acceleration. Why would you ever be in the wrong rev range?
Edit: divided by time, my bad.

[hollus]

Hmmmm, since you used a cycling example...
How does Alberto Contador measure his output? In watts. And what would happen if you forced him to run the next tour the france in a cycle with wheels significantly bigger than everyone else's? Not much, imo, he'd just run in a different gear.

[Manoah2u]

meanwhile, the comparison is flawed because we don't have a similar engine.

one is a DIESEL engine, the other is a PETROL engine. so we can't compare!

we can compare a diesel engine which has 200lbft of torque and 300hp
and compare it to a diesel engine which has 300lbft of torque and 300hp
and compare it to a diesel engine which has 100lbft of torque and 400hp

we can compare a petrol engine which has 200lbft of torque and 300hp
and compare it to a petrol engine which has 300lbft of torque and 300hp
and compare it to a petrol engine which has 100lbft of torque and 400hp

comparing a diesel to petrol does not work

please explain why this is incorrect

[langwadt]

sigh, .. Because the engines runs at different RPM they need different gear ratios to go the same speed, that ratio also multiplies the torque at the wheels. With the prober gear 200Nm@6000rpm, is exactly the same as 400Nm@3000rpm

it's like beating dead horse, this is really simple physics

yes, but you claim the only difference to both cars is FINAL gear, not the rest of the gears.

Gearbox: 7 Speed "7G-Tronic" automatic gearbox (Both cars)
Final Drive: Optimised to suit each engine

thus, we should assume 1-6 gear are the same, and only 7th gear [final drive] is different.

if that is the case, then the higher torque vehicle should accelerate faster.

but if ALL the gears (ratios) are different, then yes, then they could accelerate the same.

With final drive I understand the final ratio from crankshaft to wheels to match reasonable speed with rpms in each gear
i.e. something like a differential with a different ratio

the gears and gear box maybe the same but the over all ratio should be different,
you don't want one engine redlining at 100km/h in second gear and the other 200km/h

[mrluke]

Power is all that ever matters. Power=force*distance*time. At any given speed both cars have the same distance*time and you are left with more power = more force. It is up to you to use your gears to keep each engine where it produces about max power, which is just over 200hp in both examples. Same power and same speed -> same acceleration. Why would you ever be in the wrong rev range?

Correct.

Torque does not have a time element.

You are talking about acceleration which is a rate of change of velocity, i.e. a change in state over time. Time being the operative word which means your comparison is with power which also has a time element.

When you talk about low down "torque" what you are really referring to is the low down power, which gives a high rate of change of acceleration (known as jerk) its not simply the acceleration that is important but the rate of change of acceleration.

If you lineally accelerate to 60mph its fairly boring, when you have a really "torquey" car it feels like it just pulls faster and faster, this is due to an increasing rate of acceleration. Similarly once you get through the power band it can feel like the car has run out of puff or is becoming "breathless" although it feels like you are slowing down, your speed is still increasing. Again it is the rate of change of acceleration that you are noticing.

This is why a torquey engine can feel more powerful because you can experience a very high rate of change of acceleration whereas a "revvy" engine has to be really wound up.

[machin]

so go ahead, explain this.

So, where were we:-

(Torque at driven wheels) ∝ Acceleration

Hence

(Crankshaft torque) x (gear ratio) ∝ Acceleration

And overall gear ratio = (Crank RPM)/(wheel RPM)

Therefore (Crankshaft torque) x (Crank RPM)/(wheel RPM) ∝ Acceleration

So if we have an engine making 358lbft of crank shaft torque at 1800rpm (like the Diesel Merc in my example), travelling at a speed such that its road wheels turn at 800rpm, the overall gear ratio will be 2.25 (1800/800 = 2.25). So the torque at the driven wheels will be 358x 2.25= 805.5 lbft

If we have another engine making 250lbft of crank shaft torque at 2750rpm (like the pretrol Merc in my example), also travelling at the same speed with equal sized wheels/tyres, (i.e. driven wheels also turning at 800rpm), the overall gear ratio will be 3.44 (2750/800 = 3.44). So the torque at the driven wheels will be 250 x 3.44 = 860lbft.

If we just remind outselves that....

(Torque at driven wheels) ∝ Acceleration,

....we can see that with the two cars above they are both capable of very similar acceleration since they can generate similar torque at the wheels despite having vastly different crankshaft torque capabilities, because the wider rev range of the petrol engine allows a higher gear ratio to be used. In fact the petrol engine, despite having lower torque at the crank shaft, can actually achieve slightly higher torque at the wheels, and therefore we would expect it to have slightly higher acceleration.

So that is how you can have less crank shaft torque, but still have the same, or higher acceleration.

Now we could stop there... but I think we should go on:-

Now simple maths tells us that Force x Moment Arm = Torque, which rearranges to Torque/moment Arm = Force.

Now if we make the wording a bit more appropriate for our case this is:-

(Driven Wheel Torque)/(Driven Wheel radius) = Motive Force

Or, adding this to our earlier equation:-

((Crankshaft torque) x (Crank RPM)/(wheel RPM))/(Driven Wheel radius) = Motive Force

However, not all of this force is available to accelerate our car as some is used to overcome resistance (Rolling resistance, wind resistance, transmission resistance, gradient resistance), therefore we have:-

Force available for Acceleration = (((Crankshaft torque) x (Crank RPM)/(wheel RPM))/(Driven Wheel radius)) -(Resistance Forces)

Now we also know that F=m.A thanks to our old friend Sir Isaac Newton, which rearranges to Acceleration = Force / Mass

In our case the "Mass" term must also account for rotating inertia of wheels, tyres, brakes, drive shaft, gearing, engine crank shaft, flywheel, etc), but this means we can now substitute our "Force available for Acceleration" into our rearranged F=m.A equation:

Acceleration = ((((Crankshaft torque) x (Crank RPM)/(wheel RPM))/(Driven Wheel radius)) -(Resistance Forces)) / (Car Mass+allowance for Inertia).

So now, given sufficient data, we can actually calculate the acceleration of the vehicle. More than that we can calculate the top speed too, since at top speed the Motive Force = Resistance forces, and hence the acceleration is zero.

So there we go. Using Crankshaft torque, we can calculate the acceleration of our vehicle at any given road speed, and we can also calculate the top speed of our vehicle. But in order to do so, we also need to know, and take account of the Crank RPM[/b] at which the engine is operating in order that we can calculate the gear ratio.

"Great", you might be thinking, "So torque is used to calculate acceleration and top speed... where does power come into all this?" No, its not about sustaining speed or any of that other stuff you may've read in a boy racer car magazine. Power is simply; (Crankshaft Torque) x (the Crankshaft RPM at which that torque is generated); it is just a really convenient figure..... I'll explain:-

Lets just remember:-

Acceleration = ((((Crankshaft torque) x (Crank RPM)/(wheel RPM))/(Driven Wheel radius)) -(Resistance Forces)) / (Car Mass+allowance for Inertia).

And Top speed occurs when (((Crankshaft torque) x (Crank RPM)/(wheel RPM))/(Driven Wheel radius)) = (Resistance Forces)

Which means we can simplify the equations above by substituting Power = Crank Torque x Crank RPM:-

Acceleration = ((((Power)/(wheel RPM))/(Driven Wheel radius)) -(Resistance Forces)) / (Car Mass+allowance for Inertia).

And Top speed occurs when (((Power)/(wheel RPM))/(Driven Wheel radius)) = (Resistance Forces)


So Power can also be used for calculating acceleration and top speed??!!! YES!!!!

So when we compare the accelerative or top-speed giving abilities of an engine we can either compare the values:-

(Crank Torque)x(Crank RPM)... which is a little complicated off the top of your head.... is 258lbft x 4000rpm higher or lower than 230lbft x 4500rpm???!

Or we simply compare Power; is 196bhp higher or lower than 197bhp? -EASY!!!

The other point to bear in mind is that to determine the time taken to accelerate from one speed to another we also need to know the power (or indeed, the "crank torque x crank rpm") at all crank speeds used during the acceleration, not just the peak power or peak torque.

The sensible question you might now be asking is "Why do car magazines and brochures tell me the power at high rpm and the torque at low rpm, when really they should be telling me the power or the "Crank Torque x Crank RPM" at high and low rpm?" And the honest answer is "I don't know either -it is crazy, it is like saying 'This DVD is worth £4.00 and this other one is worth 3.65 x $2.4!!! You could wok out how they actually compare, but its not immediately obvious


Oh, and I just want to be clear; all those car performance figures on the link in my signature are independently tested by Autocar magazine.... not theoretical, not an analogy, not made up by me. Actual data.

one is a DIESEL engine, the other is a PETROL engine. so we can't compare!

A diesel engine is simply a metal box with a rotating output flange and clutch which attaches to a gearbox, as a petrol engine is also simply a metal box with a rotating output flange and clutch which attaches a to a gearbox, so the two can quite easily be directly compared to eachother if you use the correct methods to compare them.

You could also compare a gas turbine ("metal box with a rotating output flange")... or an electric motor ("metal box with a rotating output flange")... or a steam engine ("metal box with a rotating output flange")....

yes, but you claim the only difference to both cars is FINAL gear, not the rest of the gears.

"Final drive" does not mean top gear... it is the final gear ratio normally located in the differential (see below) which is, after, but in series with, the gearbox, and therefore changing that one "Final drive" gear ratio, actual changes the overall ratio in 1st gear all the way through to 7th gear, even though the gears in the gearbox themselves are physically the same.



Oh and by the way: the clutch on 99% of cars attaches to the flywheel... the flywheel attaches to the crank shaft. So "Flywheel torque" is the same as "Crank Torque" or "Torque at input to gearbox". here is a clutch attached to an engine flywheel:-

[Cold Fussion]

Posting this in the general section was always going to lead to handwaving pseudophysics by the 'experts', and this thread has not disappointed. All we need now is for someone to chime in with the torque is energy nonsense!

[Tim.Wright]

meanwhile, the comparison is flawed because we don't have a similar engine.

one is a DIESEL engine, the other is a PETROL engine. so we can't compare!

[Miguel]

meanwhile, the comparison is flawed because we don't have a similar engine.

one is a DIESEL engine, the other is a PETROL engine. so we can't compare!

http://cdn.someecards.com/someecards/us ... 149750.png

...and this is at F1T, where some of us have had physics classes as part of our higher education. Some of what I've read here is simply apalling.

[Charlatan]

I might be a bit blunt here, but Power has always been Force times Speed and that's all there is to it.

Kitchentable torque-ponderings might be interesting in itself, but is of absolutely no scientific value.

I'm surprised to behold three pages of infantile discussions on a site calling itself "F1Technical".

[Manoah2u]

meanwhile, the comparison is flawed because we don't have a similar engine.

one is a DIESEL engine, the other is a PETROL engine. so we can't compare!

http://cdn.someecards.com/someecards/us ... 149750.png

...and this is at F1T, where some of us have had physics classes as part of our higher education. Some of what I've read here is simply apalling.

exactly. it's quite arrogant to bloat around with figures whilst presenting an article or website that contradicts itself in the way it presents its information. i never had an issue with whether the given facts a true, i have a huge problem with the arrogance in the way the information is given whilst the statements contradict itself.
to make a point, you can't bring contradicting statements within the same article.

i had a pm of a very honest and great fellow member whom summed it up in almost a single sentance, and did it humble and honest without being degenerative towards any other member. i can respect that a lot.

something i can't respect, are people that think they're better then others and start with an attitude like 'here little kids, ill show you guys step by step how it works'. i can only imagine dealing with people is not some of the strongest qualities in this behavior.

[Charlatan]

Since eternity, Power has been Force times Speed, while Force is Mass times Accelleration and that's all there is to it.

Pub-ponderings over how "torque" is relevant might be intriguing in that environment, but has no scientifical value at all.

Being new here, I'm surprised to learn that a site called F1Technical even discusses this, as if it was a beauty contest.

[machin]

wait, what?

so, you claim the answer that the car with 40% more torque should accelerate faster is an INCORRECT ANSWER,
and then you try to explain why it's wrong by stating MORE TORQUE equals HIGHER ACCELERATION.



please, explain this contradictory statement. because clearly either something then must be wrong with this analogy, or there must be something wrong with the given numbers.

according to the analogy "The same is true with Torque and Engine RPM; if you can increase one or both of these parameters you will be able to accelerate your car quicker".

Go ahead, explain this.

Thank you (honestly!) for your feedback; I have updated the part of the website you refer to as follows, note new section in Bold, hopefully it is evident from this that the previous text wasn't contradictory, but maybe just not quite clear enough....

"The same is true with Torque and Engine RPM; if you can increase one parameter (without decreasing the other) or better yet; increase both of these parameters, you will be able to accelerate your car quicker."

The Diesel engine might have more flywheel torque, but it produces it at lower RPM... Hence it must use a gear with a lower overall ratio... The result is that it cannot produce any more torque (or motive force) at the driven wheels and therefore cannot accelerate faster despite having more flywheel torque.