

Horsepower vs Torque
Name: Chris G.
Status: N/A
Age: 20s
Location: N/A
Country: N/A
Date: 20002001
Question:
I was wondering if these formulas to calculate horsepower
are correct:
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
my tacometer?
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.
Replies:
Chris,
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 footpounds or as
Newtonmeters. As you made no specification, I will assume you automobile
secifications use footpounds.
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
footpounds multiplied by radians/second. It is a special property of
radians that allows this product to be footpounds/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 (footpounds)x(rpm).
1 hp = 550 ftlbs/sec = 550 (ftlbs)x(rad/sec)
1 rad/sec = 60 rad/min
= 33,000 (ftlbs)x(rad/min)
1 revolution=2(pi)radians
1 rpm = 2(pi) rad/min
1 hp = 5252 (ftlbs)(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

