The relationship between horsepower and torque is an intriguing and complicated one. In the words of Carroll Shelby “Horsepower sells cars, torque wins races.” And that’s accurate, to an extent, but horsepower and torque do work together. It’s the same for your car as much as your rival’s, but what is the significance of the cross, and where does this number come from? Jason Fenske of Engineering Explained breaks it down in a pretty comprehensive way.
Turning to his whiteboard, Fenske teaches us about power, torque, and velocity. Power is equal to force times velocity. Velocity is any given distance divided by the time it takes to cover that distance, and torque is figured by a force multiplied over a given radius value. Have we lost you yet? Just think about it for a minute, it will make sense.
You need to understand the basics of these factors before you can figure out what the significance of the cross is. The 5,252 rpm cross boils down to the math behind horsepower and torque, and also a bit of history. Scottish engineer, James Watt, developed the concept of horsepower. One metric horsepower is needed to lift 75 kilograms by one meter in one second. It’s, as you may have guessed, related to how much work a horse can do, converted to how much work can be done by a steam engine. Today, we kind of just use it to make other engine builders feel bad about their cars…or vice versa.
So, back to the point, 5,252 rpm and why horsepower and torque are mysteriously intertwined. One horsepower is equal to 33,000 foot-pounds of work per minute. Adding in the equation for torque and velocity, you’ll start to see that horsepower will always equal torque multiple by RPM, and divided by 5,252.
If you cancel out the equal variables, you’ll have horsepower and torque equaling out at 5,252 rpm. Simple enough, right? Yeah, not really for us either, but if you watch the video, you’ll have a better understanding of it all.