YACT: Torque measurement for a car?

[RaynorWolfcastle]

I come from a pretty theoretical background and I'm trying to figure how they measure torque for a car engine. Anyone that knows a bit of a physics background can tell you that torque = f x r (note that's f cross r) where f is the force applied and r is the radius (distance from centre of rotation actually).

With these two variables I can't figure out how they can make a standard, comparable, torque measurement for all cars. Do they take r as being half the wheelbase independent of that distance? Someone please enlighten me.

 

[Thegonagle]

They measure the torque of the engine, not the entire car.

I don't know what the car's wheelbase has to do with it.

 

[LunarRay]

Horsepower = Torque * RPM / 5252

Watt, the engineer sat under a tree and an apple fell on his head and he wondered .... if a horse pulled that up... and so on

 

[Ronstang]

HJD1's equation is correct. But in truth torque is the measured force and horsepower is derived from it, as the equation implies. But, if you have accurate HP stats then you can figure out torque at the given points you have HP data for otherwise the only good way to get accutate torque readings is to measure it on a chassi dyno.

 

[Ronstang]

HJD1's equation is correct. But in truth torque is the measured force and horsepower is derived from it, as the equation implies. But, if you have accurate HP stats then you can figure out torque at the given points you have HP data for otherwise the only good way to get accutate torque readings is to measure it on a chassi dyno.

 

[boggsie]

Originally posted by: Ronstang
HJD1's equation is correct. But in truth torque is the measured force and horsepower is derived from it, as the equation implies. But, if you have accurate HP stats then you can figure out torque at the given points you have HP data for otherwise the only good way to get accutate torque readings is to measure it on a chassi dyno.

... and that gives you torque at the wheel. Most cars are sold/advertised using horsepower (computed from torque) values measured at the crankshaft, not the rear-wheel.

 

[WinkOsmosis]

I want to know how it's measured also.. not like you can add resistance until the engine stops spinning...

What does wheelbase have to do with anything????

 

[LunarRay]

For purposes of this discussion, we need to measure units of force from rotating objects such as crankshafts, so we'll use terms which define a *twisting* force, such as foot pounds of torque. A foot pound of torque is the twisting force necessary to support a one pound weight on a weightless horizontal bar, one foot from the fulcrum.

 

[LunarRay]

By the way I'm an accountant... not an engineer... I had this laying around from somewhere.... I didn't want you to think it resided in my brain... I teach accounting and work counting beans not torques... what ever they are...

 

[Marshallj]

Originally posted by: RaynorWolfcastle
I come from a pretty theoretical background and I'm trying to figure how they measure torque for a car engine. Anyone that knows a bit of a physics background can tell you that torque = f x r (note that's f cross r) where f is the force applied and r is the radius (distance from centre of rotation actually).

With these two variables I can't figure out how they can make a standard, comparable, torque measurement for all cars. Do they take r as being half the wheelbase independent of that distance? Someone please enlighten me.

I have no idea what you're talking about.

They already DO have a standard, comparable torque measurement for all cars. Trucks too. They use foot pounds. "Pounds" is the force, and "foot" is the distance, 1 foot.

 

[Ronstang]

Originally posted by: boggsie

Originally posted by: Ronstang
HJD1's equation is correct. But in truth torque is the measured force and horsepower is derived from it, as the equation implies. But, if you have accurate HP stats then you can figure out torque at the given points you have HP data for otherwise the only good way to get accutate torque readings is to measure it on a chassi dyno.

... and that gives you torque at the wheel. Most cars are sold/advertised using horsepower (computed from torque) values measured at the crankshaft, not the rear-wheel.

That's all nice and everything but torque at the crankshaft is a worhtless measurement for anyone serious about comparing vehicles.....it means very little in the real world. To truly compare two vehicles you need torque at the wheels from a chassis dyno so drivetrain efficiency/gearing can be taken into account. If you want to sit around bench racing and being a magazine mechanic go ahead but at the end of the day you are just guessing.....reported torque/HP stats reported by manufacturers are a crapshoot at best as some underreport and others exaggerate.

 

[RaynorWolfcastle]

My bad on the wheelbase stuff, I thought that the torque was relative the car's wheel base (ever see a RWD car pop a weelie?). I am an EE student not a mech student (puts head down in shame)

And I know what a foot pound is, although here in the real world we use Newton meters .

Ronstag, what is a chassi(s?) dyno? I'm not familiar with this piece of equipment.

 

[Marshallj]

Originally posted by: WinkOsmosis
I want to know how it's measured also.. not like you can add resistance until the engine stops spinning...

What does wheelbase have to do with anything????

They measure it with a dynomometer, which is hooked up to the engine's crankshaft. For people who want to measure their car's power, they use a chassis dynomometer such as a Dynojet, which uses a large drum that your car's wheels go on, and it's able to measure the torque by seeing how fast you can accelerate the drum which has a known weight (I think a few thousand lbs).

 

[mAdD INDIAN]

I think he wants to know how they calculate it.

My guess is you take 'r' to be the stroke of the piston and 'F' to be the force that the piston exerts on the crankshaft. However I figure you'd need to take into account more variables, but that is the simplied form.

 

[Cyberian]

Originally posted by: boggsie

... and that gives you torque at the wheel. Most cars are sold/advertised using horsepower (computed from torque) values measured at the crankshaft, not the rear-wheel.

Yeah, we used to call that 'brochure horsepower'.

 

[Marshallj]

Originally posted by: Ronstang

That's all nice and everything but torque at the crankshaft is a worhtless measurement for anyone serious about comparing vehicles.....it means very little in the real world. To truly compare two vehicles you need torque at the wheels from a chassis dyno so drivetrain efficiency/gearing can be taken into account. If you want to sit around bench racing and being a magazine mechanic go ahead but at the end of the day you are just guessing.....reported torque/HP stats reported by manufacturers are a crapshoot at best as some underreport and others exaggerate.

Gearing shouldn't have anything to do with it. You usually choose a gear which has 1:1 gear (usually 4th gear on a manual, 3rd gear for an auto) so that gearing is not a factor. And if you have different gearing, you input it into the computer and the dyno software will compensate for the gearing. That way it's measuring your real output and not getting false results.

I've seen it on auto tranny cars where they put in the gear ratio the transmission is using for the dyno pull, and by accident the tranny downshifts and the results are all skewed. I saw a Lightning dynoing a few months ago, and his real reading was arounf 500 hp, 550 lbs of torque. But on his other pull his transmission kicked down, and the dyno sofware showed a spike past 1,000 lbs of torque. That's because the software is assuming he's in 3rd gear, but when the truck downshifted into first or second, it multiplied the torque at the wheels and the dyno sofware still assumed that he was in 3rd.

 

[Marshallj]

 

[squidman]

Usually torque is measured on a stand with rollers, using N*m units (Newton times metres). The stand would show at what RPM (1st gear of course) the car attains max torque. Try to locate a Bsoch horsepower/torque stand, if you're curious.

 

[RaynorWolfcastle]

Originally posted by: Marshallj

Originally posted by: mAdD INDIAN
I think he wants to know how they calculate it.

My guess is you take 'r' to be the stroke of the piston and 'F' to be the force that the piston exerts on the crankshaft. However I figure you'd need to take into account more variables, but that is the simplied form.

He wanted to know why they don't have a standard measurement for all cars. But little did the poster know, they DO have a standard measurement for all cars.

foot pounds and Newton meters are UNITS. If I have a wheel with a 12 ft diameter and I apply 1/12th of a pound of force perpendicular to the radius, I am exerting 1 foot-pound. Similarly if I apply 12 pounds of force at 1 inch distance on the same wheel I am still exerting 1 ft-lb of torque.

My question is about how you standardize the measurement. Where do you measure the force being applied by the engine? Is it the same for all engines or does it depend on each engine's construction and specs? That's what I'm asking.

 

[Marshallj]

Originally posted by: RaynorWolfcastle

My question is about how you standardize the measurement. Where do you measure the force being applied by the engine? Is it the same for all engines or does it depend on each engine's construction and specs? That's what I'm asking.

Manufacturers all use the same measurement, taken at the crankshaft by a brake dyno. That's why you often hear the term "brake horsepower" or "bhp" quoted.

The idea that you seem to have is that they measure a force that's a certain distance from center, but that's archaic. They don't directly measure the force that way any more. Nowadays they just measure the effects of that force and interpolate the measurements.

 

[Ronstang]

Originally posted by: Marshallj

Originally posted by: Ronstang

That's all nice and everything but torque at the crankshaft is a worhtless measurement for anyone serious about comparing vehicles.....it means very little in the real world. To truly compare two vehicles you need torque at the wheels from a chassis dyno so drivetrain efficiency/gearing can be taken into account. If you want to sit around bench racing and being a magazine mechanic go ahead but at the end of the day you are just guessing.....reported torque/HP stats reported by manufacturers are a crapshoot at best as some underreport and others exaggerate.

Gearing shouldn't have anything to do with it. You usually choose a gear which has 1:1 gear (usually 4th gear on a manual, 3rd gear for an auto) so that gearing is not a factor. And if you have different gearing, you input it into the computer and the dyno software will compensate for the gearing. That way it's measuring your real output and not getting false results.

I've seen it on auto tranny cars where they put in the gear ratio the transmission is using for the dyno pull, and by accident the tranny downshifts and the results are all skewed. I saw a Lightning dynoing a few months ago, and his real reading was arounf 500 hp, 550 lbs of torque. But on his other pull his transmission kicked down, and the dyno sofware showed a spike past 1,000 lbs of torque. That's because the software is assuming he's in 3rd gear, but when the truck downshifted into first or second, it multiplied the torque at the wheels and the dyno sofware still assumed that he was in 3rd.

Gearing does have an affect, rearend gearing that is. I understand that chassis dyno pulls are run in a 1:1 gear, usually 4th on manual trannies and 3rd on autos, and the rearend gear is also entered into the computer for compensation. But that does not mean the rearend gearing has no affect. If the gearing is improper for the engine then it will not accelerate the drum as easily as the same car with the proper gearing so the results will differ. If the same car is dynoed with no changes other than the rear end gears the torque curves will differ also, by how much depends upon how much the change was and how far from optimal the gearing is with either set of gears. Try it.....I have and it teaches you a lot.

 

[Marshallj]

Originally posted by: Ronstang
Gearing does have an affect, rearend gearing that is. I understand that chassis dyno pulls are run in a 1:1 gear, usually 4th on manual trannies and 3rd on autos, and the rearend gear is also entered into the computer for compensation. But that does not mean the rearend gearing has no affect. If the gearing is improper for the engine then it will not accelerate the drum as easily as the same car with the proper gearing so the results will differ. If the same car is dynoed with no changes other than the rear end gears the torque curves will differ also, by how much depends upon how much the change was and how far from optimal the gearing is with either set of gears. Try it.....I have and it teaches you a lot.

If you correctly input what gear ratio your drivetrain has (including the rear), the dyno software should be able to compensate for it. I'd imagine if someone wants to pull a fast one and uses a lower ratio rear and doesn't tell the dyno operator, the results will be skewed since the software will assume it's using higher gearing.

Really it's just using a mathematical formula. If you use lower gears, it will multiply the torque at the rear wheels. Therefore the software much know this so it can divide that measured torque by the gearing advantage to figure out the real torque at the wheels.

I've seen the same car using the same engine dyno'd using 2 different rear ends. One used the TT rear end, which is a 9.05" rear with a 3.69 ratio, and the other using a non-turbo rear, which is a 7.87" rear with a 4.10 ratio. The dyno'd come out pretty much the same.