Horsepower = torque x rpm / 5252

I need help figuring out how this works. Basically I don't know what values to use for the "rpm" part of the equation. :confused:

whatever rpm your motor is at

if your motor is at 5000 rpm and your torque is 200 ft/lbs, then your HP is 190.4

lets hope its higher than that

there are a bunch of posts about measuring torque and HP

Yeah I have heard more than one racing friend of mine ignore HP ratings and only rely on TQ. HP is just derived from TQ and rpm. Makes sense to me.

I'v been wondering... would HP figures remain the same at the wheels except for Torque (this excludes drivetrain losses)?

If that is true then what happens to the torque curve after gearing and tire size?

Like a 2:1 ratio... in a visual sense would a graph of an engines torque curve be compressed into half of the graphed RPM range of the engine then doubled?

kevin in the eclipse forum had alot of cool websites that explained it well. I tried to write some of it out but I made it more confusing so i'll just leave the links.
http://www.automotiveforums.com/vbulletin/showthread.php?t=210613
http://www.geocities.com/MotorCity/7177/torquehp.html

I hope those help!

The equation is useful for converting between horsepower and torque at any given RPM.

Although they are both directly related I wouldn’t say one is anymore important than the other. In fact more importantly is the power or torque curves versus the peaks. It is really the curves that give an idea of the engines potential.

Torque can be directly multiplied through by the gearing to calculate torque at the wheels. Horsepower on the other hand is unaffected by gearing and remains the same at the wheels neglecting any losses. The graph below:

http://img23.exs.cx/img23/5382/Cols.jpg

shows that Evil Result was almost correct, the power curve (not torque curve) will be compressed or stretched horizontally depending on the gear ratios (X-Axis is RPMs).

Tire size effects the amount of force that the tire exerts on the ground. For a given torque at the wheels, increasing the overall diameter of tire will decrease the force exerted on the ground. While decreasing the overall diameter of the tire will increase the force exerted on the ground by the wheel.

I love graphs...

So... even though i know this, you can make any low TQ high HP engine preform as though it was a high TQ low HP engine.

The "actual" formula is power = torque*angular velocity, according to SI the units are then watt (power), newtonmeter (torque), rad/s (angular velocity). So when you divide by 5252 that's just to convert the units.

Normally, when you measure the power from an engine you measure the torque and then calculate the power from the torque. The power will of course be as correct as the torque, any errors done when the torque was measured will also affect the power with the same amount.

If you have power you can always get torque, so from that point of view torque is "worthless", lower torque and higher engine speed will do the trick just as fine. Only the torque curve will affect the performance of the car, on a given gear the hardest acceleration will be found at torque peak, and with a wider torque curve the higher will the mean output from the engine become. However, when it comes to engines you can't eat the cake and still have it, so if you chose a wider torque curve you will most certainly lose peak power, and for a high performance car peak power is most important since you rarely drive on very low revs.

It is interesting to note that for a fixed gear vehicle the maximum acceleration in any gear occurs at peak engine torque. However, the maximum acceleration for any speed occurs at peak power. So for a given speed adjusting the gear ratios to allow the engine to run at peak power will result in better acceleration than if the engine was running at the peak torque.

So the perfect transmission should have an infinite number of gears with instant gear changes. Allowing the vehicle to opearte at peak power at all speeds.

whatever rpm your motor is at

if your motor is at 5000 rpm and your torque is 200 ft/lbs, then your HP is 190.4

lets hope its higher than that

there are a bunch of posts about measuring torque and HP


I thought so. But the numbers I get from it are always way off. I have a dyno sheet showing 154.5 ft/lb and 215.5 bhp @ 8500 rpm yet I get 250 bhp when I use formula. Am I missing something?

154.5 ft/lbs at 8500 rpm = 253 hp
215.5 hp at 8500 = 131 ft/lbs

You're not using the peak value of the torque or values on the crank versus on the wheels I hope?