Torque is a twisting force. Power is the rate of doing work. They are related by engine speed, and the relationship is exact — which means any dyno graph has a fixed, predictable shape relationship between its two curves.
The formula and the constant
Horsepower equals torque in pound-feet times rpm, divided by 5,252. That constant is not arbitrary.
James Watt defined one horsepower as 33,000 foot-pounds of work per minute. A rotating shaft turns through 2 pi radians per revolution, so converting a twisting force at a rotational speed into work per minute means dividing by 2 pi.
33,000 divided by 2 pi is 5,252.1. That is the whole derivation, and it is why the number looks so odd.
Why the curves always cross at 5,252
Set horsepower equal to torque in the formula and the rpm term has to equal the constant. So on any engine, in any units of this system, the two curves cross at 5,252 rpm.
Below that rpm the torque figure is numerically higher than the power figure; above it, power is higher. This is a property of the arithmetic and tells you nothing about the engine.
It is a useful sanity check on a dyno sheet. If the curves cross somewhere else, either the axes are scaled differently or one of them is not in these units.
Which number matters
Torque is what you feel — it is the force turning the wheels at that moment. Power is what determines how fast you can go and how quickly, because it accounts for the rate.
A gearbox trades between them. Any engine can produce enormous torque at the wheel through a low enough gear; what it cannot do is exceed its power. So acceleration over a range of speeds is a power question, and pulling away from rest is a torque question.
This is why a small high-revving engine and a large low-revving one can perform identically. The small one makes less torque and spins faster, and the gearbox reconciles them.
Peak numbers and the area under the curve
Advertised figures are peaks, and peaks are the least useful part of a curve. An engine making 300 hp at 7,000 rpm and 180 lb-ft across a narrow band behaves nothing like one making 300 hp at 4,000 with 400 lb-ft from idle.
What matters for driving is the shape — how much torque is available across the range you actually use. A flat, broad curve feels far stronger than a peaky one with the same maximum.
Turbocharged engines changed expectations here, because boost can hold torque nearly flat from low rpm, which is why a modern 2.0 turbo out-drives an older 3.0 naturally aspirated engine despite similar peaks.
Crank, wheel and the correction factors
Manufacturer figures are at the crankshaft. A chassis dyno measures at the wheels, after the drivetrain has taken its cut — typically 10 to 15 percent for a manual, more for an automatic or four wheel drive.
Dynos also apply correction factors for air temperature, pressure and humidity, and different standards correct differently. The same car can read several percent apart on two dynos on the same day.
So compare runs on the same dyno on the same day, and treat absolute numbers from different sources as roughly comparable at best.