The Hemi in the new Chrysler 300C gets 5w20!

[XS650]

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Originally posted by crashz: Torque is a rotating force. HP is a unit measure of work. Work is force x distance / time. So 69 is correct.

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Nope. Torque is a moment, a moment can be correctly expresseed as lb-ft in US customary units. Work in US customary units can be correctly expressed as ft-lb. lb-ft are not the same units as ft-lb. This is one more example of the screwed up US customary system of units resulting in people not understanding what they are doing. If people would use the correct US customary units for torque, lb-ft, then they would be less likely to make this common mistake. If you think about it bit, it becomes obvious that work and torque are not interchangeable. If you apply 100 lb-ft of torque to a lever and it dopesn't move, how much work have you done? You have dome zero work because there was no motion. If you apply 100 lb ft of torque to a lever and rotate it through one full revolution, you have obviously done 628.32 ft lbs of work. Torque and work are not the same thing at all. [ April 15, 2004, 12:58 PM: Message edited by: XS650 ]

 

[AV8R]

Sorry fellows, thought you all spoke engineering shorthand...for instance: when 69 implied "hp=torque x time" he got the definition backwards, resulting in a quantity that doesn't make sense physically; I wrote "hp x time=torque, which has the correct dimensions. The constant in the handbook formulas relating horsepower, speed, and torque arises because of the particular system of measurement used, and the constant will be different for each system of measurement, e.g. metric SI, British gravitational system, the inch,pound-force,second system used here in the USA and in Britain commonly, or cgs, electrostatic, etc. will each have their associated constants. In the British common system, the HORSEPOWER is by definition 550 ft lb of work being done each second. In the SI system, the unit for POWER is the WATT, one newton meter, or joule, of work done each second. The physical definition of POWER (horse, dog, or whatever) is: POWER = WORK per UNIT TIME. The dimensions are (force x distance)/time 69 said that POWER = TORQUE x TIME The dimensions are (force x distance) x time Get it?

 

[XS650]

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Originally posted by AV8R: Sorry fellows, thought you all spoke engineering shorthand...for instance: when 69 implied "hp=torque x time" he got the definition backwards, resulting in a quantity that doesn't make sense physically; I wrote "hp x time=torque, which has the correct dimensions.

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No it doesn't lb-ft torque is not the same as ft-lb work hp x time = work so if hp x time = torque like you say, then work would equal torque which is not true. Or to show the problem with your equation another way. If you have 150 hp for 2 seconds. How much torque is that? [ April 16, 2004, 03:38 AM: Message edited by: XS650 ]

 

[69 Riv GS]

Yes; hp = torque x time is wrong, because torque x time is still just torque, & hp is not = to torque. we know: torque = force x radius so: force = torque / radius power = force x distance per minute then: hp =(torque / radius) x (rpm x radius x2xpi)/33000 reduces to: hp = torque x rpm / 5252 and: torque = hp x 5252 / rpm Note: at 5252 rpm torque & hp values are = . At any rpm below 5252, the value of torque is greater than the value of hp. Above 5252 rpm, the value of torque is less than the value of hp. [ April 16, 2004, 08:49 AM: Message edited by: 69 Riv GS ]

 

[XS650]

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Originally posted by 69 Riv GS: : hp =(torque / radius) x (rpm x radius x2xpi)/33000 reduces to: hp = torque x rpm / 5252 and: torque = hp x 5252 / rpm Note: at 5252 rpm torque & hp values are = . At any rpm below 5252, the value of torque is greater than the value of hp. Above 5252 rpm, the value of torque is less than the value of hp.

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Nothing personal, but I'm going to save your post to use as an example of how the use of US customary units in US schools muddles peoples thinking.

 

[69 Riv GS]

Torque(lb-ft) & hp(ft-lb) are different units of measure. That doesn't mean their values can't be equal; under certain conditions. help me understand. [ April 16, 2004, 11:40 PM: Message edited by: 69 Riv GS ]

 

[XS650]

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Originally posted by 69 Riv GS: Torque(lb-ft) & hp(ft-lb) are different units of measure. That doesn't mean their values can't be equal; under certain conditions. help me understand.

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Units of HP in Customary US units are ft-lb/sec. 1 lb foot of torque is the equivilant of a force of 1 lb applied to a one foot lever arm. When you make one complete revolution of that lever arm while applying 1 lb to the end of it, you will have done 2 x PI x 1ft x 1 lb = 6.2832 ft-lb of work because you will have pushed 1 lb 6.2832 feet. One hp = 33,000 ft-lb/min of work (33,000 ft-lb/min)/(6.2832 ft-lb/rev)= 5252.1 rev/min The magical RPM where torque is numerically = hp....if you are using the Customary US system and using feet and minutes instead of other US units. If you carry the units through your calculations, it helps make things clearer. It eliminates alot of mistakes too. Something metric system users don't have to worry about as long as they can keep track of a decimal point. It's not the extra work of the US customary system that bothers me, its the way it obstructs understanding what you are doing when you calculate something.