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Torque and BHP
- Thread starter Pinhead
- Start date
Torque is a mesurement of force during a snapshot in time, whereas power is a measurement of the ability do do work across time.
Torque and power are directly relational.
To gain power with no increase in torque the engine just needs to rev higher, but without the torque dropping off (which is what vvt aims to help).
The 172s would make 276bhp from only 145lb/ft torque if they could rev to 10,000 rpm and if the torque didnt drop like it does after 5000 rpm.
BMW e46 320 Ci Sport
so to put it in laymans terms what is best to have a lot of?? how can u used torque as an indication of a cars performance? or can u only use it as a guage if u have the bhp level too?
As a practical example, will a car with 100bhp and lots of torque be better or orse than a car with 100bhp and v low torque, what will the difference be practically, really want to understand this.
Ok torque is the force without movemtn if you pust as hard as you can on a car and it doesnt move then your applying a force to it ye but its not going anywhere so youve got say 100lb toque but 0hp as its not moving.
If you then take the handbrake of the car and push it with the same 100lb torque it will move your still applying 100lb torque but as its moving your making hp as force * speed = hp.
Diesels make loads of torque at ~2k but since 2k isnt very much speed its not much hp. A 172 etc though make less forcee but make the less force faster and can rev to higher numbers.
Its like a bus hitting you at 10mph and a Saxo hitting you at 40mph. They both have the same force but there different.
Torque * rpm
----------------- = hp
5252
So if you could keep the torque going till 8k like a 172 then a diesel would make twice he hp but the shove in the back would be the same only twice as often.
Best have a look here for a description torque explained here
http://www.epi-eng.com/FND-PwrTrq.htmhttp://www.epi-eng.com/FND-PwrTrq.htm
http://www.canadiandriver.com/articles/jk/020327.htmhttp://www.canadiandriver.com/articles/jk/020327.htm
http://www.amasuperbike.com/hportorque.htmhttp://www.amasuperbike.com/hportorque.htm
But to answer your question yes you can have engine with loads of torque and little Hp (diesels) and engine litle little torque and loads of HP (F1 cars) it just depends on where the torque is prodcued. If diesels could efficiently rev to 23k then no F1 car would be petrol powered as its would be destroyed on the track.
Depends how you drive torque is force so a car with low end torque will be fast even when your not reving it ie my diesel has 50hp at 2k so when I put my foot down in normal driving if has more go.
Torque means nothing realy without knowing what revs have a look at some diesel power curves you see that they make 75% or so power at low revs thanks to the torque down there but the more you rev them they wont go much quicker as thy cannot hold the torque.
If I pushed you in yor car at 1mph with 200lb torque then you would fell a push put your wouldnt go very quick if I push with 50lb but your moving twice as fast then you fell less push but the same acceleration.
As for your second question well a 1.6 petrol and a 100dci are the same hp but the diesel make it low down so the diesel will fell quciker as in normal driving you will get ~75hp but the petrol will give you ~40hp.
If you go racing them round a track the diesel will have more adverage hp as the revs you use will 3~4k in the diesel so between 75hp and 100hp and the petrol will give you between 70 and 100hp in the top rev range as you can see the diesel has more adverage hp so is quicker.
Rich-D
ClioSport Club Member
E90 LCI 330d
Ok, so how come a turbocharged engine has more torque than a norally aspirated engine, but they could both have the same power?!
Also VTEC kinda f*cks this theory up as you get loads of power, but lower torque...
I just cannot see how they are directly related!
Ok so best example is to say this torque is force it never changes. force has no speed though you try and push a wall over thats torque.
HP is torque in relationship to speed or revs.
A turbo car and a N/A car may have the same hp but the turbo has more torque this is low down torque which means low down hp. Have a look at the image below is shows a dci it makes loads of torque low down but very little hp as the amunt of torque cannot be kept at high revs it make 50 out of a max 70hp at 2k but as the torqie drops preaty qick after 2k it cannot keep the power.
http://erc.qmuc.ac.uk/cliosport/gallery/full/1079297610__dsl.jpg
This image shows a petrol it make the same amount of torque though out the rev range its designed to do this and as it can hold the torque in high revs and as the formula says move revs with the same torque = more power.
http://erc.qmuc.ac.uk/cliosport/gallery/full/1079297676__petrol.jpg
A Honda engine can make the same hp as a 1.8T. Its the way there designed . To make hp with a small engine you have to either rev the engine more and keep the torque the same, or force more air and fuel into the piston to make a bigger amount of torque which as the formula says more torque at the same revs = more power.
Honda let the engine rev so little torque but it keeps its little push at lighening speed its like being hit by a bullet which doesnt slow down and keeps pushing at you with the same force. An engine which make hp low down is like being hit by a bus at 20mph which doesnt slow down and keeps pushing you. There both the same HP but when a bus hits you its more of a knock then being hit by a bullet.
EDD
Because the NA engine will rev higher.
No v-tec just proves it. The power from a vtec engine is made from really high revs. The torque is not a huge amount, but its high across a wide rev band, so the revs make the power.
A car that makes 100 lb/ft of torque at 8000 rpm will make 152 bhp at 8000 rpm.
A car that makes 200 lb/ft of torque at 4000 rpm will make 152 bhp at 4000 rpm.
Exactly the same power, but twice the torque is required as half the revs are being used.
LOL - they are, just takes a while to get your head around it thats all.
very very simple.
note:its fairly long so you dont have to read it, it wont hurt my feelings, but im BORED!!!!
Torque is you POWER, its is the strength of the engine at that particular rpm. BHP is the WORK DONE (remember GCSE physics), but it is a quantity that has a time limit. So 1 bhp is expressed as 33,000 lbs.ft/min.
How are they related......simple, bhp expresses the amount of work you can do at that specific BHP. For example, 50lbs.ft torque at 5000rpm = 47bhp, but 30lbs.ft torque at 10,000rpm = 57bhp.
James Watt who invented the HP as a marketing ploy to help the mining industry rate how powerful one of his steam engines compares to the horses they were using. Where a horse would pull baskets of coal up the mine shaft via a single pully, meaning there is no leverage effect.
The B in BHP comes from the method used to measure an engine torque output, more force applied as a braking force = more torque.
Torque is produced at the crank, the rotational strength. This is inturn created by the force of the piston moving down and acting upon the conrod, linear to reciprical to rotational motion. The piston is affected by cylinder pressures which are created by combustion. THe amount of pressure created as a result of combustion is directly related to VE (the more you fit in the cylinder the larger the pressure rise from the burn), so naturally the point of peak torque is also the point of peak VE (volumetric efficency).
Vtecs dont funk up the theory, they compound it. Volumetric efficiency depends on factors such as cam valve timing (affected by cam profile), gas velocities, exhaust efficiency blah blah blah. The VTEC system just works to maximise VE at all rpm points by running a less wild cam profile at lower rpm as not to let, primarily, fresh charge excape the chamber via overlap periods which are longer (in time, not crank rotation degrees) at lower rpms, and flow reversal. Vtecs make very high torque/bhp ratios, aswell as bhp/ltr ratios and they make a high % of their torque low down because the slow piston speeds means that a less wild cam profile can pass just the same amount of charge due to the larger time periods at lower engine speeds.
easy enough so far?
Turbos make more torque but can keep the same bhp as the same capacity engines because they simply run higer VE values due to the boost that fills the cylinder better than a simple 1bar atmospheric pressure. The VE values on turbo engines can exceed 100% and as a result the combustion has much higer dynamic CR values, meaning more torque at the crank pully. A 250bhp 2ltr 16V turbo makes 250bhp at circa 6500rpm, whilst a 250bhp, 2ltr 16V NA engine requires 9000rpm (Honda S2000). Turbos can increase torque and thus BHP without upping rpm by increasing boost values.
Sorry for the essay people, and poor punctuation.
Well, lets explain it this way too:
Energy = Force * distance
Lets take a simple example. You try to open a door (or move a car, for that matter). The energy required to move that door opened is a determined amount, as the door has a determined weight.
Now, you can open it in two ways:
1. Push it from near the door knob. You will need a certain force (lets say 10 Newtons and your hand will travel over a distanc of 1 meter - just another example - to get the door opened).
2. Push it from near the point where the door is fixed in the wall, when the ax of rotation is. You will need to push stronger, but your hand will travel a smaller distance, since the energy you have to deliver is the same, Force * distance. The bigger the force, the smaller the distance needed to get the same result.
In the engine something similar happens. The force of the explosion in the cylinder is applied to a rotating shaft. As the shaft rotates, the point the force is applied to travels a certain distance.
To conclude the analogy;
Force = torque
Distance = rpm
Energy = power
Since Energy = force * distance then power = torque * rpm. Or, in other words, power is directly proportional with torque and rpm. You can get more power either by increasing the rpm (keeping the torques the same) or by increasing the torque (keeping the rpm the same).
Now, what about real life and how torque and rpm affects the car behavior. The classic comparison is diesel versus petrol. A diesel engine has a big torque, but revs to a maximum of 4000rpm. A petrol engine, has a smaller torque, but it revs to a much higher rpm (6000, 7000, 8000 etc.).
How does this affect driving? Gnerally, big torque at low revs means a more confortable driving and more powerful acceleration bursts, for SHORT amounts of time. This is the regular driving. However, when racing things are a big different as there is one EXTREMELY important point, that people tend to ignore when comparing diesel cars to petrol cars. That factor is GEAR CHANGING. Indeed, the car with more torque will get faster through the revs, but as its limited to only 4000rpm, it will require more often gear changes, which - in REAL life driving conditions, with REAL drivers - mean seconds lost. If you hear that a car hit 60mph in 7s its not something special but when you hear it does it in 3rd... hmmm... it means its REALLY fast throughout 1st and 2nd, as that change from 2nd to 3rd takes some
There was somewhere on the forum a very nice analogy, with two guy riding bicycles. One is VERY strong, but cannot move his feet very fast. The other is weaker, but can move his feet VERY fast. Its exactly what torque and rpm are all about.
-BHP vs Nm
By definition, POWER = FORCE x DISTANCE ÷ TIME
Using the example where a constant tangential force of 100 pounds is applied to the rotating handle at a 6" radius, we know the force, so to calculate power, we need the distance the handle travels per unit time.
Power = 100 pounds x distance per minute
OK, how far does the crank handle move in one minute? First, determine the distance it moves in one revolution:
DISTANCE per revolution = 2 x pi x radius
DISTANCE = 2 x 3.1416 x 1 ft = 6.283 ft. per revolution.
Now we know how far the crank moves in one revolution. How far does the crank move in one minute?
DISTANCE per min. = 6.283 ft .per rev. x 60 rev. per min.
= 377 feet per minute
Now we know enough to calculate the power:
POWER = FORCE x DISTANCE ÷ TIME
so
Power = 100 lb. x 377 ft. per minute
or
Power = 100 x 377 = 37,700 ft-lb per minute
You already know that one HORSEPOWER is defined as 33000 foot-pounds of work per minute. Therefore,
HP = POWER (ft-lb per min) ÷ 33,000
So the power being applied to the crank-wheel above is 37,700 ft-lb per minute, or 1.14 HP (37,700 ÷ 33,000).
Now we combine some stuff we already know to produce the magic 5252.
We already know that TORQUE = FORCE x RADIUS.
If we divide both sides by RADIUS, we get:
FORCE = TORQUE ÷ RADIUS (a)
Now, if DISTANCE per revolution = RADIUS x 2 x pi
then DISTANCE per minute = RADIUS x 2 x pi x RPM (b)
We already know POWER = FORCE x DISTANCE per minute (c)
so if we plug the equivalent for FORCE from (a) and distance per minute from (b) into (c), we get:
POWER = (TORQUE ÷ RADIUS) x (RPM x RADIUS x 2 x pi)
Dividing both sides by 33,000 to find HP,
HP = TORQUE ÷ RADIUS x RPM x RADIUS x 2 x pi ÷ 33,000
By reducing, we get
HP = TORQUE x RPM x 6.28 ÷ 33,000
Since
6.2832 ÷ 33000 = 1 ÷ 5252
thus
HP = TORQUE x RPM ÷ 5252
wheres my point??!!
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