Todays question... Torque & BHP

[lynw]

ok... I know the SV is torquey and I know how that feels in comparison to the CBR...

but what exactly is torque? what does it measure?
and does this correlate to the power band graphs that bike mags like printing? or are they just measurements of bhp? and what exactly does the bhp figure mean? are the bhp and torque of a bike related??

please can someone explain cos I really would like to know...

[Carsick]

I'm not going to do the maths explanation, since it's been too long for me since my physics a-levels and I just can't remember it, but I will do the wordy bit.

Torque is the tendency to rotate. If you apply torque to something you are trying to rotate it.
The measurement is the amount of force at a distance from the centre of rotation (Force x Distance)

Power is energy per second (Energy/time). It's the same as effort, for example, if you're walking along a flat road at a certain speed, to walk up a hill at the same speed would take more effort, more power.

The graphs in mags will have a torque figure and a power figure.
In fact, the power can't even be measured, it has to be calculated from the torque produced and the revs.

Which brings us to how they're related.
power = (torque x revs) / k
where k is a constant that I can't remember.
(finishing my explanation)
So power is equal to the amount we are pushing, multiplied by how often we push, all divided by a constant.

[Ken McCulloch]

but what exactly is torque? what does it measure?
and does this correlate to the power band graphs that bike mags like printing? or are they just measurements of bhp? and what exactly does the bhp figure mean? are the bhp and torque of a bike related??

well in 30 seconds - torque is the strength of the push, hp is the rate at which the push is delivered. An analogy I have heard is having prop forwards pushing a Volvo. More forwards = more weight = more torque so more load (Volvo and contents) is accelerated. The prop forwards' little legs can however only run at say 15mph so this is where the power delivery collapses. Harness the same weight of horse as you had beefy blokes pushing, and you get more power, same weight = same torque but delivered more quickly, so with horses you get up to say 25 mph............ Put an engine that delivers the same torque but with a wider spread of power and a gearbox in and you can go up and down hills and much faster. Does that make sense?

[lynw]

remember someone explaining it along the lines of going up stairs...

torque was how many stairs you could go at a time... ie a higher torque bike was equivalent to going up stairs 2 at a time

then power was like saying that if you had a low torque, 1 stair at a time bike but it was higher revs it was like saying although you were doing 1 step at a time the power/revs meant you were doing it quicker so were doing it the same rate as the 2 step at a time bike...

and torque was the ease in which a bike would pull away...

is that about right? but what do those power graphs actually mean? they look pretty but Id like to actually understand what theyre saying... :oops:

[Carsick]

the stair explanation is right, yup.

What do they actually mean?
They are simply the torque output (and the calculated power output) at the full rev range of the engine.

Engines not being perfect, we have to make compromises on where we choose to make the torque and where we choose to make more revs.
Remember, power is made with either extra torque or extra revs.
But what if a modification to increase the revs changes the torque output? or vice versa.
So every change you make could change that graph, thus changing where in the rev range the torque is produced or changing the amount that it can be revved.

[lynw]

so basically if I understand this right...

the sv makes the power with torque while the cbr does it with revs...

which is why its easy to pull away on idle speed on the sv but I have to have the revs around 2-3k to pull away on the cbr...

and is this why the SV isnt such a top end performance bike like the CBR?

[Ceri JC]

Which brings us to how they're related.
power = (torque x revs) / k
where k is a constant that I can't remember.
(finishing my explanation)
So power is equal to the amount we are pushing, multiplied by how often we push, all divided by a constant.

Isn't there some crossover point at a certain number of revs, eg 4400rpm (not sure if that's the figure) where torque = bhp?

[Wiltshire7]

ive read and heard so many people descibe torque and power in different ways with different examples but tbh, its still completly a mistery when i put it in terms of an engine.

all i know is torgue means it'll pull away faster at low revs. whereas low torque will require alot of revving. hence the wan&er with an RS125 revving the nuts off his bike to make it go.

[Carsick]

Lyn, yup.

Ceri, yes, there is, and that's the constant I referred to. I can't remember the number, but for metric I think it's 5252 or something.

[Carsick]

which is why its easy to pull away on idle speed on the sv but I have to have the revs around 2-3k to pull away on the cbr...

and is this why the SV isnt such a top end performance bike like the CBR?
Just something to add onto this.
When you rev the CBR to whatever to pull away, you're probably producing about the same torque at that point as the SV does at it's pull away revs.

So, just because the SV is torquey, doesn't mean bikes that aren't considered to be particularly torquey can't produce the same amount.

In fact, I just looked at the figures, peak torque for a CBR600F is 63Nm at 10k rpm and peak torque for an SV is 64Nm at 7200rpm.

[lynw]

thank you... I feel very enlightened now... until this went in after your post... then I stopped thought about it and the light bulb is on and glowing...

its why the power band on the sv is around 7k and the cbr is around 9-10k.... yes?

[Carsick]

I feel a bit better as well.
I've never been fantastic at mechanics (the maths kind or the engine kind) and trying to explain it helped get it clear in my head.

[embee]

OK, let's go back to first principles (there's no magic! :lol: )

A petrol engine is an air pump, it uses the air to burn petrol, the amount of petrol it can burn determines how much energy you can release, the faster you burn fuel the more energy you release in a given time, which is power.

One thing to remember with petrol engines is that they run with more or less the same ratio of fuel to air (about 14.7:1 in theory) at all times, so you regulate the power by regulating the air flow with the throttles.

A diesel runs on a different principle, basically it has full air all the time and you regulate the fuel flow (it's to do with how the fuel burns).

The release of energy (burning fuel) in the cylinders generates pressure and temperature in the gas, which exerts force on the pistons. As the engine rotates the gas force pushes the pistons and so does "work". The amount of "work" depends on how high the pressure is and how much you expand it.

The amount of gas and the expansion depends on how big the engine is and how well filled the cylinders are (plus a few other odds and ends like compression/expansion ratio etc).

For convenience there is an engineering term used which describes what constant average (or "mean") pressure existing in the engine cylinders for the whole expansion stroke would have the same effect on a brake trying to stop the engine turning as does the real varying pressure, this is called Brake Mean Effective Pressure (BMEP).

The significance of this is that it allows you to compare the effectiveness of the combustion of different engines regardless of size, shape, number of cylinders etc on a fair basis.

Measured in "Bar" (1 Bar = 1 atmosphere approx, around 15psi), a decent 4-valve chamber will produce around 10Bar BMEP at 1/4 of it's max speed, 12Bar at max torque, and around 10.5Bar at max power.

BMEP takes into account the various efficiencies, volumetric (air flow as a percentage of the size of the cylinders), thermal (how well the fuel energy is released and used), and mechanical (friction, pumping of air, oil, coolant etc). There is also chemical efficiency, but you can usually assume 100% for practical purposes. If you want to compare just the burning you use Indicated Mean Effective Pressure (IMEP), which is what a cylinder pressure indicator would show (sort of :lol: ), but that's an aside.

OK, so our 4-valve petrol engine at full throttle and max torque speed is producing 12Bar BMEP. What torque is this?

Well, if the engine was V Litres swept volume, which is the air it displaces in 2 revolutions (4-stroke), the pressure times the volume equals the energy, but so does the torque times the angle, thus

BMEP (Bar) x V(Litres) = Torque (Nm) x Angle (Radians)

If you do the juggling and sort out the 2 "pi" radians per rev bit, you get

Nm = 7.958 x BMEP x V

so a 650cc engine (0.65L) and 12Bar gives

7.958 x 0.65 x 12 = 62Nm (or 6.2 Kgm approx)

Now, that's the "strength" with which the engine is trying to turn, but how quickly it's doing it determines how "powerful" it is, i.e. the engine speed comes into it. Don't get confused with "maximum torque", the engine produces "torque" at all speeds, because it is simply the way that the gas pressure shows itself.

Now, Power (kW) = Torque (Nm) x Speed (radians/sec)

But we already decided that Torque = 7.958 x V x BMEP

So, putting them together and juggling the rads/sec into RPM terms (the 7.958 comes from converting radians and revs, so cancels out eventually) we get (trust me)

Power (kW) = V x BMEP x RPM/1200

So our 650cc 4valve engine producing 10.5Bar BMEP at 9500rpm gives

0.65 x 10.5 x 9500/1200 = 54kW (72BHP)

Lastly, what determines the max power speed? Well, it's fundamentally the gas speed through the inlet valves. Once you start getting significant amount of the air flow at near sonic speeds, it becomes "choked" and going faster doesn't flow more air. This is termed the "Mach Index" (Z) in engine design circles, and the basic feature is the inlet valve area you have as a percentage of the bore size (hence the benefits of 4-valve designs).

A typical 4-valve chamber has inlet valve diameter around 0.35 of the bore diameter, so the area of inlet is about 2x (0.35 squared) of the bore area, or about 25%. Thus in simple terms the gas speed through the inlet valves is about 4 times the speed of the piston (very simplified, assuming the valves are fully open all of the time, which they're not :roll: ).

Anyway, because the valves are only part open some of the time the effective gas speed is higher than this, which is termed "discharge coefficient", and is typically about 0.47

The speed of sound (Mach1) in air around us is around 330m/sec.

You find that max power tends to come around Z = 0.5 (empirically) , and it looks like this

Z = ("mean piston speed" x "bore/valve area") divided by ("discharge coefficient" x "sonic velocity")

so

0.5 = ("MPS" x 4) / (0.47 x 330)

or max power comes around 20m/sec mean piston speed (4-valve engines). Mean Piston Speed is simply the total distance the piston travels per second, so is given by twice the stroke (per rev) times revs per second, or

MPS (m/sec) = 2 x (stroke (mm)/1000) X (RPM/60)

so our 650cc engine with a stroke of 62.6mm will reach this at

20 m/sec = 2 x .0626 x RPM/60

or speed = 9500RPM

Voila! :lol:

[Scoobs]

Blah, blah.....
Voila! :lol:

Yeah I know :roll:

[Steve H]

V-twins dont generally produce more torque than inline fours. In fact most il4 1000s will produce slightly more torque than say an aprilia RSV or Duke 999. What makes a v-twin seem to have lots more 'grunt' is the fact that the torque it produces is much lower down the rev range.
Therefore the V-twin will accelerate out of a bend quicker as it can get to the torque a lot quicker than the il4. Hence the phrase more 'Drive out of bends'. Not very scientific though! :?

[lynw]

:shock: I thought I had it there but thanks I think embee... that was a bit much for one sitting will come back to that later...

but is this basically why they could put the bigger bikes in the gp races?

[Steve H]

:shock: I thought I had it there but thanks I think embee... that was a bit much for one sitting will come back to that later...

but is this basically why they could put the bigger bikes in the gp races?

Yes. More so in superbikes the il4 s had to be 750 or restricted 1000s whereas the v twins are unrestricted 1000s. Primarily due to the fact that race bikes are always running at high revs.

[jambo]

Ah This old chestnut up again.

I'm happy just thinking of it as:
Torque: The actual twisting force at the crank
BHP: A calculated number generated from the measured torque (Force) and the rpm being multiplied together and de=ivided by a constant to give the total energy output as in if the engine's going twice as fast and producing the same force there are more bangs per seccond so more energy is being used. This is very important to calculate so that people can boast more down the pub.
:roll: :lol:

[Jabba]

There's one rather large spanner in the works that has to be remembered, too.................... gearing :thumbsup:

It's all very well having the bhp/torque curves, etc., but how fast or slow a bike accelerates, for example, is also determined by its gearing.

[lynw]

how so?

please explain... Im having an enlightening day... not much work done but enlightenment must come at a price :lol:

[jambo]

Quite Right Jabba, and now we are onto Thrust!
Thrust is the force exerted at the end of the gearing, so while the engine at 9000 rpm will produce say 64Nm of torque and 70BHp the thrust will be different in 1st to 5th gear as that force is gaered up to be pread over a different number of RPM of the rear wheel!

You can demonstrate this by opening the throttle at 6000 rpm in 2nd and 5th. At both points the engine has the same power, same torque but in a lower gear it has more thrust!

[lynw]

ah explains the cbr at 7-8k rpm in 3rd vs 7-8k rpm in 5th....

the honda certainly has some thrust... probably appropriate I said it launched down the road :lol:

and yes Im REALLY REALLY LOVING the HONDA in the snow.... my SV would be carb icing in what I was riding through last night and wouldnt start first time... :lol:

[Jabba]

You can demonstrate this by opening the throttle at 6000 rpm in 2nd and 5th. At both points the engine has the same power, same torque but in a lower gear it has more thrust!

I suspect that, because the engines can rev higher for mechanical reasons, my 600cc IL4 is noticably lower geared than the 650cc V-twin SV. I suspect that this is more than enough to negate the couple of ft.lbs of peak torque that the SV makes.

Simple question for the SV650-ers:

"What is your indicated speed in top gear at 5000rpm?"

Answers for the naked and "S" models would be interesting as, due to different sprockets, they are differently geared.

On my Hornet, 5000rpm in top is an indicated 70-ish mph.

[Carsick]

On an S it's 68 mph

[lynw]

being a new uncurvy s model... will look at what it is on the curvy model when I eventually get it back :roll:

will look at what the cbr does on way home tonight....

[Wiltshire7]

ill check my naked next time i go out on it.

i remember the 5k rpm limit though when i got my s brand new. wishing i could go over that damned 70mph limit!

[Jabba]

I suspect that my comment about lower gearing will be more valid for the lower/intermediate gears.

I have a speadsheet somewhere with the Hornet's mph/1000rpm on standard sprockets...... might be interesting to compare with an SV?

I know my bikes overgeared in 6th.......... if it would rev to peak power (12,000rpm) I'd be doing over 160mph :shock:

Well, the bike might........there's no way I could hang on at those sorts of speeds :lol:

[ophic]

don't think the SV is overgeared at all - i keep trying to find 7th :oops:

[K]

I've learned alot from reading this, thanks. :)

I've kinda known what the component 'bits' mean, I just lose it when trying to connect explainations together if you get what I mean! :oops: Until now that is.

Main reason why I really rate this site, you guys are helpful and look to educate people first, rather than put then down for lack of knowledge. It's nice.

So... how and why do BHP figures change between the engine and the back wheel?
I've heard people quote figures and then be told by others "aaahhh, but that's not at the wheel is it... blah blah... my figure is, therefore it's better... pose pose!"

I'd just like to understand what they are talking about and be able to recognise b*****ks, rather than actually participate in such ball weighing contests!

[Jabba]

don't think the SV is overgeared at all - i keep trying to find 7th :oops:

You're not alone........... :oops:

Right, the figures below gives the Hornet's gearing in mph/1000 rpm with standard sprockets. This assumes a 180/55 rear tyre with a rolling circumference of 1.94m, and 0.62 miles per kilometre.

1st = 4.7mph/1000rpm
2nd = 6.7
3rd = 8.4
4th = 10.1
5th = 11.5
6th =12.7

So at 5000 rpm in 6th, my bike has a road-speed of 5 x 12.7 mph = 63.5mph as oposed to an indicated 70ish.

This ties in with what the copper told me during a motorway trip - he compared it against his police bike's calibrated speedo and told me off for going too slowly :oops:

5500rpm would be around 70mph. Worth remembering, as is the 13 Times Table :lol:

[Jabba]

So... how and why do BHP figures change between the engine and the back wheel?
I've heard people quote figures and then be told by others "aaahhh, but that's not at the wheel is it... blah blah... my figure is, therefore it's better... pose pose!"

As a guide, knock off about 10% of the crank figures to allow for friction losses, etc.

[Dicky Ticker]

5grand=68mph and as a matter of interest 4th gear is very nearly
1000revs = 10mph Given a long enough run I just manage max revs in top gear I think the gearing is very relevant because using the same bike[600 kwaka] we are constantly changing the back sprocket on different tracks after all its no use having 160/170 top end if the bike is to high geared to get to max revs in top when if you raise a tooth or two you loose a fraction of your top speed but you accelerate much faster therefore you are much quicker
Every body on this forum seems to chance the back sprocket,is there a good reason for this because I was toying with the idea of changing the drive and leaving the back as is[1 tooth] giving my preference to better acceleration with a 5mph sacrafice on top end,after all you can,t cruise at
135mph anyway Comments to the later would be appreciated

[Carsick]

In case anyone wanted to know, the SV gear ratios can be found here http://www.powersport.co.uk/new/bikes/sv650s.html
I'm not in the mood to try and work out the speeds, though, so if anybody fancies it, feel free.

[ophic]

if you raise a tooth or two you loose a fraction of your top speed but you accelerate much faster therefore you are much quicker
although everyone seems to forget that having to change gear earlier does in fact slow you down a little.

[lynw]

hmmmm the cbr top gear at 5000 rpm was at 60mph... :shock:

now Im thinking a number of reasons for that - the FL was supposed to have 71BHP rather than the 90 most other CBRs have... also it needs the valve clearances looked at and the carbs balanced which I suspect may be afffecting its performance....

but get to 6000 rpm and im heading to 80 quite happily... and 9000 rpm... banzai!!! :lol: :lol: :lol: :lol: :lol:

[Carsick]

hmmmm the cbr top gear at 5000 rpm was at 60mph... :shock:

now Im thinking a number of reasons for that - the FL was supposed to have 71BHP rather than the 90 most other CBRs have... also it needs the valve clearances looked at and the carbs balanced which I suspect may be afffecting its performance....

but get to 6000 rpm and im heading to 80 quite happily... and 9000 rpm... banzai!!! :lol: :lol: :lol: :lol: :lol:
A knackered engine won't affect the road speed at a particular rpm. It'll only affect how quickly (or if at all) you can get to that speed.

[lynw]

then its not knackered then.... I can get to some serious speeds very very very quickly on the cbr :oops: :lol:

makes me wonder what its going to be like when its balanced and clearances are ok... :shock: