BHP- V -Torque

[emmitdb]

Greetings All,

I have permission from SWMBO to ask this question as we are in the embryonic? stages of deciding the next
tow car.
We have an 2002 Audi A6 Avant 2.5 with a multitronic gearbox. The car has plenty of BHP @ 163 but, compared with more modern turbo diesels, it seems to be short of torque which is about 226ft lbs.
My question to all the engineers out there is this.
I have seen cars out there that have less BHP but more torque than ours. Would it be better to forgo the BHP in place of torque or is it better to have the bhp and less torque. ( I do appreciate that in the perfect world you should have an abundance of both but the money pit could be a deciding factor)
Is there a 'formula' that could be attached to such a question.
I am not asking for recommendations as to which vehicle does this and that but would appreciate an answer from a suitably qualified engineer.

 

[RogerL]

For hills and acceleration, power is everything - anywhere else torque is the master - there's no simple answer but everyone else will have one.

 

[MichaelE]

Sorry i have to disagree with RogerL.
A very simple explanation torque helps with acceration and bhp will carry it to the top speeds.(Power is the rate at which work is done)

 

[Lutz]

To compare torque with bhp is a bit meaningless without knowing at which respective engine speeds they occur. A high torque at low engine speeds makes for comfortable driving, especially when towing, because you don't have to change gear so often, but it's the bhp that determines whether you can get up the hill or not. Maximum bhp normally occurs at a higher engine speed than maximum torque (for a diesel the difference is usually less than for a petrol engine) and this means that you need to change gear more often and keep the revs up more to get the same result.

 

[DeliDave_]

Signature from someone on a car forum:-
"Understeer" is when you hit the fence with the front of the car. "Oversteer" is when you hit the fence with the rear of the car. "Horsepower" is how fast you hit the fence. "Torque" is how far you take the fence with you

 

[WatsonJohnG]

Signature from someone on a car forum:-
"Understeer" is when you hit the fence with the front of the car. "Oversteer" is when you hit the fence with the rear of the car. "Horsepower" is how fast you hit the fence. "Torque" is how far you take the fence with you

Very good
Torque is the cart horse that can pull BHP is the racehorse that has speed

 

[JTQ]

Probably an easier way of looking at it is anything that happens at engine speeds above those you are comfortable using is irrelevant.
So enticingly high power figures yielded at screaming high revs are no use, at least for me.
I like a high, low speed torque well maintained to yield peak power at circa 3000 crank rpm

Also you only need to have enough “power” [the term used advisedly] to do the task at a rate you want to; any more power just enables things to happen faster. A figure often quoted for towing caravans is a minimum of 40 Bhp/tonne, I feel that whilst arguably this is ‘comfortable’ it is lower than I would like. I presently tow with 44 Bhp/tonne but would like a bit more probably about 48, but would think it an indulgence? Remember that that the power figure here must be achieved at "acceptable to you" crankspeeds.
A vehicle with such a power level would by my standards be sufficiently rewarding to drive solo on public roads.

In the final analysis if the installed power is too low towing is tedious to you and other road users. If the installed torque is too low then there is a lot of revs needed and either the auto or you will be doing a lot of gear changing “work”.

 

[xtrailman]

Greetings All,

I have permission from SWMBO to ask this question as we are in the embryonic? stages of deciding the next
tow car.
We have an 2002 Audi A6 Avant 2.5 with a multitronic gearbox. The car has plenty of BHP @ 163 but, compared with more modern turbo diesels, it seems to be short of torque which is about 226ft lbs.
My question to all the engineers out there is this.
I have seen cars out there that have less BHP but more torque than ours. Would it be better to forgo the BHP in place of torque or is it better to have the bhp and less torque. ( I do appreciate that in the perfect world you should have an abundance of both but the money pit could be a deciding factor)
Is there a 'formula' that could be attached to such a question.
I am not asking for recommendations as to which vehicle does this and that but would appreciate an answer from a suitably qualified engineer.

You should be looking at a turbo engine, diesel or petrol.
But with max torque over as wide a rev range as possible.
Preferably with max torque coming in low, say 1500 rpm, although most seem to be around 1750rpm.
Aim for around a minimum of 150 bhp, with 250 lb/ft of torque.

 

[RogerL]

Sorry i have to disagree with RogerL.
A very simple explanation torque helps with acceration and bhp will carry it to the top speeds.(Power is the rate at which work is done)

Well you'd better apply for a Nobel prize - you're trying to rewrite the laws of physics.
Power is indeed the rate at which work is done - and the more work done in a given time the faster the acceleration and hill-climbing.

 

[Dustydog]

I'm reading a book about the Midland Red Motorway Express coaches of the 60s.
The CM5T was a 6 cylinder in line turbo diesel 8.028 litres.
It churned out 138 BHP at 1,900rpm and a maximum torque of 430 lb ft at 1,400rpm.
This coach could cruise all day long at 80mph plus.
Food for thought perhaps

 

[JTQ]

I'm reading a book about the Midland Red Motorway Express coaches of the 60s.
The CM5T was a 6 cylinder in line turbo diesel 8.028 litres.
It churned out 138 BHP at 1,900rpm and a maximum torque of 430 lb ft at 1,400rpm.
This coach could cruise all day long at 80mph plus.
Food for thought perhaps

I don’t doubt it on the flat but fully loaded, even when towing my van I would not want to be behind it in hilly terrain, both from progress and pollution.

 

[otherclive]

Diesels are best for towing but you do not say what type or year of car you are looking at. If it's going to be your only car make sure that if it is a diesel with a DPF that your short journeys are counter balanced by longer ones otherwise you may find that the DPF fails to regenerate. I have a XC70 205ps which is a great towcar and in 21k has not given any DPF problems but that is because we use the smaller non DPF Note for shorter journeys. This is okay whilst we run two cars but I would seriously consider letting the XC70 go if we went down to one car and then bought a petrol XC 70. Whilst the mpg would be lower it would not have any DPF issues and over 15k miles per year would work out cheaper than the diesel as modern petrol engines are now less expensive to maintain than the highly rated diesels.

 

[RogerL]

In the situation where a DPF might be a problem, ie low mileage, short journeys, then a petrol engine is a better choice anyway.
Too many low mileage owners have jumped on the diesel bandwagon when really they should have bought a petrol car.
Modern petrol engines are becoming as expensive to maintain as diesels because they're downsizing and adding turbos but are closing the fuel consumption gap somewhat.
There's no one break-even mileage between petrol and diesel - it depends on the nature of the usage as well as the "era" of the car - a lot of fixed recommendations about what mileage to choose a diesel at were based on old diesels and old petrol engines which bear no relation to what's available today.

 

[Shirker]

All this torque and BHP is missing the point, what you need is 'oomph'.

I noticed the difference between BHP and torque for towing once when shifting a large trailer with a mate's 2.something litre petrol speedy car, when I was used to pulling it in my 1.8 litre not-as-speedy turbo-diesel. The petrol car ran out of 'oomph' far more readily than the diesel did when towing up the Air Balloon hill near Cirencester.

 

[emmitdb]

Greetings Again,
Thanks for the replies so far.
As stated |I already have a fairly gutsy vehicle. I know that diesels are better than petrols for towing (This is the PC forum after all)
My car I believe has a particulant filter and it has not caused me any problems in the 5+years I have owned it (and I do 15000 miles a year) so the short journey prob isn't one that would or should occur.
If I'm getting any feedback from here it would seem that my initial thoughts were correct. Whilst you need the BHP to start with, it is enhanced by a good supply of Torque, something that my car seems to be a little lacking in.
Keep your thoughts coming. I want to know.

 

[Whittakerr]

I don’t pretend to understand any of this technical stuff, but just looked up the performance figures for my car (2.0 diesel) which are stated as:
Max output – Kw/hp/rpm - 135/184/4000
Max torque – Nm/rpm - 380/1750-2750
I’ve not had any problems towing my challenger 570, but I am interested in what all you techies think of these figures.

 

[JTQ]

Max output – Kw/hp/rpm - 135/184/4000
Max torque – Nm/rpm - 380/1750-2750
The torque curve looks good, it is flat being 380 Nm from a respectfully slow 1750 rpm, maintained through to 2750 rpm and then only dropping a little to 322 Nm at the peak power point way up at 4000 rpm.
Whilst the peak power point might be above most users tolerable engine speed range the flat torque curve indicates nicely available pulling right through the speed range. There is no sweet spot or cliff edge to fall off, in fact a very nice power delivery where you are not going to run out of "go" if the vehicle is sensibly matched to your caravan.
For a two litre diesel it does not get much better as a tractable engine with the present state of the art.

 

[Dustydog]

Max output – Kw/hp/rpm - 135/184/4000
Max torque – Nm/rpm - 380/1750-2750
The torque curve looks good, it is flat being 380 Nm from a respectfully slow 1750 rpm, maintained through to 2750 rpm and then only dropping a little to 322 Nm at the peak power point way up at 4000 rpm.
Whilst the peak power point might be above most users tolerable engine speed range the flat torque curve indicates nicely available pulling right through the speed range. There is no sweet spot or cliff edge to fall off, in fact a very nice power delivery where you are not going to run out of "go" if the vehicle is sensibly matched to your caravan.
For a two litre diesel it does not get much better as a tractable engine with the present state of the art.

So like the Midland Red Motorway Express Coach you agree torque is the key issue??????

 

[JTQ]

So like the Midland Red Motorway Express Coach you agree torque is the key issue??????

That midland bus whilst having good low end torque for sure would not maintain it like the brilliant turbo technology in the above engine does.
You cant simply separate torque and power they are linked, power after all is only the product of torque and crank speed.
What we need is like the engine above plenty of low end torque that is maintained right through the usable crankspeed range and the secret in getting that is sophisticated turbo technology. The inevitable consequence of that flat torque curve at increasing crankspeed is also high power, they are unseperatable.

 

[RogerL]

Turbo diesels don't have naturally flat torque curves.
Turbo diesels have a natural torque curve that's actually even more peaky, at around 2,000rpm, than non-turbo diesels - but engine manufacturers get over this by fitting an even bigger turbo and then electronically limiting the torque to a level figure, typically from 1,800 to 2,500rpm.
This gives two advantages for owners - it's more economical as a much lighter transmission can be used by limiting the torque - it makes it less important to be in the correct gear.

 

[Gafferbill]

……..F1 has proved that engine characteristics are not everything!
7 gears ratios and millisecond gear changes transmit enormous power to the drive train from what are high revving small torque spread engines.
The OP’s Audi Avant is fitted with a multitronic gearbox …..a seemless gear shift, with I believe 6 ratios. It will make optimum use of the engine power and available torque in a way that another vehicle that it is being compared to may not.

You cannot just compare one set of engine performance figures against another if you are seeking a guide to actual on road performance and towing ability.

 

[otherclive]

To obtain a better torque curve ( and efficiency) a lot of modern diesels use variable nozzle guide inlets to the turbo, as was fitted when Kia uprated the Sorento engine circa 2006. Also twin turbos are becoming more common, my XC70 has twin turbos and has no peakiness in the torque delivery as the small turbo looks after the lower rev range then the larger one comes in when the revs are higher.

 

[xtrailman]

I have covered only 18K miles in nearly 3 years, actually its somewhere over 17K, and never seen a DPF light.
I know some 2008 T31 have the light coming up, but not my 2009 model.
So really i think there is a lot of misinformed assumptions regarding DPF.
In truth no engine benefits from short journeys, whether petrol or diesel.

 

[xtrailman]

Greetings Again,
Thanks for the replies so far.
As stated |I already have a fairly gutsy vehicle. I know that diesels are better than petrols for towing (This is the PC forum after all)
My car I believe has a particulant filter and it has not caused me any problems in the 5+years I have owned it (and I do 15000 miles a year) so the short journey prob isn't one that would or should occur.
If I'm getting any feedback from here it would seem that my initial thoughts were correct. Whilst you need the BHP to start with, it is enhanced by a good supply of Torque, something that my car seems to be a little lacking in.
Keep your thoughts coming. I want to know.

Petrol engines with a turbo can be excellent towcars.
I had a Audi 1.8T that gave 180bhp, and only 171 lb/ft of torque, but it performed well as a tow car. 0-60 in 8 secs, up to 145mph. Solo
The max torque was available from 1950 rpm up to 4500 rpm.
More modern petrol engines have higher torque over a even wider rev range, i can't think of a disel engine that can do that?
The main reason why diesel is the popular choice is the economy, it certainly isn't because of refinement.

 

[xtrailman]

I don’t pretend to understand any of this technical stuff, but just looked up the performance figures for my car (2.0 diesel) which are stated as:
Max output – Kw/hp/rpm - 135/184/4000
Max torque – Nm/rpm - 380/1750-2750
I’ve not had any problems towing my challenger 570, but I am interested in what all you techies think of these figures.

Your figures are around the same as the BMW X3, i would be very happy with those figures.
But if it is a X3, I've read owners are getting no where near 50mpg, more like low 40's ?