Horsepower vs Torque
Name: Chris G.
I was wondering if these formulas to calculate horsepower
1hp = 550 ft*lbs, takes 1 hp to move 550lb 1 foot per 1 second..
hp = torque x rpm / 5252 ?
torque = hp x 5252 / rpm ?
Where do they (who ever came up with the formula (Watt?) ) get the # 5252?
Do I locate the revolutions per minute # from my vehicle specs or is it on
Basically I am just trying to calculate my '93 Ford Thuderbird's
horsepower (v6 3.8 engine).. i know the torque (between 35 and 45 on this
particular engine))but I'm not sure if I am getting the correct RPM.. and
if 5252 is what I am suppose to divide by.
One horsepower is an estimate of the power a standard workhorse can exert:
550 ft.lbs/sec. Before applying any formula, we must first identify the
units of torque on you engine. Torque may be listed as foot-pounds or as
Newton-meters. As you made no specification, I will assume you automobile
secifications use foot-pounds.
The power exerted by a rotating object is the torque it exerts multiplied by
the speed at which it rotates. In standard English units, this would be
foot-pounds multiplied by radians/second. It is a special property of
radians that allows this product to be foot-pounds/second: a radian is a
distance around an arc divided by the length of the radius (feet per foot).
We start with 1 horsepower. We want to get to (foot-pounds)x(rpm).
1 hp = 550 ft-lbs/sec = 550 (ft-lbs)x(rad/sec)
1 rad/sec = 60 rad/min
= 33,000 (ft-lbs)x(rad/min)
1 rpm = 2(pi) rad/min
1 hp = 5252 (ft-lbs)(rpm)
As for source of rpms, that varies from moment to moment. The number of
rpms will probably be greatest in the lowest gears. When rpms get too
great, a vehicle is usually shifted to a higher gear and a lower rpm for the
motor. The torque tends to be greater in lower gears, when the car is
trying to speed up. Once at cruising speed, all the engine needs to do is
keep the car moving.
Look at the greatest rpm listed on the scale of your tachometer. Use this
as a reasonable maximum. Multiply this by your engine's greatest torque.
This is an estimate of your vehicle's maximum horsepower. Actual value can
vary with speed, with how well oiled the car is, even with humidity.
Dr. Ken Mellendorf
Click here to return to the Physics Archives
Update: June 2012