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Excursion: HP and Torque Different?

1385 Views 6 Replies 6 Participants Last post by  Bernhardt

Check it out. Go to details. Then Engines and Fuel Economy. It says the 5.4 Liter Triton V8 for it puts out 255hp and 350lb-ft torque. The V10 puts out 310hp and 425lb-ft torque. And the Diesel V8 puts out 235hp and 500lb-ft torque.
How come the 5.4 Excursion has 5 less hp, but 5 more lb-ft torque? And the v10 has 10 more hp and 15lb-ft torque more? And the diesel is the same.

1999 Ford F150 XLT ,Cloth Seats, CD Player, 4.6 Liter Triton V8, 3:55 LS, Towing Package, 4 Speed Auto, K&N Airfilter, and soon Superchip and K&N generation II
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Engines can be tuned to produce more or less power depending on several factors.
The question on HP vs torque. Remember that HP is a mathematical oproduct of rpm X torque at that speed divided by a constant. If you hold torque constant and increase rpm, HP goes up. Gas engines have the ability to turn faster than diesels hence they will show higher HP even though their torque is less. Some gas engines with very high specific outputs ( a couple hundres HP per liter) develop this power only at a very narrow band of engine speed. Let the revs drop too low and there is very little power, but keep it in the power band and they run well. Diesels, on the other hand, dont't care; the power is all there from idle on up to the top of their ( somewhat limited) speed. Diesels are great for use with auto trans as they ( AT) require substantial torque to accelerate well.
Here's the truth:

Torque: Force applied times the perpendicular distance from the point force application to the axis of rotation (i.e. socket = axis, wrench length = perp. distance)

Jerry applies 200 pounds of force to the end of a wrench one foot long. Torque at socket is equal to 200 lbf x ft (commonly written as 200 lb-ft)

Work (the physical definition) is force (lb, Newton, etc.) times distance applied (ft, meter, etc.), or torque (lb-ft, N-m) times angle of rotation (in radians, whose angle measure has no defined physical unit like length or mass or force, 360 degrees of rotation = (2 x pi) radians).
Work done is energy exchanged. Units of energy (and work) are lb-ft, BTU, joule, etc.

Jerry pushes with 200 pounds of force on a car that travels ten feet. Jerry therefore exerted 2000 lb-ft (2.57 BTU, 2712 Joules, 648 Calories) of energy.

Jerry applies a torque to a wrench equal to 200 ft-lbs and the wrench spins 573 degrees, or ten radians (about 1.6 revolutions). Therefore, Jerry exerts 2000 lb-ft (2.57 BTU, etc.) of energy.

You now know the definitions of torque and work. Power is simply the amount of work done divided by the time required.

Jerry exerts 2000 ft-lb of energy in 20 seconds. therefore, he is capable of producing 100 ft-lb/s of power. 1 horsepower is equal to 745.7 Watts, .707 BTU/s, and 550 ft-lb/s. Therefore, Jerry produces .182 horsepower.

Final formula:

HP=Tq x rpm x (1min/60sec) x (6.28 radians/1 revolution) x (1hp/(550 ft-lbf/s))

boiled down, HP=(Tq x rpm)/5255

Theoretically, power dictates how effectively one can tow (do some torque and final drive ratio calculations to find the torque applied to the wheels. Don't keep both motors in the same gear. Spin the gas motor at its power peak in a lower gear.). The only reason diesels have such a following is that they produce good power and live long at lower engine speeds. I know little about how rigorously gas motors are tested under maximum loading conditions (300 horses, 4000+ rpm, etc.). Engine life might be limited. It is pretty well known that diesels can hold their water in the engine life at max output category. my two lincolns.

J. Vekved
1984 F-250hd 4x4
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Ohhh my God. Not another physics lesson

Greg Heizer
[email protected]
96 F250 PSD XLT SC LWB 4x4 3.55
1997 26Ft Wilderness 8,000lbs plus a few logs for the fire
Sorry, just graduated from college w/ degree in Mech. Engineering. By this point the stuff is practically second nature. Surely time will rob me of this expensive knowledge. It's a phase. It'll pass.

J. Vekved
1984 F-250 4x4 4-sp.
Good stuff Vek!

But ........ is Jerry STRONG or not! 0.187
hp doesn't seem like much?!
If that's Jerry's max output, then he's not that strong. If, however Jerry can maintain it for say an hour or two, then he's a stud. A typical man(not out of shape lard b*tt) can sustain about .1HP.
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