Power to Weight versus Torque to Weight

[murph7355]

Tony - 😬. I'd have loved to have seen his face.

 

On the bike thing AMMO, why wouldn't the bike guys have fed the ECU with actual road speeds? I'd have though this an easier and more reliable thing to implement than switches on the gearbox. You could also more finely hone the map. In fact why not measure airspeed at the induction system. Easier still I would have thought.

 

 

[AMMO]

Andy

 

Road speed is logged on a separate data logger which is used by the suspension guys usually . The road speed can be logged by an ECU like the Pectel if neccessary as it has loads of channels that can be configured to do what you like. The ECU already has air pressure and air temperature sensors inside the airbox to help it make it's mind up. This is more accurate than trying to predict what air is going to be in the air box at different speeds. Most airboxes don't go that positive anyway.

 

I think what Peter says is correct, much more important to make sure you have cold air. When testing airboxes on the flowbench all I'm interested in is that the airbox causes no or very little restriction so the engine can get on with it's job of consuming as much air as it wants.

 

Anyway we seem to be going off subject. I'm a power to weight guy if anyone remotely cares. High power and low weight is where it's at. Never been to bothered about torque figures but very interested in the spread of power. Maybe peak torque is sort of interesting. Much more interested in overlaying various power curves to try to predict performance and to analyse improvements on the same vehicle after modifications have been carried out.

 

On bikes more power usually makes you go faster. Sometimes more power, especially if as a result of increased capacity, is accompanied by more torque. You can make more power with less torque. More torque sometimes makes you pull out of corners faster but it may also lead to loss of traction and suspension problems. I'm sure the same applies to cars.

 

Everyone here is interested in going fast to some extent. There is a limit to how fast you can personally go. Your limit is your confidence, abilty and skill. How many times have we heard "if I had 20 bhp more I could win?". Truth is there are those who can and those who can't.

 

If I had 20 bhp more I would still be as slow as I am now around the bendy bits.

 

One last parting shot before I go to beddy byes with a cup of cocoa (Snetterton track day tomorrow). Engine tuned on rolling roads go pretty well. Engines that have been properly optimised on the track from datalogging figures go better.

 

Buona notte e ciao.

 

AMMO

[scooby dooby doo]

rereading it my post came out more positive than i meant it to. Yes you can get a few percent more air but it would be a lot of effort get it working nicely and the results will be very small.

Drag is also an important consideration.

FWIW I'm sure I've heard that you need 150 mph to make ram effect REALLY useful.

 

HOOPY R706KGU CYCLE WINGS AEROSCREEN K SERIES CUCUMBER

[Graham Perry]

The different power mapping in different gears thing is actually quite common. Large construction equipment and some of the latest trucks have this so that they don't get too much wheelspin in low gears on a wet surface.Apparently it also protects the gearbox. I suspect its only a matter of time before it may well be built into the mapping of some of the supercars and possibly F1, but its not something the manufacturers would want to shout about.

[R2D2]

Peter,

 

Sorry for the delayed reply been away for the weekend.

 

You have completely missed the simple point that I am trying to make.

 

The driving force at the contact is a torque. Torque is a tangential force measured at a specific distance from the centre of rotation. I am not sure why this isn’t clear.

 

Power is calculated from torque in the following manner:

 

Torque in lbsft and speed in revs/minute to calculate horsepower:

 

Horsepower = torque x revs/minute/minute/60 x 2 x pi x 1/550

 

In the first line, torque is in lbsft, speed in revs/minute. The third term just converts RPM to revs/s. The fourth term converts revs to radians. The last term is the conversion factor from lbsft/s to horsepower

 

I can only see one force in this equation so power must be calculated. Again I can’t understand why you struggle with this concept.

 

I did not state that power was not real, this is clearly an assumption on your part and you shouldn’t really misquote what was written.

 

You omitted to make a comment about gearing in your statement about 200Hp and 5000/7000 rpm.

I believe that this comment was needed for accuracy and clarity.

 

Thank you for your description of a gear lever, I had been puzzled by this for some time.

 

With regard to wheel sizes, the driving force that produces acceleration can easily be calculated from the power curve in the following manner:

 

Starting from the full power or torque curves of the engine with respect to rpm, and by knowing the gearing and tyre size calculate the driving force in every gear at any road speed.

 

It is quite normal that in any gear the torque produced at the engine’s maximum rpm is always higher than at any rpm in the next gear and this is why shifting gear at maximum power gives better acceleration than shifting at the peak torque.

 

To calculate the actual driving force you need the wheel size so trying to compare different cars with just data about power to weight is simplistic and is about as useful as “mine’s bigger than yours” comments.

 

Your final comment about comparing two engine characteristics in the same vehicle is obviously quite valid. You have effectively eliminated variations in driving force due to gearing and wheel size.

 

I am not trying to support the torque to weight argument, I am trying to say that power to weight is also insufficient as a sole criteria.

 

I would accept that the highest power to weight is the most significant number up when standing against the bar, but real cars need more thought.

 

 

Edited by - chris flavell on 16 Sep 2002 11:56:44

[kjp]

power = force * velocity (1)

 

force = mass * accelaration (2)

 

accelaration = Power / ( mass * velocity)

 

Power to weight seems pretty important.

[Peter Carmichael]

Chris,

 

I know you played back my words at me, but if you can't see the failing in this:

The driving force at the contact is a torque. Torque is a tangential force measured at a specific distance from the centre of rotation. I am not sure why this isn’t clear.

...then we are at an impasse.

[Peter Carmichael]

Torque is never a force.

[Tony Martyr]

Torque is the product of a force

But that is by the way - I, like AMMO am a power to weight believer

I was at a Steam fair this weekend and it is interesting to work out the torque to weight ratio of some of the steam engines there. These produce maximum torque at near zero rpm which makes for an interesting power curve !

[Paul Ranson]

Many agricultural vehicles would have great torque/weight ratios. It's nonsense.

 

I was fortunate enough last weekend to get a brief drive in a racing car with a 2.5l V6 and about 475BHP. You open the throttle and it goes. Keep it over 8000 and I expect life gets quite exciting...

 

I believe this car was recently involved in some Autocar 0-100-0 type testing at Bruntingthorpe, maybe some plausible figures will be published.

 

I think modern engine management coupled with multi-valve cylinder heads and low inertia valve trains have made most of the 'torque wins races' cliches moot. The edge of the driveable envelope is at a very high specific horsepower per litre. An F1 engine pulls from 5000 to 18500, and leaving a hairpin at 5000rpm in first probably still invokes the traction control.

 

Paul

[windy1]

All this talk of equations is doing my head in

 

There is a simple equation that I can relate to:

 

Power sells engines, Torque wins races

 

Bike engined cars have pretty similar power to car derived ones yet they are not nearly as quick (for a given weight) because the torque delivered is less.

I would definitely say torque is a far more useful characteristic than ultimate power. Cars such as the Lotus Carlton & Audi R8 have torque in bucket loads.

 

Windy

[R2D2]

Torque is a measure of how much a force acting on an object causes that object to rotate

 

In other words, torque is the cross product between the distance vector (the distance from the pivot point to the point where force is applied) and the force vector.

 

If torque is never a force how does it produce an acceleration?

[Paul Ranson]

Because a linear acceleration is the result of a force, not a torque?

 

Fact remains, 'torque to weight' is tosh. To work out how a torque will accelerate a vehicle you also need an rpm. Torque and rpm means 'power'.

 

Paul

[Peter Carmichael]

For Torque, substitute Apple pie

For Force, substitute Apple

 

What you are saying is: Apple pie is Apple

 

torque is the cross product between the distance vector and the force vector

Well this is a very mathematical and correct description, but it seems that you ignore its meaning in everything else you write.

 

If torque is never a force how does it produce an acceleration?

Because you divide by perpendicular distance to the line of action and your force pops out again. A fundamental of mechanics is the conversion of linear to rotational motion (pistons, conrods) and back again.

 

By the way, is pressure a force?

 

Edited by - Peter Carmichael on 17 Sep 2002 11:29:31

[Highlander]

Quote.

An F1 engine pulls from 5000 to 18500, and leaving a hairpin at 5000rpm in first probably still invokes the traction control.

 

I think you're missing the point here. Yes in first from 5000 rpm the traction control is probably used but that's because the 'thrust' available is enough to overwhelm the rear wheels, and the thrust available is dependant on the torque, gearing and rolling diameter of the driving wheels. Now in second gear the thrust available would be less because the gearing has been increased.

Conversly if you increase the engine torque with the same gearing and rolling diameter the thrust, and hence accelerative force increases.

So torque to weight is very valid.

[Paul Ranson]

Conversly if you increase the engine torque with the same gearing and rolling diameter the thrust, and hence accelerative force increases.

 

You've also increased the power.

 

So torque to weight is very valid.

 

Torque to weight makes diesel cars look very fast. It's tosh. It might even encourage one to think an unloaded articulated truck was a good basis for a drag racer.

 

Paul

 

Edited by - Paul Ranson on 17 Sep 2002 18:35:41

[scooby dooby doo]

didn't 5th gear / top gear do a race between tiff in a BMW 330 and Jenson Button in a 330D.

 

Button is certainly the quicker driver but he lost...

 

and the 330D has ENOURMOUS torque compared to the 330 but it only has about 160 (?) bhp vs 230bhp for the petrol version.

 

HOOPY R706KGU See you at Cadwell

[Peter Carmichael]

Right.

 

What is your specialist subject 🤔

 

Whatever it is, you are wrong, OK.

 

So far, in my estimation the people who have it clear in their minds are:

 

Me

Paul Ranson

Ammo

Tony Martyr

kjp

Oliver Sedlacek

 

The dissenters are:

Windy1

Highlander

Chris Flavell

 

The characteristics of the former group are postings related to physics and agreeing in detail with each other.

 

The characteristics of the latter group are postings containing conjecture with some confused references to the underlying physics but a general guidance by what a bloke in the pub once said.

[Highlander]

Peter C

 

Sorry, but I do understand the physics and I'm not quoting someone down the pub.

 

The point I was trying to make to Paul about the F1 needing traction contol in 1st was about the thrust issue which is dependant on gearing. Nothing else. Surely what said ties in with the physics you posted earlier.

 

The only thing I'm confused about is what we're arguing about

[Bilbo]

Hi Peter

 

Interesting discussion just started reading this tonight, think I need to re-read it all again.

 

As to your question is pressure a force, bit puzzled why you asked I am assuming its related to Chris's comment "If torque is never a force how does it produce an acceleration? "

 

Anyway its the ratio of the force acting on an area relevant to the total area, equal in any direction. Therefore the force exerted is the total force acting on that area. ( good definition 🤔 )

 

So I take it your point is that torque is the resultant not the force 🤔

 

Putting it VERY simply I have always considered torque to be what gets you going and BHP is what gets you there. So performance is a combination of the 2 relative to the gear your in at the time.

 

So considering one without the other is misleading. Diesels have loads to torque relatively less BHP so they will climb a hill no trouble but when climbing the hill will not accelerate as they have insufficient BHP. Petrol cars of a similar cc have relatively less torque but they will be able to climb the a similar incline but also obtain a higher speed. However the Diesel will lift more mass ( best say mass to avoid arguments ) up the hill in a low gear as a result of having more torque ❗ .

 

Similarly if you want a car to toe a trailer it needs an engine that produces high torque.

 

So it really depend on what your doing with your car and its mass..... or have I missed something important 🤔. Then again is might just be the definition of performance...........

 

Anyway as to my argument ... what's the point in considering one without the other as a overall guide to performance relative to weight err mass of a car . Then again I did say perhaps I should re-read this again.

 

 

1988 200 bhp, 146 ft lbs, 1700cc Cosworth BD? with Brooklands and Clamshell wings, Freestyle Motorsport suspension. Q 979 CGY

 

[Graham Perry]

Having driven many an unladen truck with big torque and comparitivly modest power to weight ratios of say 55bhp/tonne, I can vouch that in anything but the top few gears they can be a real handful as as soon as you squeeze the loud pedal. The wheels easily spin even though they have about two tonnes of chassis weight pushing down, so torque is an issue but trucks are at a different end of the performance envelope to cars, so I think comparisons are not really valid

[AMMO]

All this talk is all good and well. Is this a force? Is that a leverage ratio applied to the torque of radian of the gizmo? (Don't want to stop you boys discussing this stuff, you seem to be having a good time).

 

I'm not clever enough to converse at this level. Some of you guys could probably explain how a TV remote control changes from BBC2 to ITV and know the exact wavelength and what the CAA directive is to stop remote control interference with aviation traffic.

 

Me, I adopt a simpler approach. I press 1 and BBC 1 comes on. I press 4 and I get channel Four. I really don't feel the need to know how it all happens (life is so short, all those beautiful women and so little time). Some of the lads that I know in Essex can build race winning engines and don't know how to switch on a scientific calculator, let alone use one.

 

When I think of performance engines I think of an eight year old boy. He has shorts and knobbly knees. He has a satchel and a bit of a dishevelled look about him. If you really want to be stereotypical in a sort TV meeja sort of way, he has National Health round specs which he pushes up the bridge of his nose with his index finger.

 

His is the most intelligent question of all:

 

"What'll do mister?".

 

That's what it's all about.

 

 

AMMO

[Paul Ranson]

The point I was trying to make to Paul about the F1 needing traction contol in 1st was about the thrust issue which is dependant on gearing.

My point was that the most extremely tuned engines there are can be driveable, and that 'driveable' is really what 'torque wins races' means.

 

I mentioned my experiences in the 2.5l hillclimb car because it was a pretty cool thing and because this is an engine that could never be described as 'torquey' but which has set many outright records in a sport which traditionally has detuned engines for 'torque'.

 

Paul

[Highlander]

Quote

 

Diesels have loads to torque relatively less BHP so they will climb a hill no trouble but when climbing the hill will not accelerate as they have insufficient BHP.

 

Surely on an incline, or on the flat, with the same gearing, mass and wheel diameter, the vehicle with the larger force vector acting up the hill will accelerate faster (F=MA, A=F/M). So with more torque and hence greater thrust at wheel contact point, the diesel will out drag the petrol up the incline?

[Bilbo]

Hi all

 

Thinking bit about it a little more, irrelavent to discussion, the only thing that moves the car is the turning of the crank. So as that's a rotation then its torque.

 

We all talk about BHP because that's what a RR measures at the wheels but in fact brake horsepower on a RR must be a function of revolutions per minute and pounds/feet of torque. After all that's all it can measure 🤔

 

So Power (work done) err BHP = revs x torque

 

Reaching for text book [works never that far a way ... boring !]

 

BHP = power that must be exerted in lifting 33,000 pounds at the rate of one foot per minute. Not much help here.....

 

So when ever a dyno or RR measures BHP it is in fact calculated from the torque. In fact the only thing a engine produces as an output is TORQUE. In fact BHP is only relavant to gearing....

 

(waits to be flamed)

 

1988 200 bhp, 146 ft lbs, 1700cc Cosworth BD? with Brooklands and Clamshell wings, Freestyle Motorsport suspension. Q 979 CGY