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jamesnj 09-19-2002 03:28 AM

Torque/HP
 
Will someone please explain the difference between torque and hp? I know the difference generally but can't say I understand it. Is torque basically when the guy puts his foot down how fast the car moves of the line? Is horsepower the ability of the engine to get the car up to "top" speed?

Also need to know the difference between hp and bhp.

Thanks

jcantor 09-19-2002 07:14 AM

James,

I'll give it a shot although my automotive engineering classes were over 10 years ago so I'm probably a bit rusty.

Torque is rotational force or twisting. Measured in Ft-Lbs or N-m, it is an indication of the amount of force that the engine is able to generate and transmit to the wheels. In this case, the units of measurement are actually quite helpful in envisioning what torque is. Say you have a car that is rated at 200 Ft-Lbs. This means that at a distance of 1 ft from the center of a rotating shaft, it can provide 200 lbs of force. If you assume that the radius of the wheel and tire on your car is approximately 1 ft (very approximately), then you have 200 lbs of force being applied by the tire to the pavement. It is torque that pins you to your seat when the light turns green. :D

Horsepower is the engine's ability to produce torque over time - torque x rpm (there's also a conversion factor here to make the units work out but I don't recall what it is) and is most important in determining the car's top speed.

There's an old saying 'Torque wins races; Horsepower sells cars'. While this isn't true of all types of racing, it is definately true at the drag strip. Most important, is actually the shape of the engine's torque curve when plotted against engine speed - the flatter and more even it is, the better the car will 'pull' from all speeds and in all gears.

You also asked about the difference between hp and bhp. BHP stands for brake horsepower and I believe is a term that goes back to the original methods of measuring horsepower through the use of a hydraulic brake mounted on the engine output shaft. It is a measure of the engine's output only and does not account for losses elsewhere in the drive train. Net horsepower, which has been used here in the US at least since 1972, is measured at the drive wheels and is a much more realistic measure of the total performance of the car. HP is a general abreviation for horsepower and may refer to net or brake horsepower depending on the writer.

I hope this helps. Anyone who has additions or corrections, please chime in. As I said, it's been awhile since I dealt with this stuff regularly.

jlc

PS - James, do I gather correctly you are from NJ?

Lebenz 09-19-2002 10:27 AM

Horse power sells cars but torque moves them

Ali Al-Chalabi 09-19-2002 11:28 AM

Horsepower is the power that the engine develops, at a given RPM it is the only number that will tell you how fast the car will accelerate or top out at.

There are two components to horsepower, RPM and torque. Torque is one of the two factors that determine how much power an engine develops.

If you know torque and RPM, you can find horsepower, and vice versa.

The formula is: Horsepower = (Torque x RPM) /5250

For example, my C280 puts out 199 ft/lbs of torque at 3750 rpms. From the formula, we can calculate that the horsepower at this rpm is 142 hp.

Also note from the formula that for every engine in the world, torque and horsepower are always equal at 5250 rpms.

If one car puts out 400 ft/lbs of torque at 2000 rpms and another puts out 200ft/lbs of torque at 4000 rpms, there is the same exact power being geneated. Both engines will be identical in performance.

Don't let anyone tell you that their car is faster simply because it has more torque, it may be faster but that is only if has more average horsepower throughout the engine's useful range. If torque was the only thing that moved a car, then you wold accelerate harder at 3,000-4,000 rpms than at higher rpms, and we know that this is not the case.

Glen 09-19-2002 12:19 PM

Take a look at this:
http://www.datsuns.com/torquehp.htm

bobbyv 09-19-2002 01:07 PM

power measured in a straight line = (force x speed). Hence 1 HP = 550lbs x 1ft/sec = 550ft-lbs/sec. In this case, the "ft-lbs" in the numerator is actually "work", where (work=force x displacement)

torque is the rotational equivalent of force. As mentioned earlier, it is measured by the force (lbs) exerted along a given radius (ft).

the measure of torque in the English system is actually ft-lbs/radian but since radian is dimensionless, this simplifies to ft-lbs. To differentiate it from Work=ft-lbs, torque is quoted in "lb-ft".

rpm is the rotational equivalent of linear speed. Since power=(force x speed), rotational power = (torque x rpm).

BHP (brake horsepower) came from the output measured on a "brake dynamometer".

the engine torque is transformed by the drivetrain and tires into linear force onto the road surface, as the vehicle goes down the road. Along with this transformation, there are frictional losses

this linear force exerted by the driven tires then determines linear acceleration of the vehicle (force = mass x acceleration) - the higher the force (from the torque), the higher the acceleration.

with the vehicle at constant speed, the linear force exerted by the driven tires equals the total of the aerodynamic & frictional drag forces. The frictional drag increases about linearly with vehicle speed. The aerodynamic drag however, increases with the SQUARE of the vehicle speed.

since power = force x speed, the power required to defeat aerodynamic drag is then proportional to the CUBE of the speed. This means that you need EIGHT TIMES (i.e., 2 cubed) more power just to double your top speed.

the moral of this long-winded discussion is that torque is responsible for acceleration, and power is responsible for top speed. But the important thing is that you get diminishing returns from increases in power, since the bulk of it gets used for defeating aerodynamic drag, which increases dramatically with speed.

SteveH 09-19-2002 02:48 PM

Well, I've read all the replies and find this all very interesting but I've got something to throw in here.

Having owned a Chevy 4x4 I was told by fellow 4x4 enthusiasts to NOT go with the biggest - free flowing intake and exhaust I could find. I had planned on a throttle-body spacer, K&N air filter and 3" exhaust. Apparently this was a no-no. This was because while it would increase horsepower, it would put the torque on the higher RPM range instead of the low RPM range, which is where all the slower "rock crawling" power is needed. Needless to say I didn't do anything, and have a nice - stock - Blazer 4x4 (which is for sale by the way... www.geocities.com/stevehurwitz)

With that in mind, one needs to apparently choose whether you want off-the-line power or top speed or something in between. If you want to smoke your tires you aparently don't want the most free flowing intake and exhaust, while for top speed you would.

Or do I have this completely wrong?

Benzman500 09-19-2002 04:21 PM

you guys are all wrong just ask any ricer ;) that 100hp intake will show you;) ;)

Her is a question for everyone how do you preffer a car? all hp or torquey? Me I like the torque of my BMW's and the 500

Benzman500 09-19-2002 05:18 PM

O come on you see them doing 140 all the time... Ohh no wait their speedos don't even break 120 :)

TANK 09-24-2002 11:51 AM

Let me make it easy for you - real world
 
All micro-analysis aside, ever wonder what the difference is between a V8 in a mustang and a 4cyl like an s2000 that put out the same 250+-hp?

The concept of torque is easily recognizable here. I don't have exact numbers but that 4.6 liter V8 in the mustang puts out about roughly 300 or so ft pds of torque at say 3000 rpm. Whereas, the s2000 puts out maybe 200 or so and way out at a much higher rpm at that. Now, take both cars and drop automatic transmissions in them. Normally these cars would have a 0-60 within perhaps .05 secs of one another. The mustang's 0-60 time may slow by a couple of tenths +-. The s2000 0-60 time will slow by at least a second. Meaning the mustang will REALLY blow away the s2000 this time leaving more than just a .05 sec difference. The s2000 will eventually catch up to the stang and perhaps meet or match on top speed, but doesn't produce enough torque/grunt to pull it up to the high end of the pwer band quickly enough when it starts from the lowly idle of 500-1000 rpm.

Now put those same two cars on a steep hill and run them to 60 again. The mustang will pull away even more. Now add some weight/passengers to the cars and the s2000 will be reduced to lawn tractor speed while the mustang keeps clicking off reasonably good times. This is the real world difference between and application of torque and horsepower.

Not sure? Just take a look at any of the motor trend specs on something with a big v8 like a mustang, z28 or vette. You don't see much diff in 0-60 between auto and stick. On the old eclipse turbo, small v-6's, or even the old 300zx, the lack of torque shows greatly when you slap an automatic on one of them. The end up do 8 second 0-60's at best because they don't have any low end hard pulling power.
;)


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