Why Do They Always Cross At 5252?

[Forged]

Torque and horsepower curves that is.

I had a discussion with mates regarding dyno graphs and why in most cases an engine's torque and power curves always cross at 5252 rpm?

Remember there was a theory about it. Can anyone explain it for us?

[sh@un]

it's because horsepower = (torque x revs) / 5252

so it's 1-to-1 when the revs match the equation denominator

the cross over point is different if you use kW&Nm, or kW&ftLbs, or PS&kg.m, or whatever variation...

[mikel]

power = torque x revs.

which is why peak torque occurs lower in the rev range that peak power - power will keep rising after peak torque so long as the revs rise faster than the torque drop off.

There's no magic number - it just depends on the characteristics of the engine.

There may be a tendency for the engines you've been looking at to have a crossover around 5252 rpm, but the scales are different (ie kW vs newtons), so you could cross them over any where (or not at all) just by fiddling with the scales on the "y" axis.

Also remember the torque figure is not Nm (newton metres) but newtons (N) as there's too much variation in gearing between every car (tyre/wheel size, diff ratio etc), so the torque reading would change by changing wheels, but the horepower won't (much).

I'd love to read more about this theory - I may be missing something.

Edited August 14, 2007 by mikel

[saliya]

Torque and horsepower curves that is.

I had a discussion with mates regarding dyno graphs and why in most cases an engine's torque and power curves always cross at 5252 rpm?

Remember there was a theory about it. Can anyone explain it for us?

The longer version of Sh@un's answer:

http://auto.howstuffworks.com/question622.htm

and yes, it's a 'magic' number by virtue of the formula.

Regards,

Saliya