Torque vs Power - some clarity...

[Mad Manny]

To work out Power from Torque is the same - we need to know distance and time. the distance in the torque figure has to be altered to give us a linear distance to work in the Power equation.

So to translate it from a rotational to a linear is pretty easy - the circumference of a wheel is 2 x pi x radius

In effect - by multiplying it out, you've now translated the figure to Energy. 2pi Nm(torque) = 1 Nm (Energy) = 1J

So, we now know Force x distance (for 1 revolution) and just need to add time to get to Watts.

We measure speed of work in a motor in rpm (generally)

so, to add that into the equation we must divide the rpm by 60 to get to seconds AND we must multiply by the number of revolutions in one second as our figure is only for 1 revolution.

we'd also like the figure to be in kW (not Watts) so we divide by 1000

so, the final equation to get from Torque to Power (metric) is:

Power = Torque x 2pi x rpm/60/1000

As Pi is constant as well as the 60 and 1000 we can shorten it to:

Power = Torque x rpm /9549

As can be deduced - High Torque at low rpm (e.g. A big Diesel making 500Nm at 1500rpm is making only78kW) can make the same power as a motor making low torque at high rpm e.g. a proper F1 motor that revs to 19000rpm but makes only 300Nm is making 597kW

Basically... for Manny ... This means... that for any rpm less than 9549 the Torque figure will be higher than the power figure
at 9549 rpm power figure and torque figure will be equal
And over 9549rpm the Power figure will be higher than the Torque figure.

Now hopefully I never have to see anyone declaring that you convert torque to power by using the stroke of the engine ever again!

I may be confused...

Power = Torque x rpm /9549
Lets take the Toyota 86
Bore & stroke = 86mm x 86mm - Boxer motor
1998cc
147kW @ 7000rpm
205Nm @ 6400rpm

Applying your formulae;

205 x 6400/9549 = 137kW - but the 86 is claimed to produce 147kW.
Are you saying the Power (kW) at 6400 is 137kW?

Do I then used the power peak (rather than the Torque peak)in the formulae?

Like so; 205 x 7000/9549 = 150kW (closer to the claimed 147kW).

Let's do another one;
Lexus NX 300E
86mm x 86mm - Straight 4
1998cc
175kW @ 4800rpm
350Nm @ 1650rpm

Apply formulae: 350 x 1650/9549 = 60...
So lets use the power peak;
350 x 4800/9549 = 175kW

BINGO!!!

Right, last one:
Subaru WRX
86mm x 86mm Boxer
1998cc
Turbocharged
197kW @ 5600rpm
350Nm between 2400 & 5200rpm.

Formulae: 350Nm x 5600/9549 = 205kW (bit higher than claimed - but close enough).

So its Torque x Power (not Torque) peak rpm divided by 9549

Or have I messed it up?

[Apocalypse]

To work out Power from Torque is the same - we need to know distance and time. the distance in the torque figure has to be altered to give us a linear distance to work in the Power equation.

So to translate it from a rotational to a linear is pretty easy - the circumference of a wheel is 2 x pi x radius

In effect - by multiplying it out, you've now translated the figure to Energy. 2pi Nm(torque) = 1 Nm (Energy) = 1J

So, we now know Force x distance (for 1 revolution) and just need to add time to get to Watts.

We measure speed of work in a motor in rpm (generally)

so, to add that into the equation we must divide the rpm by 60 to get to seconds AND we must multiply by the number of revolutions in one second as our figure is only for 1 revolution.

we'd also like the figure to be in kW (not Watts) so we divide by 1000

so, the final equation to get from Torque to Power (metric) is:

Power = Torque x 2pi x rpm/60/1000

As Pi is constant as well as the 60 and 1000 we can shorten it to:

Power = Torque x rpm /9549

As can be deduced - High Torque at low rpm (e.g. A big Diesel making 500Nm at 1500rpm is making only78kW) can make the same power as a motor making low torque at high rpm e.g. a proper F1 motor that revs to 19000rpm but makes only 300Nm is making 597kW

Basically... for Manny ... This means... that for any rpm less than 9549 the Torque figure will be higher than the power figure
at 9549 rpm power figure and torque figure will be equal
And over 9549rpm the Power figure will be higher than the Torque figure.

Now hopefully I never have to see anyone declaring that you convert torque to power by using the stroke of the engine ever again!

I may be confused...

I am in no doubt that you exist in a perpetual state of confusion....

Power = Torque x rpm /9549
Lets take the Toyota 86
Bore & stroke = 86mm x 86mm - Boxer motor
1998cc
147kW @ 7000rpm
205Nm @ 6400rpm

Applying your formulae;

205 x 6400/9549 = 137kW - but the 86 is claimed to produce 147kW.
Are you saying the Power (kW) at 6400 is 137kW?

correct! do bear in mind that torque peak in not necessarily power peak as power is a function of torque and rpm.
Do I then used the power peak (rather than the Torque peak)in the formulae?

Like so; 205 x 7000/9549 = 150kW (closer to the claimed 147kW).

Que?

if you wish to calculate Torque from Power, you must rearrange the formula. Power = Torque x rpm / 9549 THEREFORE Torque = Power x 9549/rpm (Grade 6 maths)

so, at 7000 rpm the motor is making 147kW which means it is making 147x9549/7000 = 200N.m at that point in the rev range.

again. Power peak is not Torque peak.


Let's do another one;
Lexus NX 300E
86mm x 86mm - Straight 4
1998cc
175kW @ 4800rpm
350Nm @ 1650rpm

Apply formulae: 350 x 1650/9549 = 60...
So lets use the power peak;
350 x 4800/9549 = 175kW

Que?

Basically... what has happened here is that the torque curve is so flat your bastardised use of the equation has come up with the right answer. Shakes head sadly. you can't pick and choose the numbers. Torque is Torque and Power is Power...




BINGO!!!

Right, last one:
Subaru WRX
86mm x 86mm Boxer
1998cc
Turbocharged
197kW @ 5600rpm
350Nm between 2400 & 5200rpm.

Formulae: 350Nm x 5600/9549 = 205kW (bit higher than claimed - but close enough).

cries in disbelief

So its Torque x Power (not Torque) peak rpm divided by 9549

Or have I messed it up?


yes.

you cannot pick and choose the numbers from wherever you like.

Sweet LAWD. Manny De Freitas shadow finance minister. God help us.

[Quack]

Well Alex in fairness Manny is halfway there, but torque peak and power peak is hardly ever at the same point.

Power peak is that sweet spot between rpm and torque.

To understand it a bit better you ideally should look at the graphs, most modern engines will make a fair amount of torque over a wide rpm range. So normally your power peak will occur just as the torque drops of at the high end of the rpm range.

I need some crayons to explain it!‍

And lets not confuse the issue even more by stating that both peaks happen at full throttle!

In the off road environment, not even the most enthusiastic drivers drive at full throttle all the time!

The drivability of an engine depends a lot on the torque made at partial throttle, but as I’ve said this will just confuse the issue even more!
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[Mike Nel]

Bring the crayons. You guys "torque"to much

[Apocalypse]

Well Alex in fairness Manny is halfway there, but torque peak and power peak is hardly ever at the same point.

Power peak is that sweet spot between rpm and torque.

To understand it a bit better you ideally should look at the graphs, most modern engines will make a fair amount of torque over a wide rpm range. So normally your power peak will occur just as the torque drops of at the high end of the rpm range.

I need some crayons to explain it!‍

And lets not confuse the issue even more by stating that both peaks happen at full throttle!

In the off road environment, not even the most enthusiastic drivers drive at full throttle all the time!

The drivability of an engine depends a lot on the torque made at partial throttle, but as I’ve said this will just confuse the issue even more!
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It would appear that Manny has eaten all the crayons .

[Quack]

Only the blue ones


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[Mad Manny]

So, who supplies the rpm that you must use in the Formula?

[Mad Manny]

And lets not confuse the issue even more by stating that both peaks happen at full throttle!

Quoted Peak Power & Peak Torque are ALWAYS measured at:
* Full Throttle,
* at the Flywheel,
* at Sea Level...
Note That ambient temperatures also play a role.

So when you take your 120kW motor, with 86 000km on the clock, to a Tuner in JHB & they show you are graph saying they got you 148kW you know the graph is tuned because:
* It's at the wheels(unless they claim they worked it back).
* You are at Altitude.

[Apocalypse]

So, who supplies the rpm that you must use in the Formula?

350Nm @ 3000 rpm

40kW at 6000rpm

you can't decide to use 350N.m @ 6000rpm and expect to come up with 40kW....

[Apocalypse]

So, who supplies the rpm that you must use in the Formula?

350Nm @ 3000 rpm

40kW at 6000rpm

you can't decide to use 350N.m @ 6000rpm and expect to come up with 40kW....

A graph is a series of plotted points.

so you might see 30kW at 1000rpm, 40kW at 2000rpm, 50kW at 3000rpm 55kW at 4000rpm. that forms your curve.

from that you can caluculate Torque using the Formula : P=Txrpm/9549 which is T=Px9549/rpm

so, 286N.m@1000rpm , 190Nm@ 2000rpm, 159Nm@3000rpm, 131N.m @4000rpm

as we can see - power peak is at 4000 rpm and Torque peak at 1000rpm.

unless it can rev to 5000rpm - even is torque is less, the increased rpm (rate of work) would make for a higher kW figure

[KurtG]

Manny, the computers on the dyno can compensate for altitude. It’s very clever

[Apocalypse]

I think I've offended Manny.

[Apocalypse]

Manny, the computers on the dyno can compensate for altitude. It’s very clever

well, they don't compensate for it.

they add in a factor based on atmospheric pressure and temperature.

so - e.g. to comeback to DIN , you must multiply your results by a figure that assumes the motor would make x% more power if it was 20degrees at 1 bar (20 degrees at sea level) instead of where you happen to be. When it's 35deg ambient and 1700m above sea level that conversion is so thumbsucked you might as well have asked Zuma to give you a figure.

plus... many of the factors are set by the operator..... which tells you how reliable a dyno run can be if the operator is unscrupulous....

this is why you'll often find that (as grips mentioned) the torque @rpm can quite often give a higher power figure than the peak power on the run.... which is impossible....

[ThysleRoux]

Thank Alex = even I should be able to do the calculation - if I can wrestle the last (blue) crayon from MalMan's hairy paw......

[Mad Manny]

Yet I'm the only one who has applied the formula.

The rest just claim understanding.