Discussion
I'm sure this is a stupid question, but........
Can someone explain to me what the difference is between torque and bhp - how is it measured and how come they don't have a relationship with each other? ie some cars have more BHP than torque and some cars the other way round! uh! :confused:
Can someone explain to me what the difference is between torque and bhp - how is it measured and how come they don't have a relationship with each other? ie some cars have more BHP than torque and some cars the other way round! uh! :confused:
To be fair there are a thousand threads on this topic around if you do a search.
In a nutshell, if you take the analogy of an engine lifting a weight, the torque of the engine would determine how heavy a weight it could lift, whilst power is how fast it could lift it. In terms of a car, a simple answer is that you need good torque for good acceleration and high power for high speeds. This is why rally cars tend to be tuned for torque whilst F1 cars are tuned for power.
Torque and power are directly related. Power is torque divided by time taken. However, maximum torque and maximum power tend to occur at different engine speeds so that one cannot be determined from the other. The fact that sometimes the figures quoted for bhp and torque are similar is purely coincidental when using lb-ft and bhp as units.
The figures are measured with the use of a dynamometer which is either connected to the output from the engine or via the wheels of the car. Commonly the dyno consists of a brake (hence bhp) that applies an opposing torque to the driven shaft. The values of this torque and the engine speeds it occurs at are used to produce a torque/power curve.
The different engines behave in different ways due to their design. An engine with a long stroke will generally generate more torque than a similar engine with a shorter stroke because the distance between big end and crankshaft centre line will be greater (and torque is force x distance). However, the longer conrods might be heavier, increasing the inertia of the piston and thereby reducing the power produced.
The characteristics of the same engine may also be adjusted to suit the application. For example, the V6 diesel engine in the Jaguar S-type and the Land Rover Discovery 3 are essentially the same but the Jag's has higher max bhp and lower torque. The reasoning behind this would be that the Land Rover is a lot heavier than the s-type and requires more torque (especially when off road) but that the extra power would be wasted in a car that aerodynamically could not reach high velocities.
In a nutshell, if you take the analogy of an engine lifting a weight, the torque of the engine would determine how heavy a weight it could lift, whilst power is how fast it could lift it. In terms of a car, a simple answer is that you need good torque for good acceleration and high power for high speeds. This is why rally cars tend to be tuned for torque whilst F1 cars are tuned for power.
Torque and power are directly related. Power is torque divided by time taken. However, maximum torque and maximum power tend to occur at different engine speeds so that one cannot be determined from the other. The fact that sometimes the figures quoted for bhp and torque are similar is purely coincidental when using lb-ft and bhp as units.
The figures are measured with the use of a dynamometer which is either connected to the output from the engine or via the wheels of the car. Commonly the dyno consists of a brake (hence bhp) that applies an opposing torque to the driven shaft. The values of this torque and the engine speeds it occurs at are used to produce a torque/power curve.
The different engines behave in different ways due to their design. An engine with a long stroke will generally generate more torque than a similar engine with a shorter stroke because the distance between big end and crankshaft centre line will be greater (and torque is force x distance). However, the longer conrods might be heavier, increasing the inertia of the piston and thereby reducing the power produced.
The characteristics of the same engine may also be adjusted to suit the application. For example, the V6 diesel engine in the Jaguar S-type and the Land Rover Discovery 3 are essentially the same but the Jag's has higher max bhp and lower torque. The reasoning behind this would be that the Land Rover is a lot heavier than the s-type and requires more torque (especially when off road) but that the extra power would be wasted in a car that aerodynamically could not reach high velocities.
Frik said:
The different engines behave in different ways due to their design. An engine with a long stroke will generally generate more torque than a similar engine with a shorter stroke because the distance between big end and crankshaft centre line will be greater (and torque is force x distance).
But in order to maintain the same capacity, the bore must be reduced as the stroke is lengthened. This results in a smaller piston area and therefore a smaller force on the piston, which exactly cancels out the extra leverage of the longer crank throw. The torque therefore remains the same.
However the long-stroke, narrow-bore engine has less valve area in relation to its capacity than the short-stroke, wide-bore version, so the short-stroke one breathes better and can produce more power at high revs. The shorter stroke also means a lower piston speed and acceleration so the high revs produce less mechanical stress.
Frik said:
However, the longer conrods might be heavier, increasing the inertia of the piston and thereby reducing the power produced.
The inertia of the piston does not affect the power produced. The energy that goes into accelerating it, you get back when decelerating it. But the force needed to accelerate/decelerate the piston is increased, which results in a lower rev limit for the same mechanical stress.
By similar engines I meant those with equal bore rather than equal capacity.
It's probably worth adding as a further distinction that torque can be considered as related to the size of the explosion whereas power is related to the quantity of explosions per second. Therefore the more explosions you can get in per second, the more power you can generate.
F1 engines run to 19,500 rpm so they can produce very high power outputs but only because of the exotic materials they use (and the reduced time between rebuilds) in order to cope with the much higher mechanical stresses, as mentioned by Pigeon.
Pigeon said:A slight heat-related brain melt problem at my end there.
The inertia of the piston does not affect the power produced. The energy that goes into accelerating it, you get back when decelerating it. But the force needed to accelerate/decelerate the piston is increased, which results in a lower rev limit for the same mechanical stress.
It's probably worth adding as a further distinction that torque can be considered as related to the size of the explosion whereas power is related to the quantity of explosions per second. Therefore the more explosions you can get in per second, the more power you can generate.
F1 engines run to 19,500 rpm so they can produce very high power outputs but only because of the exotic materials they use (and the reduced time between rebuilds) in order to cope with the much higher mechanical stresses, as mentioned by Pigeon.
Power = 2*П*N*T where N is the engine rotational velocity (similar to revs) and T is torque.
So the relationship is there between torque and Power, but it is dependent upon RPM.
To this end an engine that has lots of low down torque but no rev range (a turbo diesel) has relatively low power output but a (relatively) gutless petrol (think V-TEC) has lots of power as it revs round to 8000 rpm plus.
If you do the maths of this example, a lot of revs mean lots of easy power (the V-tec peak torque at 6000 rpm plus, but still producing useful torque after that up to 8000, compared to the diesel that runs out of puff at 4000 rpm, so the diesel has to have twice as much torque (at half the revs to match the VTEC power).
You need a huge increase in torque to get the same power as a result.
Have a look at bike engines. Fit ones have 170 bhp and only 90-100 lb/ft torque - 'cos they rev so high.
Hope this helps.
So the relationship is there between torque and Power, but it is dependent upon RPM.
To this end an engine that has lots of low down torque but no rev range (a turbo diesel) has relatively low power output but a (relatively) gutless petrol (think V-TEC) has lots of power as it revs round to 8000 rpm plus.
If you do the maths of this example, a lot of revs mean lots of easy power (the V-tec peak torque at 6000 rpm plus, but still producing useful torque after that up to 8000, compared to the diesel that runs out of puff at 4000 rpm, so the diesel has to have twice as much torque (at half the revs to match the VTEC power).
You need a huge increase in torque to get the same power as a result.
Have a look at bike engines. Fit ones have 170 bhp and only 90-100 lb/ft torque - 'cos they rev so high.
Hope this helps.
Whoooohhhhhhhh....... way too much information for my small brain here - this'll take me a few coffee's to get my head round now
OK, to try and get a simple answer, can I rephrase my question:
If my car has got 250 bhp and 280 lb/ft.
My friends car has got 280 bhp and 250 lb/ft
Who is going to win a 1/4 mile race at Santa Pod.
Assume that the cars weigh the same and the same traction and drivers are as incompetant as each other and both drivers are as fat as each other and the aerodynamics are identical and I'm not colour blind and there will be no atmospheric differences between the lanes .........
Reason I ask, is I'm going to race my friend in his RX7, I've got a TVR Chimaera 500. I think my car is more torque than the RX but less bhp
OK, to try and get a simple answer, can I rephrase my question:
If my car has got 250 bhp and 280 lb/ft.
My friends car has got 280 bhp and 250 lb/ft
Who is going to win a 1/4 mile race at Santa Pod.
Assume that the cars weigh the same and the same traction and drivers are as incompetant as each other and both drivers are as fat as each other and the aerodynamics are identical and I'm not colour blind and there will be no atmospheric differences between the lanes .........
Reason I ask, is I'm going to race my friend in his RX7, I've got a TVR Chimaera 500. I think my car is more torque than the RX but less bhp
Ultimately, with reasonable gearing, the car with more power will win assuming equal weight and drag and rolling friction etc.
HP is usually gained in many ways, so a small engine can develop huge power.
However, a small engine can never really develop a huge amount of torque assuming no forced induction.
I think torque is in the order of 100lbft per litre for a naturally aspirated car theoretical max. 2bar boost would get that to 200lbft theoretical max, though 80% is usually what is achieved.
Power however is only really limited by rpm's!
Dave
HP is usually gained in many ways, so a small engine can develop huge power.
However, a small engine can never really develop a huge amount of torque assuming no forced induction.
I think torque is in the order of 100lbft per litre for a naturally aspirated car theoretical max. 2bar boost would get that to 200lbft theoretical max, though 80% is usually what is achieved.
Power however is only really limited by rpm's!
Dave
Mr Whippy said:
I think torque is in the order of 100lbft per litre for a naturally aspirated car theoretical max. 2bar boost would get that to 200lbft theoretical max, though 80% is usually what is achieved.
Dave
Depending on RPM's and cam duration! Curious about the amths which arises at 15psi=200ft/lb, even an approximation. I agree though.
ian187 said:Unfortunately that isn't enough information. Those numbers are "just" the highest points of the cars' power and torque curves. You need to know the shape of the curves too. You need to know the car's mass, its gearing and either the torque curve or the power curve (they contain exactly the same information) in order to calculate the potential acceleration (and this is ignoring the available grip from the tyres too).
...
If my car has got 250 bhp and 280 lb/ft.
My friends car has got 280 bhp and 250 lb/ft
...
19560 said:
and this is ignoring the available grip from the tyres too
IMHO you've hit the nail on the head there. Ian forget numbers and graphs, just give it a go. If you can set off quickly without spinning the wheels then you should win; this may be a lot easier to say than do...
and front wheel drive, rear wheel drive, 4 wheel drive ....
Get down to the Pod to find out
In simple terms an engine produces torque..... it also revs.
BHP is a calculation:
(Torque x rpm)/5252 = BHP
The torque curve plotted against RPM will vary massivelly depending upon engine but what they will always have in common due to this formula is that:
At5252rpm BHP=Torque.
above 5252rpm BHP > torque
below 5252rpm BHP < Torque
acceleration and speed is a much more difficult thing to predict as there are a huge number of variables... weight, traction and gearing being the biggest on the strip.
BHP is a calculation:
(Torque x rpm)/5252 = BHP
The torque curve plotted against RPM will vary massivelly depending upon engine but what they will always have in common due to this formula is that:
At5252rpm BHP=Torque.
above 5252rpm BHP > torque
below 5252rpm BHP < Torque
acceleration and speed is a much more difficult thing to predict as there are a huge number of variables... weight, traction and gearing being the biggest on the strip.
Frik said:
torque can be considered as related to the size of the explosion whereas power is related to the quantity of explosions per second.
This topic has been covered thousands of times and I've read pretty much all of them.
I hereby award Frik the medal for the most concise summary of the lot.
ian187 said:
If my car has got 250 bhp and 280 lb/ft.
My friends car has got 280 bhp and 250 lb/ft
Who is going to win a 1/4 mile race at Santa Pod.
Assume that the cars weigh the same and the same traction and drivers are as incompetant as each other and both drivers are as fat as each other and the aerodynamics are identical and I'm not colour blind and there will be no atmospheric differences between the lanes .........
Reason I ask, is I'm going to race my friend in his RX7, I've got a TVR Chimaera 500. I think my car is more torque than the RX but less bhp
Alot of time can be found by perfecting your reactions and launch technique. Launch when you see the three yellow lights appear, don't wait for the green. Often close drag races are won on the launch. Launch using about 3,000 rpm, try not to wheelspin, feed the power in once the rear hooks up and then stamp the throttle to the floor in each gear. Rev to the redline in each gear. Make perfect gearchanges.
All the above sounds simple, buit when you are grinning and looking across at your mate its real easy to f#ck one small bit up and lose half a second!
It'll be a close race.
The BHP and lb ft numbers you quote are maximums. You won't be at your maximum power or torque for the majority of your drag race, you'll be accelerating through the rpm range. You need to look at the power and torque the engines make throught the rpm range to compare. Also, as others have already mentioned, the tyres you use and the gearing also make a big difference. Thats why the first modifications you make to a drag car are the diff ratio and some sticky tyres.
I'd fancy the TVR for the drag race. If your mates RX7 is stock, then it'll run a mid to low 14 second 1/4 on a perfect run, if you perfect your run, you might go low 14s or even a high 13 if you are very lucky.
_Al_ said:
torque can be considered as related to the size of the explosion whereas power is related to the quantity of explosions per second.
This topic has been covered thousands of times and I've read pretty much all of them.
I hereby award Frik the medal for the most concise summary of the lot.
Can I have silver please.............
I was told by an ex boxer that..
Torque = how hard you hit
Power = how many punches you throw in a round
I’m very confused by many of the above posts even as an engineering student?!!
I’m just going to lift something from a previous post I made first to clear up absolutely what torque is and how an engine creates torque:
So if you think about it torque is mechanical advantage, a force that keeps something rotating and nothing else!
Now power is a different thing entirely, and we should start off by not talking about power itself but instead about energy.
Power = Energy/Time (Energy is sometimes called “Work” or “Work done”, I hate it but other will try)
But how do we define Energy in a car engine? Obviously petrol contains the energy and we can easily measure it but our engine are not 100% efficient. We could do it by figuring out our engines efficiency but that’s a long winded and silly process with questionable results as the engine in your car has varying efficiency.
Something far more useful is that Energy = Force * Distance. So if you walk 100 meters and you produce a net force of 10 Newton’s with your legs you have used 1000 (10*100) Joules of energy (Often referred to as a Kilo joule).
Now torque does effect force however from the engine to the car wheel. The amount of force you car wheel exerts on the road is governed by Torque = Force * Distance so Force = Distance/Torque. So evidently your car needs Torque to move, but force isn’t enough because you need to expend energy to keep the force applied.
We can say that Power = Force * Distance/Time. Since Energy = Force * Distance and Power = Energy/Time
From this it should become apparent that when we measure an engine against a break we are actually measuring its ability to apply a force while revolving at a certain speed, this is power!
I’m just going to lift something from a previous post I made first to clear up absolutely what torque is and how an engine creates torque:
speedy_thrills said:
First of all the easiest way I can describe Torque is a rotational moment (a moment about a rotating axis) or mechanical advantage. It has bugger all to do directly with valve area or rotational inertia as previously mentioned.
Moment = Force * Perpendicular Distance, which when you think about it is true! Trying to undo the nuts on a wheel with spanner with a short handle is really difficult isn’t it? You have to exert a larger force to create enough moment to get the nut moving (however once the nut is undone having a short spanner is good thing because you can twist faster than you can with a long handle).
So if Vixpy weighs 100kg and he hangs off a spanner handle 0.5 meters long the moment is 100*9.81 (to turn kilograms to newtons) * 0.5 he is generating 490.5 N/m’s of torque. The same principle applies to the Imperial system in ft/lbs (but I was brought up under the sin of the metric system).
The same mechanics apply in an engine, but there are a few complications. You see unlike Vixpy hanging on his spanner handle an engine is a real moving thing and is often hard to measure forces.
Directly and simply we can say that the Torque generated by the engine on a power stroke = the force acting down the control rod (due to the cylinder) * the Perpendicular distance from the con rod to the axis around which crank rotates.
Obviously there are further complications because:
1. An engine only creates a positive moment during the “Power/ignition” stroke, the rest of the time the engine is dragging and creating negative moment through the exhaustion, induction and compression stroke.
2. We cannot actually measure the force acting through the control rod directly.
3. The force acting through the control rod varies through the engines cycle.
4. The perpendicular distance between the control rod and the axis around which it turns it constantly changing.
However we can still calculate this by using our heads.
To still manage to solve if we can say that:
1. Positive moment – Negative moment = Total moment throughout the stroke, if it is greater than positive zero the engine will move and create excess torque (that which is measured on a dynamometer) if the result is zero the engine will not move and finally if the answer is negative we have messed up somewhere because the engine wants to rotate backwards.
2. This is complicated, really complicated. We will break the engine cycle down into 4 parts for each stroke of the cylinder the piston performs. Since Force = Pressure* Area, the mean effective pressure (MEP) or average pressure as it could be known acting on top of the cylinder in each cycle * The total area of the bore = The force exerting down onto the piston during each stroke. But due to angular changes we have to use between the control rod and (If I get this wrong can some one correct me please) the tangent of the angle between the vertical centeroid of the half circle created during the stroke and the axis around which crank rotates * the force acting through the piston = The total force acting through the control rod. So we have solved the problem using some basic mechanics and trigonometry.
3. As mentioned above we use the Mean/average pressure acting on the cylinder all the way through each of our 4 strokes to solve this problem.
4. Again this is why we used the centeroid or the half circle created by our one stroke for our angle in our calculations.
So if you think about it torque is mechanical advantage, a force that keeps something rotating and nothing else!
Now power is a different thing entirely, and we should start off by not talking about power itself but instead about energy.
Power = Energy/Time (Energy is sometimes called “Work” or “Work done”, I hate it but other will try)
But how do we define Energy in a car engine? Obviously petrol contains the energy and we can easily measure it but our engine are not 100% efficient. We could do it by figuring out our engines efficiency but that’s a long winded and silly process with questionable results as the engine in your car has varying efficiency.
Something far more useful is that Energy = Force * Distance. So if you walk 100 meters and you produce a net force of 10 Newton’s with your legs you have used 1000 (10*100) Joules of energy (Often referred to as a Kilo joule).
Now torque does effect force however from the engine to the car wheel. The amount of force you car wheel exerts on the road is governed by Torque = Force * Distance so Force = Distance/Torque. So evidently your car needs Torque to move, but force isn’t enough because you need to expend energy to keep the force applied.
We can say that Power = Force * Distance/Time. Since Energy = Force * Distance and Power = Energy/Time
From this it should become apparent that when we measure an engine against a break we are actually measuring its ability to apply a force while revolving at a certain speed, this is power!
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