HP v Torque

[ian.stewart]

On another forum I visit someone stated that a torque curve plot allways crosses the power curve around 5200rpm, and showed a Rolling road plot to prove it, OK, bit of BS here, untill I dragged out the last RR sesion results I have and my torque/HP cross at 5200 as well, and I have just checked a few posted graphs on other sites and all apart from one all seem to cross over at 5.2k, Why????
Ian

[Notso Swift]

5252 actually

The formula to convert HP and torque is
HP = (work done per minute) divided by 33000
Work done is 2Pi x torque x rpm

you can use 2Pi and 33000 as constants so divide them out and the figure is 5252.
So the rpm may change, the hp my change and the amout of torque may change, but the 5252 is always the same.

EDIT: the one that doesn't cross at 5252, is that in ft lbs, because if it is it has to, or it is simply wrong

[johnny yuma]

This one has had me intrigued for a while.If you draw the graph with non-"natural" scales for the vertical datums for power and torque with rpm plotted across the bottom surely you can make the curves "cross" anywhere.How is the graph most naturally represented--is the graph drawn with a crossover at 5252 as the starting point and the dots then filled in ? There cant be anything signifigant about 5252 rpm surely ?

Is there another crossover point if we graph Kw onto NM instead of bhp onto lbs ft. I'm looking at a metric Mitsubishi dealers brochure which has the crossover point at 2400 rpm.[mivec 2.4 litre petrol,graph ends at 6500 rpm]

[Tim Falce]

Originally Posted by ian.stewart

On another forum I visit someone stated that a torque curve plot allways crosses the power curve around 5200rpm, and showed a Rolling road plot to prove it, OK, bit of BS here, untill I dragged out the last RR sesion results I have and my torque/HP cross at 5200 as well, and I have just checked a few posted graphs on other sites and all apart from one all seem to cross over at 5.2k, Why????
Ian

Mine crosses at about 4700rpm. What happens with an engine that wont rev to 5200?

[crogthomas]

Power is simply the rate at which work is done. Work in this case being Torque and the rate being engine speed (rpm)

A dyno measures Torque and rpm, you then multiply them (and your 5252 constant to account for engine geometry) to get Power.

Thus Power is directly related mathematically to torque. Inescapably linked you could say.

If you plot Power in HP and Torque in lbsft, both ON THE SAME SCALE, you will find they cross at 5252rpm. If they don’t you are being lied to.

If you engine wont rev that high, they wont cross. Simple.

And yes, there will be an equivalent for KW and NM scales, about 9500rpm I think (??).

[MGDavid]

[QUOTE=crogthomas].....Thus Power is directly related mathematically to torque. Inescapably linked you could say.
If you plot Power in HP and Torque in lbsft, both ON THE SAME SCALE, you will find they cross at 5252rpm. If they don’t you are being lied to.
.....QUOTE]
Absolutely, inescapably linked, simple nnnit ;-)

But changing the units of measurement won't alter the point at which the curves cross, will it?

[crogthomas]

Originally Posted by MGDavid

But changing the units of measurement won't alter the point at which the curves cross, will it?

Im afraid it does. The constant 5252, accounts for the units being used as well. Imperial units being imperial mean they never play nicely. Therefore if you change your units of measurement you will also have to calculate a different constant.

[Chucky]

For this to work, power muct be measured in HP, torque in ft/lb and they have to be scaled the same.

I think it's 9246 if power is in Kw and torque is Nm. (Can someone verify?)

If your dyno cannot do this, then I don't know how it's calculating power and shouldn't be trusted.

[Notso Swift]

Like most dyno's, really

[johnny yuma]

So the only signifigance of 5252 rpm is that if your graphed curves don't cross here,or 9450 rpm for metrics,someone has stuffed the math or the plotting ?

Do dynos produce plots or just torque readings at various RPM FOR CONVERSION TO POWER OUTPUTS ?

[Chucky]

They derive the power figure form the amount of torque they measure and the speed of the engine/rollers (dependent on the type).

If they don't follow that calculation then it just isn't being done properly.