Wow, I just have to jump in here.
First, we need to define some terms so we're all talking the same language. I know this is booooooring but stick with me here. When we're done, you'll only have to remember one simple formula.
Work
Work is done by moving a force through a distance.
W = F x d
If you raise a 10 lb. weight from the floor to your chest, you've done about 40 ft-lb of work.
W = 10 lb x 4 ft = 40 ft-lb
Power:
Power is the rate of doing work or work per unit time. The faster you work, the more power it requires.
P = W/t
For example, if it takes 2 sec. to raise that 10 lb. weight 4 ft. to your chest, on the average, you've used 20 ft-lb/sec. of power.
P = 40 ft-lb/2 sec. = 20 ft-lb/sec
Horsepower:
One Horsepower is defined as 550 ft-lb of work per sec.
Hp = P/550
In the example, we expended an average of 0.036 Hp.
Hp = 20/550 = 0.036 Hp
Horsepower is unit dependent and only has meaning in the English system of units. Power in the Metric system is measured Kilowatts like electricity. 746 Watts = 1 Hp
But what about Torque:
Torque is a twisting motion or a force acting at the end of a lever arm like a torque wrench.
T = F x L
The correct units for torque are lb-ft to differentiate it from Work with units of ft-lb. Work and Torque are not interchangeable!
Engine Example:
Let's assume we have an engine turning at 3000 RPM and producing 300 lb-ft of torque. In this case, think of 300 lb-ft of torque as a 300 lb force acting at the end of a 1 ft lever arm. As the crankshaft rotates, the lever arm rotates 3000 turns in 1 min. For a 1 ft lever arm, the end travels about 6.28 ft. for each revolution (circumference = 2 x Pi x L) or about 18850 ft/min at 3000 rpm.
The Power generated is 300 lb x 18850 ft/min or 5,655,000 ft-lb/min. Using our Horsepower formula, this is about 171 Hp.
Remember this:
Once you have worked through all this, all you really need to remember is that for an engine:
Hp = RPM x Torque/5252.1132
(Hp = RPM x Torque/5250 is close enough.) Torque must be in lb-ft.
Fred Aldrich
www.GeoCities.com/~69_chevelle
[This message has been edited by Fred Aldrich (edited 05-27-99).]