[doughboy]

A bit techy this thread.....but as we don't have a UK tech list i'll put it here.

If you're too clever / bored of this subject then please stop here.

And it's not anti-diesel before anyone says so.

Everyone these days jumps on the torque bandwagon (no newton metres here, we're British so lb/ft only please) but does everyone understand the issues here? I'm no expert, but i wanted to try and clear up a few points....

Quoting 'power' is fairly straighforward because power out (at the wheels) = power in (at the crank) - losses on the way (tranmission etc).
So 'crank power' is a good figure with which to compare one car with another without having to know anything else, assuming losses are roughly similar.

Now torque on the other hand, is a force that is multiplied by gear ratios and wheel diameters etc, so quoting 'crank' torque figures is fairly useless for comparing different car models. It shows the engine turning force delivery characteristic, but that's it.

After all it is torque at the road wheel which overcomes friction to propel the vehicle forwards. The wheel torque / wheel speed relates directly to wheel power, so wheel torque is a more useful figure. Crank torque is not however.

I've compared 2 cars, 330i and 335d, in the graphs below. The final drives and gear ratios are taken into account for each car and using power/ torque figures for standard engines it shows the wheel torque in each gear 1st to 6th. Underneath are calculated top speeds in each gear at given rpms and tyre sizes (same wheels/tyres for both). You can see the wheel power is of course the same in each gear, but wheel torque decreases dramatically as higher gears are selected. As petrols rev higher you can select a higher gear later, thus maintaining higher wheel torque for longer.

This is the important point: Diesels have higher ratio gearboxes to overcome the narrower rev range, this in turn multiplies to a lesser extent the wheel torque compared to a lower geared petrol car. So often the higher crank torque figure of a diesel engine is lost due to the higher gearing at a given speed.

You can see that the 335d has an enourmous wheel torque advantage in 1st at around 4,750 lb/ft (although I believe that torque is limited in 1st/2nd gears by the ECU to stop mass wheel spin and broken parts), but this advantage is soon lost through the gears.

This shows that after 1st gear, and by using the correct gear selection of 2nd 3rd and 4th in the 330i (30 - 130mph), the 335d has little wheel torque advantage over the 330i. Again, it is the higher 'power' of the 335d which wins over in the end. (and giving the 330i a sound thrashing while the 335d wafts along!)

The diesel of course has high torque available from a point in the revs where you are more usually operating at, i.e. general cruising speeds, so you 'feel' the car is more 'powerful' when you press the throttle. Of course, changing down two or three gears in the petrol car has the same effect, but seems a lot more effort and 'thrashing' the car.

I'm not trying to put down any engine or fuel type, just saying that when using crank torque figures, there's a lot more to it than just that number.

I for one couldn't give a toss about engine torque, the power curve shows the torque by definition, its an irrelevance really IMO except to show if you can be lazy with your gear changes.

But i suppose the derv's have to have a figure to beat the petrols with? ;-)

Please reply if you can be bothered to read all that guff!

PS also attached 335i v 335d, you can see they are much closer and in sixth they have nearly the same drive ratio and thus same wheel torque....the higher revving 335i going on to produce more power.

[- Paul -]

I read it - It made no difference to me - the 320d is faster than the 320i - end of.

[biscuits]

I read it too.... mine still goes round corners faster

[doughboy]

Originally Posted by - Paul -

I read it - It made no difference to me - the 320d is faster than the 320i - end of.

Absolutely correct Paul - because the 320d has more 'power' than the 320i, so given the same weight it will be faster.

Crank Torque, as an arbitrary figure, has got nowt to do with it.

[xenon]

Originally Posted by - Paul -

I read it - It made no difference to me - the 320d is faster than the 320i - end of.

7 bhp more power but 60kgs heavier. I'm sure that will makes it *loads* faster... Or, for a '05 320d, 7 bhp less and 77Kg more. Proper rocket ship now

[biscuits]

Originally Posted by xenon

7 bhp more power but 60kgs heavier. I'm sure that will makes it *loads* faster...

Doesn't sound like a tractor though....

[- Paul -]

Originally Posted by xenon

7 bhp more power but 60kgs heavier. I'm sure that will makes it *loads* faster...

Not loads faster - just faster - I'm only going off the official BMW figures released for the cars though

what is interesting though is the 318i vs 318d figures - both have the same power but the petrol IS faster. (Unless its an automatic)

[doughboy]

Originally Posted by - Paul -

what is interesting though is the 318i vs 318d figures - both have the same power but the petrol IS faster. (Unless its an automatic)

That's because the 318d weighs 75kg more than the i (1 x 12 stone passenger permanently on board) so the acceleration rate will be slower, but top speed in theory will be the same.


Come on.... somebody start quoting newton/foots or whatever!

[- Paul -]

Why is the D faster in auto though?

[xenon]

Presumably the ratios of the auto-box better suit the characteristics of the d.

[creepy coupe]

Reciprocating mass is also a factor, put the same weight components in each engine and the end results will be very different, put heavier parts in the petrol engine and it will gain torque, and lighter parts in the derv burner and that will gain HP

[dxb335d]

Torque is not just a by product that makes a car easier to drive quickly. It does help with acceleration.

Two cars, idenctical weight and size and BHP but one has 100ibs more torque, That one will be quicker.

[AlanQS]

I think what Doughboy is on about Carl is this: A 335d in say fourth gear at 50mph will walk away from a 335i in similar situation.

I assume the point is that, in real life, the 335i would drop to third or second and then the 335d would not walk away at all.

So, to that extent, the argument is sound, that the extra torque primarily allows you to be lazy with the gear change.

It also possibly allows you to maintain acceleration even when heavily laden - better than a petrol would and it certainly aids in fast but relaxed driving - now we're back to lazy gear changing again. (335d is maybe a bad example since it is automatic - but it applies to other models in the range.)

So, is there any other intrinsic benefit in the diesel's characteristics - don't say better fuel economy - currently with short distances being travelled, I have absolutely no chance of ever recovering the premium for diesel engine from fuel savings.

[creepy coupe]

If we go back a good few year diesels were very rarely fitted with turbo chargers and fitted into commercial load lugging vehicle, with a very low top speed.

Since fitting them to car became the thing to do, making them more driveable and refined has lead to Turbo charging and six speed gearboxes, every generation of new diesel engine has more further from there original concept, engine parts have got lighter and smaller which works against the diesels strengths, so force feeding has become a must do.

Apart from not having to change spark plugs and HT leads on a damp morning, I can’t think of another advantage.except for tank range, but that's almost the same as MPG

[doughboy]

Originally Posted by dxb335d

Torque is not just a by product that makes a car easier to drive quickly. It does help with acceleration.

Two cars, idenctical weight and size and BHP but one has 100lbs more torque, That one will be quicker.

+100lbft more torque at the same rpm will mean more power, so it will be faster yes.


If you had two 335i's and you chip one for +100lb/ft, that increases the BHP anyway, so it will be faster as it has more power.

Unless it made more torque at less revs (like a diesel), thus making the same BHP lets say.

In that case it would be in a higher gear to keep the revs at the lower level at the same speed, thus helping to cancel out the crank torque increase.

TO BE FASTER IT NEEDS MORE POWER! - derived from crank torque via appropriate gearing!

[dxb335d]

Ok then explain why a 380bhp 335i is as quick as a 414bhp M3 but with similar weight. Because one has over 400ibs torque and the other 295ibs.


My car wouldnt bang in 13.3's all day long with a mere 345bhp and 1600+kg's Its the torque thats hauling its ass up the strip.

[- Paul -]

Here's some info I posted a while ago! Back in 2006 actually but the info is still the same......

http://www.e90post.com/forums/showpo...4&postcount=17

Torque & BHP Technical Information

What is all this talk of torque?
Torque is the power of a rotating force, which is the product of one of two equal, opposite, and parallel offset forces and the distance between them. When this is applied to a car this means the effort exerted on a shaft to move the vehicle along. When torque is great enough to move a shaft through a given distance in a given time this is expressed as power and measured as horsepower.


Eh?
OK, if you have a centre nut on one of your wheels and then put a socket on it with a 1ft bar (bear with me on the imperial). 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.


What about overtaking?
The samne 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.

Why is torque important if I tow a caravan etc?
The more torque you have the more weight you can move forward from a standstill. Or in other words if the wheel you are trying to turn is stiffer it may not turn at al with 50 lb ft - it requires more force to turn it. Therefore if you increase the force by 50lb and the wheel turns then need 100 lb ft torque to move it. Apply that to a car with a trailer, you require more force to propel that whole unit (car and trailer) forward than you would with just the car. So once again with more torque you can move your car and trailer off from a standstill quicker and with less effort than you could before.


What about wheel spin?
With more torque available the quicker you can move the wheels from stationary. When this happens the wheels can spin before the vehicle has chance to move and you of course waste energy and move off slower. Therefore you need to alter your driving accordingly if the torque has been increased.


OK before I nod off, how is torque calculated?
The theory (oh no - not that word again!) is that torque has nothing to do with engine speed (revolutions per minute (RPM)). 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. That's the theory.....

In reality the MEP (you should have read the previous paragraph) 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! Phew - that was easy.


BRAKE HORSE POWER

What is "Horsepower" or HP?
Lets start at the dawn of time - or at least the beginning of mechanical devices. Any "engine" was obviously going to be compared to the ability of the then main power sources to do labour - horses, men and oxen. As most of the devices were used drive industrial equipment the natural comparison was therefore the 2/1 Favourite at Ascot. So, even the pioneers of the time realised that marketing of the new fangled machin'rey was important so they likened the power of their devices to a certain number of horses.

Moving on from this early start, 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 but you are not here for a history lesson!


So what is "brake" Horsepower or BHP
Well simply this is power that has been measured on a brake or normally known now as a Dynamometer. This device provides a load for the engine to "drive" against and then measures the torque produced by the engine, which if it is then multiplied this by crankshaft revolutions per minute and adjusted with the standard figure, it provides a horsepower figure

[zltm089]

Originally Posted by - Paul -

Not loads faster - just faster - I'm only going off the official BMW figures released for the cars though

what is interesting though is the 318i vs 318d figures - both have the same power but the petrol IS faster. (Unless its an automatic)

I think the petrol would only be faster, if coming out of a corner, the driver has managed to keep the revs at higher rpm...as In low revs the diesels would tend to be faster due to more torque...hence, let say if coming out of a corner...ur in 3rd gear..in a diesel car and at 3000 rpm....you would probably feel the pull.....whereas in a petrol, u 'd be more likely to hav to kick down... based on my experience with Hondas where these cars have very low torque, hence the need to keep the rpms high within the vtec range, to actually "feel" the power....


I'm only referring to how to drive your car here, and not about all the theory of power/ torque/ bhp theory....

[acerboo]

lost me im afraid but i will never buy another petrol car unless its an aston. one interesting point reading a test on an m3 e92 the writer said he could have done the long distance trip overall quicker in a 335d because he would not have had to keep stopping to refuel!

[dxb335d]

Me and Jerome were very close the other day, E92M3 vs 335d tuned.

[biscuits]

Originally Posted by dxb335d

Me and Jerome were very close the other day, E92M3 vs 335d tuned.

I wish I had understood what the feck you were doing on the A14 Sunday when you pulled along side, beeping your horn... we'd have seen how close you were then

[zltm089]

Originally Posted by dxb335d

Me and Jerome were very close the other day, E92M3 vs 335d tuned.

LOL ...I was close as well!!! well a few cars away....