Why does this happen on every single graph? Seems a little more advanced than tech.

 

(Torque x Engine speed) / 5,252 = Horsepower

 

This is palagerized from Musclecarclub.com. It's pretty good if you want the derivation of the magic number 5252:


"Torque is measured using a dynamometer. The torque generated is measured at different RPMs and the result is plotted on a graph. Then, horsepower is calculated by taking the torque at each RPM, and converting it using the following formula:


Horsepower = Torque X (RPM/5,252)


This formula is the result of combining several formulas into one. First, 1 horsepower is defined as 550 foot-pounds per second. The units of torque are pound-feet. So to get from torque to horsepower, you need the "per second" term. You get that by multiplying the torque by the engine speed. But engine speed is normally referred to in revolutions per minute (RPM). Since we want a "per second," we need to convert RPMs to "something per second." The seconds are easy -- just divide by 60 convert minutes to seconds.

Now what we need is a dimensionless unit for revolutions: a radian. A radian is actually a ratio of the length of an arc divided by the length of a radius, so the units of length cancel out and you're left with a dimensionless measure. You can think of a revolution as a measurement of an angle. One revolution is 360 degrees of a circle. Since the circumference of a circle is (2 x pi x radius), there are 2-pi radians in a revolution. To convert revolutions per minute to radians per second, you multiply RPM by (2-pi/60), which equals 0.10472 radians per second. This gives us the "per second" we need to calculate horsepower. We need to get to horsepower, which is 550 foot-pounds per second, using torque (pound-feet) and engine speed (RPM). If we divide the 550 foot-pounds by the 0.10472 radians per second (engine speed), we get 550/0.10472, which equals 5,252. So if you multiply torque (in pound-feet) by engine speed (in RPM) and divide the product by 5,252, RPM is converted to "radians per second" and you can get from torque to horsepower -- from "pound-feet" to "foot-pounds per second."

Of course if you use SI Metric units of Newton meters and kilowatts the curves don't cross. The engine doesn't know how you measure it, nor does it care.

 

Thanks! That's the clearest derivation of horsepower that I have ever read.

Rich Krause