Power to Weight versus Torque to Weight

[Anthony Micallef]

There was a letter in this months Evo which stated that Torque to weight ratio was more important than power to weight. Anyone care to comment?

 

The Happy 7 Owner 😬 😬 😬

 

[Peter Carmichael]

Utter tosh. I may reply to Evo.

[dino ferrana]

Peter as soon as I read that article I though of you (to quote the advert). I wondered how long it would take you to pick up on it!

[Graham Perry]

I thought the same, knowing that this is one of Peters favorite subjects from previous threads

[edmandsd]

🙆🏻

[Graham Sewell]

Surely the whole argument is null and void. At a particular engine speed the torque and power have a direct relationship, you cannot increase one without increasing the other. So therefore the power to weight ratio at 7500rpm is directly related to the torque to weight ratio at the same speed. Conversely, the reverse applies at the max(torque) engine speed. To say that one figure is more important than the other is irrelevant as they both convey the same information but at two different engine speeds.

 

Low tech luddite - xflow and proud!

[Joe 90]

At any specific speed, the car with the higher power to weight can be geared to also have a higher torque to weight ratio (at the wheels, where it matters). The arguement for high torque is irrelevant because torque and power are proportinal. What is beneficial is a wide torque spread. If anyone has any torque curves they can send me, I am quite interested in publishing an analysis of torque spread.

 

 

99,000 miles so far

[scooby dooby doo]

its utter rubbish as power or torque to weight tell you about what happens when the engine is at its peak (power or torque) and the car is unladen.

 

So a very peaky engine appears to give you a great power to weight but might actually be quite slow on a track as you spend your whole time trying to get to the peak point and changing gear to try and stay there.

 

I don't think you can really use a single number to rate a car like this. except perhaps laptime on a known track, with known conditions etc etc etc.

 

HOOPY R706KGU CYCLE WINGS AEROSCREEN K SERIES CUCUMBER

[Peter Carmichael]

Hoopy, I will step in to defend "power to weight", but I agree with everybody so far that a spread of power (torque) is the important thing.

 

While the peak power to weight is only available at peak power revs, it does tell you something concrete about the vehicle's performance potential.

 

Torque to weight does not allow you to tell anything much about a vehicles performance. You need extra information: the revs at which the torque peak is achieved or the gearing of the vehicle. I assert that this is then a facile exercise because the combined consideration of torque and revs derives the power figure. In order to presume that "torque to weight" is more useful than "power to weight" we have had to close our eyes to the simple bit of maths that demonstrates that both quantities are linked by the rpm at which the torque is achieved.

 

Power is the more useful and more fundamental quantity. 200 horsepower developed at 5000rpm has exactly the same accelerative potential as 200 horsepower at 7000rpm. The same is not true of torque.

 

Oliver,

 

I have a normalised comparison of power spread for over 70 engines that have been tested at Dave Walker's rolling road. Do you want me to forward it to you?

[R2D2]

Power and torque are always related by a very simple formula.

 

Horsepower / rpm * 5252 = Torque in lbsft or kW/rpm * 9549 = Torque in Nm

 

This is why dyno curves for torque and power always cross at a fixed point depending on the units being used.

 

There are ways of considering the problem of engine tuning.

 

One is to find ways of increasing torque without increasing rpm and the other is to maintain torque and increase rpm.

 

The arguments will go on for ever about the "best" result.

 

I think Power to Weight must be the most important but with the cavaet that they are appropriately geared.

 

There also used to be an arguement that Tractive Effort was the most important measurable of performance as this value takes gear ratio into account

 

 

 

 

 

[scooby dooby doo]

Peter - I'm not sure that knowing the peak potential of the vehicle is *that* useful. if that's what you're interested in then surely a quote of max longitudinal G attained down a drag strip would work better 🤔

 

I think we usually end this argument saying that you need a dyno plot, gearing details, etc etc etc to get the full story...

 

HOOPY R706KGU CYCLE WINGS AEROSCREEN K SERIES CUCUMBER

[Peter Carmichael]

Hoopy,

 

I agreed with you on that bit: The peak potential is only one part of the story.

 

My point was that:

- given a bald power figure you know something about vehicle performance

- given a bald torque figure you know nothing about vehicle performance

- given a torque figure and the rpm at which it is achieved means you can derive a power figure and hence you know something about vehicle performance

 

Simple as this:

 

Accleration = power/mass/speed (with a factor thrown in for using non-SI units)

 

Try doing that with torque...

[scooby dooby doo]

ah ❗ i see what you mean. agreed

 

HOOPY R706KGU CYCLE WINGS AEROSCREEN K SERIES CUCUMBER

[R2D2]

Trying to find a "single characterising parameter" for car performance has been the Holy Grail of car magazines for years. CCC published their article justifying Tractive Effort in about 1971.

 

I think some basic definitions are needed to try to gain a better understanding of some of the arguments.

 

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

 

The relationship between torque and angular acceleration is analagous to Newton's 2nd Law of Motion, which simply states that acceleration is proportional to the applied Force.

 

To get to a definition of Power we need to define Work.

 

Work is the application of a force over a distance and Power is the rate at which work is being done.

 

 

The only force present is torque.

 

To be able to say that the accelerating potential of 200 HP is idential regardless of rpm can only be true if the gearing of the vehicle is different in both cases.

 

A car is accelerated by the torque available at the rear wheels (assuming ideal grip) so gear ratio needs to be considered as a critical part of the debate.

 

Horsepower is a great way to consider an engine but dyno's are only really capable of measuring torque, power is only calculated.

 

I don't think it matters wheter you consider torque to weight, power to weight or any other single characterising parameter, none of them tell the whole picture without looking at the power band, gearing, wheels sizes.

 

In the end you just need to drive the car to see if you like the performance.

 

 

 

 

Edited by - chris flavell on 13 Sep 2002 17:05:20

[Peter Carmichael]

Chris,

 

I read and reread what you have written but I am fairly convinced you are talking tosh.

 

The only force present is torque.

 

Tosh. A car is not accelerated by the torque at the rear wheels. It is accelerated by the force at the contact patch if you are getting pedantic. The ability to sustain a force pushing against the moving ground is dependent on power being supplied. Your torque is not being delivered at zero speed, is it?

 

All this talk of power "not being real" because you have to calculate it is utter tosh and really winds me up. It has a physical definition as you outline. If you measure torque and speed you have measured power. If you don't measure speed at the rolling road or on the dyno you might as well be counting petals on a daisy because it equally tells you nothing about the engine's performance.

 

And you have hit on exactly my point while missing my point.

To be able to say that the accelerating potential of 200 HP is identical regardless of rpm can only be true if the gearing of the vehicle is different in both cases.

 

... and yes, under your left hand you will find a gear lever so you can arrange for the gearing to be different. QED.

 

none of them tell the whole picture without looking at the power band, gearing, wheels sizes.

 

And once again tosh. With a consideration of power to weight you can assert something about vehicle performance without any concern about wheel sizes.

 

Now lets dig a bit into the thorny gearing issue. Say you have a close ratio gearbox. I look at my power curve and see that at 7000 rpm the engine is developing 220 horsepower (obviously I am looking back at the glory days) and it sustains above 220 horsepower until 9200. The gearing allows me to keep engine revs above 7000rpm always. Therefore I always have 220 horsepower available. Therefore I don't need to consider gearing or wheelsize to calculate accelerative force at the contact patch, but I did consider power band. I think you will find that everybody had already agreed that power band was important.

 

🙆🏻Just realised that i am talking tosh. The other thing I need to know is the road load equation (RLE), particularly for aerodynamic losses...

 

Now I know this is a robustly worded response, but you have completely sailed over some very clearly expressed reasoning:

 

- Torque to weight is inane.

- Power to weight tells you something.

[scooby dooby doo]

Therefore I always have 220 horsepower available.

---

 

except when changing gear

 

HOOPY R706KGU CYCLE WINGS AEROSCREEN K SERIES CUCUMBER

[edmandsd]

In drag racing you'll find that a car with a peak of say 200hp at 8,000 rpm is always faster than a similar car with a peak of say 200hp at 5,500 rpm. The second car would always be referred to as the torquey car and yet is inevitably slower than the one that's referred to as top endy. I appreciate that this isn't particularly technical, more of an observation really. Give me a 300hp ex touring car engine over a 300hp Rover V8 anyday....

 

Home of BDR700

[scooby dooby doo]

I think we need to give the link here again. it explains why edmandsd is right.

 

I don't thinkits the original article - its on a BMW site so the examples are BMWs in some cases. But its the best I could google.

 

 

And following on from my previous comment - I've asked some ofthe quaiffe sequential buyers for some numbers. ie how much time to you save between gears with the sequential box.

 

HOOPY R706KGU CYCLE WINGS AEROSCREEN K SERIES CUCUMBER

[Tony Martyr]

Interesting exchanges. Also interesting that both contributors and the link state that dynamometers only measure torque and not power - nowadays this is a bit disingenuous because although power is a derived term it is derived from speed, torque and a constant and a dynamometer measures both speed and torque. Modern machines, which are systems rather than single lumps connected to an engine, compute and record power in near real time. Indeed dynamometer systems (as they now are) compute corrected power by taking into account other measured factors. Although all control the dyno has over the engine is torque derived it was the old sluice gate machines that just measured resisting torque, now modern units have optical encoders that can observe changes in rotation acceleration within a single revolution.

[Peter Carmichael]

This contributor is very clear:

If you don't measure speed at the rolling road or on the dyno you might as well be counting petals on a daisy

[Morls]

"Trying to find a "single characterising parameter" for car performance has been the Holy Grail of car magazines for years."

 

It's much simpler when Karen's magazines, (Cosmo, Red, Company etc.), test cars....

 

 

"The new ******** is available in Blue, this season's must have colour"

 

 

And as for rolling roads....

 

With ram air being fitted to some Blackbirds, and the poor location (in a low pressure area?) of many 7 filters / airboxes, Isn't a "road speed" fresh air supply also not important for the whole story. Do many RRs have this?

 

Has anyone ever attached a manometer to the area around the filters and coasted at high speed?

I wouldn't be surprised to see a negative pressure on the filter elements of a (obviously non-running,) traditional XF / VX Twin-Sidedraft-stuck-out-the-bonnet-side type layout.

 

 

 

Mark

[scooby dooby doo]

someone tried it on se7ens.net and another person verified the numbers theoretically. I think it came out to a couple of % more air once you got over 100 mph, so 5 bhp or so...

 

HOOPY R706KGU CYCLE WINGS AEROSCREEN K SERIES CUCUMBER

[Peter Carmichael]

Generally the sort of lash ups that people try to get a bit of positive ram effect probably cause more drag than the extra power they may generate. A cold air feed is far more valuable.

 

If there were huge low pressure areas developing, the engine would perform significantly differently in different gears - you only get the high speeds that cause the low pressure effects in high gears, so the rolling road observed readings would be close enough for 1st 2nd and 3rd and probably start to drift out in 4th 5th and 6th. As the low pressure area develops an injected engine with no correction will drift into running a rich mixture which will make it feel sluggish revving to the redline. So what are you going to do on a rolling road with a road speed airflow? Map separately for each gear?

 

I would like to know the hard data for such situations, but they are minor effects considering that rolling road tuned engines have been demonstrated to work pretty well. I think it is an ideal area to apply some empirical engineering to eliminate obvious faults and give an engine the best sportign chance to breath in a decent slug of cold air. It is no more complicated than that unless you want it to be...

[AMMO]

It might interest you to know that Superbikes have gear switches on the selector drum so the ECU knows what gear the engine is in. The ECU is mapped differently for different gears (usually only for 1st, 2nd and 3rd. This is mainly so the bike doesn't spit you off when you crack open the throttle). This is also to take into account the different airspeed into the airbox. A friend who ran a carburetted Kawasaki Superbik said that the engine didn't run properly until he was in third.

 

It is now more frequent for advanced bike rolling roads to have fans which relate to road speed. I think these can only go up to around 150 mph. Superbikes tend to be quite a bit faster than this. I recently spoke to a team manager who I tested airboxes for who told me that they recently purchased a rolling road with this facility. Whilst running the bike on the rolling road you have to crouch behind the fairing because it is so uncomfortable to try to sit up.

 

Four cylinder bikes tend to be mapped the same for all cylinders. Twins have dual lambda probes and the cylinders are mapped individually. I'm sure that cars are also using these facilities. Pectel ECUs have the ability to be mapped for different gears and individually for each cylinder. Pectel is now indirectly owned by Ford. Ford World Rally cars use Pectel. Draw your own conclusions.

 

AMMO

 

Edited by - Ammo on 15 Sep 2002 13:00:51

[Tony Martyr]

Speed following fans are a manditory requirement for almost all emission tests but they only have to simulate the airflow up to about 120 kph from memory and have to have a standard nozzle size to give a given flow into the front of the car. The problem with bike testing is that lots of machines use the ram effect and the energy required to give an airflow of over 100kph over the full frontal profile of a bike is large and rises exponentially with speed. This is why special wind tunnels are built where the test object sits in the throat of a recirculating system, even so it take a lot of power. Recent units for cars have been supplied with 1MW drives for the fans.

It takes less energy to move the object through the air than the air past the object.

I comissioned an out of doors facility once where we forgot the bolt the 60kW road speed following fan down. On the first auto run the - robot driven - car accelerated to 100 kph and the fan took off like a rocket until it came to the end of its cable and disappeared in a big arc out of site. We heard the crash and had a very shaken and pissed off delivery van driver appear in the control window. He had been parked and eating his lunch at the time.