Torque VS BHP
13 June 2002, 05:14 PM
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Embarassingly my wife asked me to explain this to her last night. I tried but could not really get the message over. I thought I understood.
Could someone "in the know" please explain in 3/4 lines the difference so that I can salvage some respect in my own home.
Following this, the offside rule, the meaning of life and beer goggles.
No, seriously, what exactly is bhp and torque and the difference.
ALso, on another note, what is a PLC, can anyone define one of these for me?
Could someone "in the know" please explain in 3/4 lines the difference so that I can salvage some respect in my own home.
Following this, the offside rule, the meaning of life and beer goggles.
No, seriously, what exactly is bhp and torque and the difference.
ALso, on another note, what is a PLC, can anyone define one of these for me?
13 June 2002, 07:00 PM
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I asked the same question a few months ago - pitty search is fecked - hint 
I think the conclusion was that they are both a different way of measuring the same thing (what the engine is producing). I remember someone say that they are linked in that (ignoring gearing, etc) if a engine produced 100lbft torque at 2500rpm it would generate the same power at that point as an engine that produced just 50lbft torque at 5000rpm. That is why oil burners have lots of torque low down but poor power outputs and why more powerful cars can produce so much power but with less torque (the key is that it's higher in the rev range).
This is second hand info so I stand to be corrected or amended
I think the conclusion was that they are both a different way of measuring the same thing (what the engine is producing). I remember someone say that they are linked in that (ignoring gearing, etc) if a engine produced 100lbft torque at 2500rpm it would generate the same power at that point as an engine that produced just 50lbft torque at 5000rpm. That is why oil burners have lots of torque low down but poor power outputs and why more powerful cars can produce so much power but with less torque (the key is that it's higher in the rev range).
This is second hand info so I stand to be corrected or amended
13 June 2002, 07:23 PM
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Failing that try the following link, http://www.seatcupra.net/power.shtm . An idiots guide to power and torque! I needed it when I had to explain to someone else, and seeing as I have the technical understanding of a semi-retarded hamster, I found it useful. 
13 June 2002, 09:00 PM
#5
Torque * RPM = Power in BHP
------------
5250
Power is overall work, ultimately you need power to move, as any amount of torque will be no good without some rate of application (rpm in our case).
When you see the following written:
"The great thing is engine A has 100ftlb at only half the speed of engine B".
Except for acceleration it isn't as good as it seems, as if you have to run engine B at double the spead to get the same torque, you can run a gear that's half as long, meaning at the same road speed you will have double the torque of engine A (well increase drive losses will rob some of that).
A human is good for about 1HP, with a suitably long lever, could generate 1000ftlb of torque easily, but a 300ftlb scoob will accelerate somewhat faster than a pushbike.
All said though, driving an engine that's only any good over 5000rpm is rather a pain.
Paul
[Edited by Pavlo - 6/13/2002 9:13:26 PM]
[Edited by Pavlo - 6/13/2002 9:14:51 PM]
------------
5250
Power is overall work, ultimately you need power to move, as any amount of torque will be no good without some rate of application (rpm in our case).
When you see the following written:
"The great thing is engine A has 100ftlb at only half the speed of engine B".
Except for acceleration it isn't as good as it seems, as if you have to run engine B at double the spead to get the same torque, you can run a gear that's half as long, meaning at the same road speed you will have double the torque of engine A (well increase drive losses will rob some of that).
A human is good for about 1HP, with a suitably long lever, could generate 1000ftlb of torque easily, but a 300ftlb scoob will accelerate somewhat faster than a pushbike.
All said though, driving an engine that's only any good over 5000rpm is rather a pain.
Paul
[Edited by Pavlo - 6/13/2002 9:13:26 PM]
[Edited by Pavlo - 6/13/2002 9:14:51 PM]
13 June 2002, 10:36 PM
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PLC - Public Limited Company: Limited liability company with a reasonable (I can't remember exactly) percentage of its shares available for trade on the UK stock exchange.
Limited liability company: the owners are limited to the amount of money they have invested in the company as opposed to Lloyds names who have unlimited liability
Limited liability company: the owners are limited to the amount of money they have invested in the company as opposed to Lloyds names who have unlimited liability
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13 June 2002, 11:32 PM
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Lifted from the above mentioned site, Brilliant Even I understand it now
Take the time to read it, it's worth it
Lets invent two characters:-. Arnie and Splinter. These guys epitomise two extremes of engine genre. Arnie has whopping leg muscles…thighs thicker than Tara-Palmer-Tomkinson. He is however a little clumsy and uncoordinated, so whilst he can REALLY “pump dem pedals”, he can only manage slow rotations. Anything else is too much for his feeble mind and coordination. (whisper…he’s the turbodiesel).
Splinter conversely has spindly “twiggy legs” - He can barely stand up from his seat when his cup of Horlicks is full to the brim. Splinter however has been surgically enhanced with “VAJ” (variable-ankle-jeometry) that endows him with an uncanny ability to “spin da cog”. Although he can only manage a feeble push, he can maintain the coordination needed to rotate the pedals at…oh god knows how many rpm…it’s so fast I can’t count. You’d really need to see this guy. (whisper again….he’s the VTEC)
We line them up on the drag strip and Whooaa! Arnie storms into the lead, his rear tyre barely able to contain the thrust created by those burly legs. But…oh dear….his coordination is going, and he’s quickly forced to change up into second, then third…and look at this! Splinter is staging a comeback, still in first with his legs flailing like ….a flaily thing. Whilst Arnie is producing major leg thrust, he’s currently using 3rd gear, which only doubles the torque his legs generate before it reaches the rear wheel. Splinter is still in first, so his weedy leg strength continues to be subject to 5-times multiplication…and he’s reeling Arnie in!
It’s a similar story when they are up to maximum speed. By enormous coincidence they appear to have an identical flat-out pace. Arnie is pumping his top gear firmly and steadily ~ wind resistance and friction conspire against him – he can go no faster. Splinter is right up there with him, but he’s still in 3rd - legs whizzing round.
If we assume they have the same drag coefficient & weight etc, they are both producing the same POWER at this point. Arnie via high leg torque and low rpm, Splinter via low leg torque and high rpm. Their two power outputs are equal, and each calculation is valid.
It’s exactly the same story when you look at the torque arriving at the rear wheel of the bike ~ Arnie is making high leg torque, but it’s being blunted by the high gearing he has to use. Splinter uses low gearing to maintain the multiplication of his leg muscles and provide an identical torque at the rear wheel.
Back to the world of cars. You should now be able to understand why TDI’s are often quoted as having “massive in-gear thrust”, whilst it’s acknowledged that they suck fat ones when it comes to both “0-60” and top speed. Most TDI’s barely get to 60 in third (let alone second), so in addition to requiring an extra gear change over most petrol models, they pass “the magic 60” in 3rd ~ which is by definition higher geared than 2nd, so the prodigious torque is multiplied to a lesser extent before it get’s the chance to arrive at the drivewheel and “do the business”.
As diesels operate using compression ignition they don’t require spark plugs. This omission unfortunately makes advancing the spark somewhat difficult ~ the main cause of their high rpm torque defecit. Lack of power (torque at high rpm) alone blunts their maximum velocity potential, but the coup-de-grace is that (with a rev ceiling around 4,500rpm) they have to use higher gearing ~ which again reduces the torque reaching the wheels. These two traditional performance metrics are therefore unreflective of the “real world” performance of a modern TD…it’s low-rev torque very noticeable on the road, but fairly irrelevant in the two above circumstances.
A VTEC engine with 140lbs.ft max torque CANNOT punch as hard as a TDI with 200lbs.ft. In a VTEC however you will choose to accelerate with a lower gear than the TD, so the torque it does have is multiplied to a greater extent before it reaches the wheels. A TDI might feel impressive in 4th~ certainly more impressive than a VTEC in 4th, but the VTEC driver would undoubtedly pop it into 3rd (or even 2nd) to make better progress. The TDI has no such option.
Real Life Example – for the average TDI, gearing multiplies engine torque by a factor of approximately 12 in 1st gear, but only 2.5times in 5th gear. A TDI with 200lbs/ft will therefore produce a peak of 500lbs.ft at the drivewheels in 5th.
A VTEC shares pretty similar gearing in 1st, but its’ lower top gear multiplies engine torque by a far higher 3.7times(approx) in 5th. Its’ 140lbs.ft engine torque peak therefore equates to a peak at the drivewheels of 518lbs.ft in top.
Assuming they are the same weight etc and each car is at peak-torque rpm, you can see that the VTEC will accelerate marginally harder in top than the TDI - despite its apparent torque deficit.
As a rule - lower gearing makes better use of what torque you have available, but leads to unfashionably (and often uncomfortably) high-rev cruising. If you like that – fine. If you don't. what do I care? Your decision ~ there is no “best”.
Just out of interest – lets look at a (current) F1 car example-
Assume 800hp at 18000rpm. Using the equation in the 1st paragraph, we can see that it is only producing 233lbs.ft at this point. Not a lot really….a 1.9TDI VAG unit produces up to 235lbs.ft in el-boggo-standard form.
However…..if we want it to do 210mph, then our mega-revvy F1 engine only needs a top gear pulling 12mph per 1000rpm…..which is barely higher than 2nd gear in most road cars. Hence the “torque multiplier” from the gearing is massive, and you can still spin those fat rear slicks in top gear. Massive power allows vast exploitation of gearing. (I would guess gearing ratio in top is approx 6.5:1….so the engines’ 233lbs.ft becomes 1500+ lbs.ft to be shared between the rear wheels).
What if you could extend the rev range further? Okeydokey - let’s double it to 36000rpm. With this, the same car could use gearing of only 6mph per 1000rpm (about the same as 1st in a road car) and still manage 210mph. All 800 horses are still required (and produced), although only 116.5 lbs.ft torque is required to create it at this astronomical engine speed. Has the torque arriving at the wheels changed? (No). You can see why F1 engineers love engines that rev.
It aint simple….but then the best things never are. When this all clicks home in your noggin it’s as satisfying as your first clean heel&toe downshift, and you’ll never look at engines in the same way again.
Knowledge is power as they say. Or should that be “torque x rpm”?
Even I understand it now Take the time to read it, it's worth it
Lets invent two characters:-. Arnie and Splinter. These guys epitomise two extremes of engine genre. Arnie has whopping leg muscles…thighs thicker than Tara-Palmer-Tomkinson. He is however a little clumsy and uncoordinated, so whilst he can REALLY “pump dem pedals”, he can only manage slow rotations. Anything else is too much for his feeble mind and coordination. (whisper…he’s the turbodiesel).
Splinter conversely has spindly “twiggy legs” - He can barely stand up from his seat when his cup of Horlicks is full to the brim. Splinter however has been surgically enhanced with “VAJ” (variable-ankle-jeometry) that endows him with an uncanny ability to “spin da cog”. Although he can only manage a feeble push, he can maintain the coordination needed to rotate the pedals at…oh god knows how many rpm…it’s so fast I can’t count. You’d really need to see this guy. (whisper again….he’s the VTEC)
We line them up on the drag strip and Whooaa! Arnie storms into the lead, his rear tyre barely able to contain the thrust created by those burly legs. But…oh dear….his coordination is going, and he’s quickly forced to change up into second, then third…and look at this! Splinter is staging a comeback, still in first with his legs flailing like ….a flaily thing. Whilst Arnie is producing major leg thrust, he’s currently using 3rd gear, which only doubles the torque his legs generate before it reaches the rear wheel. Splinter is still in first, so his weedy leg strength continues to be subject to 5-times multiplication…and he’s reeling Arnie in!
It’s a similar story when they are up to maximum speed. By enormous coincidence they appear to have an identical flat-out pace. Arnie is pumping his top gear firmly and steadily ~ wind resistance and friction conspire against him – he can go no faster. Splinter is right up there with him, but he’s still in 3rd - legs whizzing round.
If we assume they have the same drag coefficient & weight etc, they are both producing the same POWER at this point. Arnie via high leg torque and low rpm, Splinter via low leg torque and high rpm. Their two power outputs are equal, and each calculation is valid.
It’s exactly the same story when you look at the torque arriving at the rear wheel of the bike ~ Arnie is making high leg torque, but it’s being blunted by the high gearing he has to use. Splinter uses low gearing to maintain the multiplication of his leg muscles and provide an identical torque at the rear wheel.
Back to the world of cars. You should now be able to understand why TDI’s are often quoted as having “massive in-gear thrust”, whilst it’s acknowledged that they suck fat ones when it comes to both “0-60” and top speed. Most TDI’s barely get to 60 in third (let alone second), so in addition to requiring an extra gear change over most petrol models, they pass “the magic 60” in 3rd ~ which is by definition higher geared than 2nd, so the prodigious torque is multiplied to a lesser extent before it get’s the chance to arrive at the drivewheel and “do the business”.
As diesels operate using compression ignition they don’t require spark plugs. This omission unfortunately makes advancing the spark somewhat difficult ~ the main cause of their high rpm torque defecit. Lack of power (torque at high rpm) alone blunts their maximum velocity potential, but the coup-de-grace is that (with a rev ceiling around 4,500rpm) they have to use higher gearing ~ which again reduces the torque reaching the wheels. These two traditional performance metrics are therefore unreflective of the “real world” performance of a modern TD…it’s low-rev torque very noticeable on the road, but fairly irrelevant in the two above circumstances.
A VTEC engine with 140lbs.ft max torque CANNOT punch as hard as a TDI with 200lbs.ft. In a VTEC however you will choose to accelerate with a lower gear than the TD, so the torque it does have is multiplied to a greater extent before it reaches the wheels. A TDI might feel impressive in 4th~ certainly more impressive than a VTEC in 4th, but the VTEC driver would undoubtedly pop it into 3rd (or even 2nd) to make better progress. The TDI has no such option.
Real Life Example – for the average TDI, gearing multiplies engine torque by a factor of approximately 12 in 1st gear, but only 2.5times in 5th gear. A TDI with 200lbs/ft will therefore produce a peak of 500lbs.ft at the drivewheels in 5th.
A VTEC shares pretty similar gearing in 1st, but its’ lower top gear multiplies engine torque by a far higher 3.7times(approx) in 5th. Its’ 140lbs.ft engine torque peak therefore equates to a peak at the drivewheels of 518lbs.ft in top.
Assuming they are the same weight etc and each car is at peak-torque rpm, you can see that the VTEC will accelerate marginally harder in top than the TDI - despite its apparent torque deficit.
As a rule - lower gearing makes better use of what torque you have available, but leads to unfashionably (and often uncomfortably) high-rev cruising. If you like that – fine. If you don't. what do I care? Your decision ~ there is no “best”.
Just out of interest – lets look at a (current) F1 car example-
Assume 800hp at 18000rpm. Using the equation in the 1st paragraph, we can see that it is only producing 233lbs.ft at this point. Not a lot really….a 1.9TDI VAG unit produces up to 235lbs.ft in el-boggo-standard form.
However…..if we want it to do 210mph, then our mega-revvy F1 engine only needs a top gear pulling 12mph per 1000rpm…..which is barely higher than 2nd gear in most road cars. Hence the “torque multiplier” from the gearing is massive, and you can still spin those fat rear slicks in top gear. Massive power allows vast exploitation of gearing. (I would guess gearing ratio in top is approx 6.5:1….so the engines’ 233lbs.ft becomes 1500+ lbs.ft to be shared between the rear wheels).
What if you could extend the rev range further? Okeydokey - let’s double it to 36000rpm. With this, the same car could use gearing of only 6mph per 1000rpm (about the same as 1st in a road car) and still manage 210mph. All 800 horses are still required (and produced), although only 116.5 lbs.ft torque is required to create it at this astronomical engine speed. Has the torque arriving at the wheels changed? (No). You can see why F1 engineers love engines that rev.
It aint simple….but then the best things never are. When this all clicks home in your noggin it’s as satisfying as your first clean heel&toe downshift, and you’ll never look at engines in the same way again.
Knowledge is power as they say. Or should that be “torque x rpm”?
14 June 2002, 05:24 AM
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Torque is a measure of "(twisting) force".
Power is a measure of "work done per unit time".
Torque is a measured value (like length, temperature, mass, time etc).
Power is a calculated value (by dividing a measured value over time).
Ultimately, the torque vs RPM relationship is the main thing that determines how fast a vehicle accelerates, assuming a certain set of gear ratios.
Needless to say, oil burners don't accelerate well because they make their torque very low down in the RPM range, and then run out of breath (as the "spark" can't be advanced, stated in the above article). VTEC's on the other hand, accelerate decently well because they manage to hold their lowish torque all the way to 8000RPMs.
Power is related to "area under the torque-RPM curve". The bigger the area, the more the power.
[Edited by RT - 6/14/2002 5:34:06 AM]
Power is a measure of "work done per unit time".
Torque is a measured value (like length, temperature, mass, time etc).
Power is a calculated value (by dividing a measured value over time).
Ultimately, the torque vs RPM relationship is the main thing that determines how fast a vehicle accelerates, assuming a certain set of gear ratios.
Needless to say, oil burners don't accelerate well because they make their torque very low down in the RPM range, and then run out of breath (as the "spark" can't be advanced, stated in the above article). VTEC's on the other hand, accelerate decently well because they manage to hold their lowish torque all the way to 8000RPMs.
Power is related to "area under the torque-RPM curve". The bigger the area, the more the power.
[Edited by RT - 6/14/2002 5:34:06 AM]
14 June 2002, 10:45 AM
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I think I understand.
Thank you my friends. You have helped me salvage some of my pride.
On the PLC thing I'm not sure I agree with this and will explore further. I am an ACA and have never had a satisfactory explanation to this.
Thank you my friends. You have helped me salvage some of my pride.
On the PLC thing I'm not sure I agree with this and will explore further. I am an ACA and have never had a satisfactory explanation to this.
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