Is this true?

[karabiner98k]

BMW 320is E30 was made in 1980s. This car had a 2.0L engine with 192PS@ 6900 RPM & 210nm@ 4900 RPM. I read in several articles that this car develops 180nm of torque from 3500 - 6700 RPM. Here is one of them:
Italian M3

I also saw a torque curve diagram of it in a German magazine. Here is the torque curve of this engine (S14B20):
http://commondatastorage.googleapis....l/36364439.jpg

According to the relationship between horsepower and torque, this engine which develops 192PS@ 6900 RPM should have 198nm@ 6900 RPM too. But according to that article and that diagram this engine has about 178nm@ 6900 RPM which means it should have 172PS@ 6900 RPM not 192PS!!!
How could this be true?? There is about 20nm difference.
Please help me with this problem.

[Sledgehammer]

This video may help you clear some things up.

[ame="http://www.youtube.com/watch?v=Pj2NOTanzWI"]http://www.youtube.com/watch?v=Pj2NOTanzWI[/ame]

[demonrunning07]

I think the problem is that you're confusing power and torque, two completely different measurements. A very very watered-down explanation of this difference is that torque is a measurement of how hard your engine can turn the car's wheels. That is, how much work can be achieved by turning the wheels. Power on the other hand adds an additional element of time. That is, how quickly a given amount of work can be achieved.

There are a lot of different factors that determine how much horsepower an engine generates in relation to the amount of torque. For example, diesel engines and turbocharged gasoline engines typically produce more torque in relation to power than do naturally aspired gasoline engines. Likewise, high-revving naturally aspired engines produce significantly more power in relation to torque, simply by virtue of the fact that they're turning faster (that's the element time). That's why power typically increases across the entire RPM band. Take F1 engines for example. They're small and produce only about as much torque as an Audi TTS but because of their prodigious RPMs they produce over 800 hp.

Here are the scientific explanations of power and torque:
http://en.wikipedia.org/wiki/Power_(physics)
[ame="http://en.wikipedia.org/wiki/Torque"]Torque - Wikipedia, the free encyclopedia[/ame]

I hope that helps answer your question.

[Matra et Alpine]

BRILLIANT find Sledgehammer - passed to my SO who is a math teacher ... let's see if she uses it

That is one fo the best parodies of the BBC output for schools programs from the 60s and 70s.
They really were that plummy voice and similarly stupid examples.
A great excuse for the teacher to skive off for 30 mins and for us to catch up with gossip while it was playing

[henk4]

I had a feeling of Deja Vu....

[Matra et Alpine]

^^^ did your schools "steal" our broadcasts as well as all you at home watching the BBC transmission but not paying the fees ??

[henk4]

^^^ did your schools "steal" our broadcasts as well as all you at home watching the BBC transmission but not paying the fees ??

no, it was just so Pythonesque....

[pimento]

There's 9 episodes in total in series 1 of Look Around You (8 + pilot). Definately worth tracking down. Series two is totally different and less good. It's mostly Peter Serafinowicz and Robert Popper I think.

[karabiner98k]

I think the problem is that you're confusing power and torque, two completely different measurements. A very very watered-down explanation of this difference is that torque is a measurement of how hard your engine can turn the car's wheels. That is, how much work can be achieved by turning the wheels. Power on the other hand adds an additional element of time. That is, how quickly a given amount of work can be achieved.

There are a lot of different factors that determine how much horsepower an engine generates in relation to the amount of torque. For example, diesel engines and turbocharged gasoline engines typically produce more torque in relation to power than do naturally aspired gasoline engines. Likewise, high-revving naturally aspired engines produce significantly more power in relation to torque, simply by virtue of the fact that they're turning faster (that's the element time). That's why power typically increases across the entire RPM band. Take F1 engines for example. They're small and produce only about as much torque as an Audi TTS but because of their prodigious RPMs they produce over 800 hp.

This is one thing i don't understand at all.

Here are the scientific explanations of power and torque:
http://en.wikipedia.org/wiki/Power_(physics)
Torque - Wikipedia, the free encyclopedia

I hope that helps answer your question.

Hello, my friend.
I didn't confuse power and torque. I know everything you said about them. My problem is very simple:

Here is the relationship between torque and power.
Power (hp) = Torque (lb-ft) X RPM / 5252

Now look here:


320is E30 has 192PS@6900 RPM.

My problem is that according to the above formula when you calculate torque at 6900 RPM, the result is 198nm@ 6900 RPM (assuming this engine has 192PS).
It means that this engine shound produce 198nm of torque @6900 RPM to be able to achieve 192PS @ 6900 RPM but when i look at the diagram it has about 178nm @ 6900 RPM. According to the formula by 178nm@6900 RPM you can only achieve 172PS.
How could this engine produce 192PS@ 6900 RPM while it has only 178nm@ 6900 RPM??

This is one thing i don't understand at all.

[Matra et Alpine]

Are you doing the conversions along the way and getting them right ?
PS-> hp and lb/ft -> nm and back ?
The graphs also looks "created by man" rather than measured by machine and so they may have introduced errors and simplifications

[demonrunning07]

units, your problem is units.
I've never seen that equation but from my limited experience with physics I can tell you that theoretically it cannot work unless you're using the same units across the entire equation. If the torque figures you're using are in Newton meters (Nm) and your'e using RPMs (# of rotations per 1 minute) to express the rate at which the engine is turning, then you would have to use the units Nm/min (Newton meters per 1 minute) to express power. If you were to convert the rotations per minute into rotations per second, then the equation would yield a power figure in Nm/s which is the same as a Watt. Dividing that figure by 1000 will convert it into kilowatts (kW) which is also used in the specifications you included or can be easily converted into horsepower using a simple online unit converter.

That's what I know from solving physics problems. As for that equation, I have no idea how it's derived or what kind of results you can expect. But you might as well give it another try making sure you use correct units and see if that will help it to better align with the factory's graph of the torque/power curves.

Again, here's what the equation should look like:
Power (kW)=[Torque(Nm)xRPS]/1000

I don't see where dividing by the constant 5252 would come into play. Maybe try it first without this step and if your answer seems off by about 4 orders of magnitude then try dividing by 5252.
Hope that all helps.

[Matra et Alpine]

demon the division by 5252 is THE "fudge factor" because HP was a created unit of work of one horse. So the ratio between HP and lb/ft has to have that in to work. AND that's based on a rounded figure of 33,000 ft·lbf/min not the real amount.

Further evidence of the insanity of any continued use of imperial measurements

[demonrunning07]

yeah, I figured the 5252 had something to do with unit conversion and was probably linked to having used the English system. That being noted, it looks like he may have also tried to cross English and Metric by using Nm for torque and PS for power. Another big no-no.

Any idea where I can read a little more about this "golden ratio" between power and torque?

[Matra et Alpine]

wiki but best of all any engineering depts studies/reports.
HowStuffWorks is good high school intro stuff

Read up on Watt and the original definition of HP and then the "golden ratio" 5252 comes clear. ( it's all dependant on the 33K ft-lbf/min )
The ratio itself comes from the fact that power is work over time. Torque is the force to deliver that power.

[demonrunning07]

I knew the last bit from Gen Physics I, just didn't that there was a formula that relates torque to power at a given RPM.