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BHP / Torque


Philj

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Hi,

I was wondering if people in the know could educate me on Power/BHP/Torque.

Whenever people are talking about engines, phrases like X lb/in at Xk RPM are bandied about and sometimes there is talk of tuning for torque at different RPM's. Whats is the relationship between torque and bhp ? How much of torque/bhp does it take to make 2 tons of land rover move ? How much of this BHP / Torque is 'lost'(?) in the transmission ?

Thanks in advance,

Phil

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One way to think about it is this:

If you tied a long rope to the crank pulley of the engine and hung a weight off the rope, Torque is how much weight you could hang off the rope before the engine wouldn't turn, but BHP is how fast it can lift it.

Torque is caused the by the bank in the engine which is caused by a mixture of air and fuel. The more air and fuel the bigger the bang and the more torque you get. However as the engine gets faster (ie the pistons start going up and down quicker and quicker) it can't pull in as much air and fuel as before so on each stroke the power begins to drop- BUT because you are getting more strokes per minute the total amount of bhp your getting still goes up

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I think this is like one of those illusion patterns you stare at, .you either see the picture or you can't :lol:

Torque is easiest described as turning power. Once you know that - this is the easiest description of BHP's relation:

An engine produces torque and brake horsepower is simply the product of torque multiplied by engine speed and couple of constants.

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Fixed it for ya:

Torque is easiest badly described as turning power.

Descriptions like that are what perpetuates confusion. :rolleyes:

Torque is torque, power is something very different. Torque is badly described by using the word 'power' in the definition. Power is a physical term in its own right and should not be used to mean 'kind of, oomph'.

Shackleton - I know what you are trying to say, but definitions need to be clear because the people who read them are not clear on the meaning of the word - by definition! :P

Torque is the moment (turning force) that the engine can apply to the crankshaft. Think of it as how hard it would be to stop the flywheel turning by grabbing it (!).

Power is the RATE of doing WORK. Pugwash explained that well.

Sorry for being retentive! Here endeth the lecture. ;)

Al :)

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Power is the RATE of doing WORK. Pugwash explained that well.

Except shouldn't it be...

Torque is caused the by the bank in the engine which is caused by a mixture of air and fuel. The more air and fuel the bigger the bang and the more torque you get. However as the engine gets faster (ie the pistons start going up and down quicker and quicker) it can't pull in as much air and fuel as before so on each stroke the power torque begins to drop- BUT because you are getting more strokes per minute the total amount of bhp your getting still goes up
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Amin to what Al says :) If you mess up with wrong words the whole thing will be a greater mess.

If someone "lost" the basics learned in school, it's advisable to try and "find" them 1st. By the way, in which grade do these things (Mechanics) get studied in UK?

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Duely put in my place :D

Yeah the picture's fading, fading, it's gone.

Does anyone really need to understand the concepts anyway, I think not LOL, key go in, car go.

EDIT: We don't get mechanics as a standard subject here as you guys obviously do. We're not the big industrial powerhouse you guys are see... I'm self taught and self professed :D

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Mark90 - yeah, sorry, I meant the power description about the rate of lifting the weight was good.

Shackleton - I figured you knew what you meant, I just wanted to state it clearly! No offense meant. ;)

Cipx - ironically, these terms are used from our teenage years but in all honesty I waffled like the best of them until I got to my PhD where my somewhat intimidating Serbian supervisor's favourite phrase was 'Vot are you talking about you idiot?'. You can get a long way before someone kicks your backside into communicating accurately. It gets more and more important as the concepts get harder and harder. :rolleyes:

Al.

:)

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Shackleton, I meant the core theory of Physics called Mechanics, other cores being electromagnetism (including optics), relativity, thermodynamics, quantum mechanics.

Al, I would agree to that (up to point, maybe). I was fortunate enough to have good teachers on one hand, kinda like physics on the other and have my father kick me in the right place at the right time on top of those :D.

I studied the basics needed for a proper understanding of the phenomena we're talking about in the 9th grade. 1st time in 7th grade but not enough, repeated them twice at higher levels after that.

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Shackleton, I meant the core theory of Physics called Mechanics, other cores being electromagnetism (including optics), relativity, thermodynamics, quantum mechanics.

Ahh that's where you're going wrong. Mechanics isn't Physics, Mechanics is Maths that physicians have stolen :D:P

I did the maths and physics of mechanics at A-level, so that was from 16 onwards although i never did any "mechanics" ni the automotive sense- i think the closest you get is with CDT (craft design and technology) which never taught us anything useful- didn't even teach us how to weld :(

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Ahh that's where you're going wrong. Mechanics isn't Physics, Mechanics is Maths that physicians have stolen :D:P

I did the maths and physics of mechanics at A-level, so that was from 16 onwards although i never did any "mechanics" ni the automotive sense- i think the closest you get is with CDT (craft design and technology) which never taught us anything useful- didn't even teach us how to weld :(

NERD FIGHT!

GO NERD BOY, TORQUE HIS HEAD OFF, GO NERD BOY, IT'S YOUR BIRTHDAY

:lol:

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Hi,

I was wondering if people in the know could educate me on Power/BHP/Torque.

Whenever people are talking about engines, phrases like X lb/in at Xk RPM are bandied about and sometimes there is talk of tuning for torque at different RPM's. Whats is the relationship between torque and bhp ? How much of torque/bhp does it take to make 2 tons of land rover move ? How much of this BHP / Torque is 'lost'(?) in the transmission ?

Thanks in advance,

Phil

Do you want the 5 minute version or the full half hour ?

In Putting this together I would like to thank Thor Racing, Puma Racing, and Jeremy Fearn.................. also I only know petrol engines .......... although the same principles apply to stinkin dervs I don't have too much experience in tuning them ................

Firstly look at my signature .................. a very famous quote from Carrol Shelby, probably the greatest race engine designer / builder of all time.

Torque ........... I have always liked this description.............

OK, if you have a centre nut on one of your wheels and then put a socket on it with a 1ft bar. The car will be at rest until you put a weight on the end of the bar to try and turn the wheel. So if you now put 50lb on the end of that 1ft bar and the car moves, the force that has moved that car from a standstill, at that speed, is 50 lb ft. So if the engine of that car produced the same amount of twisting force at its peak it would have been said to have a Torque of 50lb ft.

Now if you take that theory one step further and double the weight on the end of the bar and let go, the wheel would rotate again, only this time because the amount of force pushing the bar down was greater, the wheel would move off quicker and with more ease. Apply this once again to the engine producing the force and it would have been said to have a Torque of 100lb ft.

So you can see the more Torque you have the quicker the wheel would move off from stationary.

This is all fine and good on a light car as it means my 0-62 time will be decreased?

Yes the theory does indeed point to that, however do bear in mind that if a car produces 100 lb ft of torque and your car does 0-62 in 10 seconds, it is NOT going to do 0-62 in 5 seconds if you give it 200 lb ft of torque due to the inefficiencies of the engine, transmission etc, but it will be significantly reduced.

The same theory applies to overtaking. The more force you can use to make the wheels go from say 50mph to 70mph the less time it will take to get there.

OK ............. we have Torque nailed so lets look at HP (Horse Power)

Lets start at the dawn of time (about the same time that Les Henson and Pugwash were born). Any "engine" was obviously going to be compared to the ability of the then main power sources to do labour - horses & men. As most of the devices were used drive industrial equipment the natural comparison was therefore the horse. So, even the pioneers of the time realised that marketing of the new fangled machinery was important so they likened the power of their devices to a certain number of horses.

A famous engineer called Captain Thomas Savoury reasoned that if it took eight to ten horses to operate a mine pump 24 hours a day (two horse working at a time with the next pair taking over when the last two became tired), then a mechanical device that did the same job in the same time had 10 - 12 hp! You can move on through history with this through the likes of James Watt el al

So Horsepower has been determined into a measurement of mechanical power that is clearly defined ........ 1Hp = 33000 lbs/ft per minute or if you care to make the calculation 745.69987158227022 Watts

OK .......... Brake Horse Power..........

This is power that has been measured on a brake (better known as a Dynamometer). This device provides a load for the engine to "drive" against and then measures the torque produced by the engine. A simple calculation of engine RPM against the measured Torque provides the HP figure............ or the BHP figure, which is the same number but a better description of how it was derived.

OK ......... The Calculation..............

BHP(or HP) = Torque (ft/lbs) x RPM/ 5252

How is Torque produced ...........................

This is the million dollar question that vexxed engine designers for years...............but is now very well understood.........

The pure theory is that torque has nothing to do with engine speed . The torque figures depend on the mean effective pressure in the cylinders (MEP), which is calculated by taking away the total of the average pressures on the induction, compression and exhaust strokes from the average pressure on the expansion strokes.

Theory is all very well but in reality the MEP of an engine decreases at high speed and the torque drops off. So the MEP is calculated from the Brake Horse Power (BHP) figures for an the engine, taking into account the inefficiency of the engine, so now the MEP becomes the Brake Mean Effective Pressure or BMEP, which is measured in lb in sq.

So taking this one step further, if you understand MEP .............. then you can design your engine to have a peak torque in the rev range that you need it. With LR engines for off road use we need as much toque as possible at low RPM.............

So what are the transmission losses ..................

Often you see transmission losses expressed as a % ................... in reality this is only relative to the engine power at the flywheel. The transmission losses generally expected for a '90' or '110' are about 70bhp, but this figure fluctuates wildly ............. which is why you can do 3 runs on a rolling road and end up with different results of BHP / Torque at the wheels. The losses have many variables ............ gearbox(s) oil viscosity, diff oil viscosity, transmission temperature, tyre tread pattern, tyre pressure, tyre heat, wheel bearing adjustment, brake adjustment .............. and so on..................but I can tell you that you can reduce the losses to as low as 40bhp if multigrade EP oils are used, slack wheel bearings and the transmission hot......... but that is unusually low............. normally its 50 - 90 bhp...........but its important not to confuse BHP with torque.

Finally the final engine tune has a dramatic effect on Torque and BHP ................... engine timing is a 'torque issue' .......... engine should be tuned for MBT......... which is known as Minimum Best Torque...........in other words you want the minimum timing advance that produces the best torque figure.

I hope this helps the understanding ................... I guess from the engine tuner / rebuilders point of view the most important area to understand is MEP and how we can improve that figure by increasing the CR, better breathing, cam profiles etc ..................that way we get the torque into the correct rev range for our application........................

HTH .........

:)

Ian

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I studied 'mechanics' in school in physics classes and then motor vehicle mechanics at city and guilds level in 1984 and again as a mechanical engineering degree in 1986. But I really started in 1972 when my Dad gave me a carburetter off his Land Rover to play with in my toy box and then an encyclopedia of science when I was 8 years old.

I can explain torque and power and bhp and so on but I need to use my hands to demonstrate and a whiteboard to draw on. I did once make a cutaway of a Ford Escort engine and gearbox to demonstrate this at college for my C&G and then I left it with the college as a teaching aid.

From what I remember, as a rule of thumb, if you want high speed performance you want your peak torque high in the rev range and close to the peak power and then set your gear ratios so that they are close and keep the engine speed between the peak torque rpm and peak power rpm.

If you want good economy you have your peak torque low in the rev range and peak power high on the rev range and set your gear ratios wide, again to match the peaks.

If you want to tow heavy loads then you need to have a wide, flat torque range across the engine speed range and set your gear ratios to keep the engine running within the flat of the torque curve. The power of the engine will be a lesser influence except to limit how fast you can tow that load.

As has been said Power = Torque x RPM x some constant to sort out the units.

Torque is the turning effort, Work is the turning effort actually turning, power is the rate that turning effort does the work.

You can't have any power without some torque but you can have torque without any power when rpm is zero and no work is done as in the stall situation with a fluid flywheel.

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Thanks for all the replies.

Often you see transmission losses expressed as a % ................... in reality this is only relative to the engine power at the flywheel. The transmission losses generally expected for a '90' or '110' are about 70bhp, but this figure fluctuates wildly ............. which is why you can do 3 runs on a rolling road and end up with different results of BHP / Torque at the wheels. The losses have many variables ............ gearbox(s) oil viscosity, diff oil viscosity, transmission temperature, tyre tread pattern, tyre pressure, tyre heat, wheel bearing adjustment, brake adjustment .............. and so on..................but I can tell you that you can reduce the losses to as low as 40bhp if multigrade EP oils are used, slack wheel bearings and the transmission hot......... but that is unusually low............. normally its 50 - 90 bhp...........but its important not to confuse BHP with torque.

The Land Rober 2.25 Petrol only produces about 70bhp - how does it even move ! (I assume this is why people are so keen on free wheeling hubs?)

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Philj - its losses in the system due to friction etc, so its always going to be a percentage of whats put in, not an absolute value. The percentage will not be constant, but will change with temperature, wear etc, but its always going to be some fraction of whats put in, rather than a fixed threshold, below which no work is done in actually rotating the tyres.

Cheers, Al.

:)

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Philj - its losses in the system due to friction etc, so its always going to be a percentage of whats put in, not an absolute value. The percentage will not be constant, but will change with temperature, wear etc, but its always going to be some fraction of whats put in, rather than a fixed threshold, below which no work is done in actually rotating the tyres.

Cheers, Al.

:)

Yes Al, that is all well and good, but answer the man's question - how does a 2.25 actually move..??? :lol::lol::ph34r::P

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Ahhh I see, so the engine is attracted to some other mass, (presumably a large beard and a flat cap :P ), and is inextricably drawn towards it?

So they only work in reverse then as the beard and flat cap would be behind the engine? Or is there another one in front with a red flag?

Coat on and running! :ph34r:

TTFN.

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