GTRs too slow on the straight?

[biggie]

Hi :)

I was just wondering about one thing concerning the GTRs... it always occurred to me that their top speed seemed to be a little low, taking into account their absolute power of ~500 bhp.
On the longer straights, the GTRs seem to have only a minor advantage over the much heavier and less powerful FZ50. In the end of the Aston National start/finish straight, the difference is only 20-25 Km/h - of course with 0 downforce on the FZR.

Shouldn't it be possible for those monsters to hit the 300 Km/h-barrier quite easily or am I missing something? It seems they're completely lacking the "oomph" above 250 Km/h...
I didn't want to post this in the bug reports section because I was merely wondering and maybe there's something I didn't take into account.

See the attached screenshots for demonstration.

biggie

[colcob]

Even with zero wing angle, the GTR's are still generating significantly more drag than the FZ50 with no wings at all.
If you look at the aero section, set both wings to zero, you can see that the undertray and both wings are still generating a fair bit of drag over and above the body drag.
Also, the power to top speed relationship is exponential, so to acheive 25kph higher top speed once you are already up at 250kph requires a lot more extra power than increasing the top speed from 150-175kph for example.

[nikimere]

well when you consider a Ferrari 550 Maranello road car has a top speed of 199MPH, and the race car will probably only do a little over 200 if even that.
it's not about top speed in race cars, more about acceleration and corner speeds.

[Vain]

He's talking about km/h.

Vain

[xaotik]

for Vain:

well when you consider a Ferrari 550 Maranello road car has a top speed of 320.2Km/h, and the race car will probably only do a little over 321.8 if even that.
it's not about top speed in race cars, more about acceleration and corner speeds.

:P

[xaotik]

HOn the longer straights, the GTRs seem to have only a minor advantage over the much heavier and less powerful FZ50. In the end of the Aston National start/finish straight, the difference is only 20-25 Km/h - of course with 0 downforce on the FZR.

And two stupid questions from me:
1. Is this tested with no wind?
2. Have you tinkered with the gear ratios of the GTR?

EDIT
Never mind - just noticed that you have the hotlap record on Aston National with the FZR... :)

[nikimere]

He's talking about km/h.

Vain

km/h is just a unit. that doesn't affect my point!??

[Vain]

I thought you mistook km/h with mph and justified the achieved speeds as good-as-it-is.
Nevermind.

Vain

[Mogar]

Well, the cars on Brazilian stock car championship have about 450hp and 1200kg, and their speed at the end of main straight on Interlagos track (where F1 takes place on Brazil, a track with tight curves but very long straights) is around 240km/h, so I think that the final speed of the GTR class cars (except FXR) is pretty on par with reality

[biggie]

Even with zero wing angle, the GTR's are still generating significantly more drag than the FZ50 with no wings at all.
If you look at the aero section, set both wings to zero, you can see that the undertray and both wings are still generating a fair bit of drag over and above the body drag.
Also, the power to top speed relationship is exponential, so to acheive 25kph higher top speed once you are already up at 250kph requires a lot more extra power than increasing the top speed from 150-175kph for example.

Yes, I am aware of the undertray force. But I wasn't thinking that the drag would be so excessive that almost twice of the power/weight-ratio would result in only 25 Km/h more topspeed.


[...] it's not about top speed in race cars, more about acceleration and corner speeds.

True, but I was just wondering because I kept imagining that GTR-like cars could achieve higher top speeds. Guess I might have to reconsider ;)

[xaotik]

But I wasn't thinking that the drag would be so excessive that almost twice of the power/weight-ratio would result in only 25 Km/h more topspeed.

If you look at the formula for calculating drag you see that the value also includes the velocity of the object (car in our case) squared...

Stolen from a physics-related site:
F_drag = 0.5 c p v^2 A
c = drag coefficient
p = density of air
v = terminal velocity
A = frontal area

[qwejibo]

Heres some real life downforce/drag figures for comparison to LFS cars(at 80.5m/s = 290km/h, 180mph)

1999 Ferrari Modena (street car) = -1892N downforce, 2570N drag

1997 McLaren GTR (long tail, Le Mans) = -7927N downforce, 3602N drag

2001 Penske-Reynard-Honda (Indycar, road setup) = -17680N downforce, 5941N drag

And for fun: 1992 Allard J2X (IMSA GTP) = -35352N downforce, 5892N drag (!!!)

And the LFS S2 cars:

LFS S2 GTR class (min/max) = -7154/14074N downforce, 3292/5238N drag

Formula XR(min/max) = -5541/11639N downforce, 2365/3965N drag

Formula V8(min/max) = -7387/15519N downforce, 3046/5179N drag

As you can see the LFS cars have realistic drag figures for the downforce they are generating.

The GTR class have high max. downforce for a GT style car, probably similar to the old JGTC cars when they had underbody tunnels(before 2001 iirc).

[Bob Smith]

Yes, I am aware of the undertray force. But I wasn't thinking that the drag would be so excessive that almost twice of the power/weight-ratio would result in only 25 Km/h more topspeed.
Power to weight is irrelevant, I'd say it's the most useless statisitic you can have about a car IMO.

Why?
Top speed can be judged by power vs. aerodynamics
Low speed acceleration is all about torque-to-weight (well, and grip)

Power to weight tells you nothing really.

It is the ~35% hike in power that you need to look at. Tuning the FZ50 to give the same output as the FZ50 GTR would give it a top speed of about 190mph. The FZ50 GTR can only manage about 180mph with wings at zero (no high nose cheat), so about 10mph is being lost to the undertray.

[skiingman]

Low speed acceleration is all about torque-to-weight (well, and grip)


:huh:

Perhaps if your gearing is off...

Power limits the acceleration of the car. Not torque. 1500Nm of torque does you little good at 400rpms.

Last I checked, F1 cars get off the line in a serious hurry despite a tall first gear, an on/off clutch, and less torque than my road car. 200-250lb-ft

The venerable PT6 is another great example. The little PT6a produces
(depending on rating) 675ish hp and more than 1600lb-ft of torque at the output shaft at 2200rpm. At 30000 rpms at the input shaft of the planetary reduction box, its producing 118lb-ft of torque. Honda territory.

Unless fuel efficiency or ultra-longevity is important, engine speed doesn't much matter. Therefore, torque doesn't matter. The only thing that matters is the product of torque, a constant, and engine speed....horsepower.

So I would say that it is torque to weight thats an unbelievably useless figure. Its rarely published, perhaps because its pointless.

[colcob]

Sorry but I think you're wrong. Currently delivered torque absolutely dictates your acceleration.

Torque/effective wheel radius = delivered linear Force at contact patch.

Acceleration = Force/Mass

Therefore as weight = mass*g

torque/weight is directly analogous to force/mass which equals acceleration.

Therefore torque to weight is the most accurate way of defining a cars acceleration potential. The reason its never quoted is perhaps because people dont understand it.

[axus]

Mass is irrelevant in the equation of top speed. The only thing it affects is how quickly you get to the top speed of the car, but mass only really affects low speed acceleration because power is what helps you overcome drag and accelerate at high speed. Here is an article that might explain everything, although a bit technically - The Physics of Racing (http://www.miata.net/sport/Physics/06-Speed.html)

[colcob]

My god I know that. We were talking about acceleration just then.

[StewartFisher]

Mass might have a very small effect on the top speed in the sense that a heavier car will put more load on the bearings, increasing rolling resistance.

I think I agree with Bob and Colcob about torque being more important than power for low-speed acceleration but I can't quite convince myself why.
Since engines tend to produce peak torque and peak power at different RPM, which point gives the best acceleration? If a car had constantly-variable transmission, would it be better to hold the engine at peak torque or peak power while accelerating?

[ajp71]

Last I checked, F1 cars get off the line in a serious hurry despite a tall first gear, an on/off clutch, and less torque than my road car. 200-250lb-ft


This would only apply to the FO8, all the other engines are production based.

[Ball Bearing Turbo]

Hmm...

I understand the reasoning behind the favor of Torque : Weight,

But perhaps the rate at which the torque is being applied gives a better idea of how much energy is being put into the system, therefore HP: Weight should be more important....

Who cares if you've got all the force in the world if it takes a vast amount of time for it to be applied?

Drag cars are built for HP which probably says something.

My 2 cents

[Stregone]

How about just saying how fast a car accelerates if you want to tell someone how fast a car accelerates?

[spoop]

An engine will always accelerate fastest at the torque peak in any given gear. Horsepower comes into play when you factor in gearing. Lets say you have engine a and engine b. They are similar but engine a makes 200lb-ft of torque at 2000 rpm and then suddenly drops off and engine b makes 200bl-ft up to 4000 rpm. So, engine b makes twice the hp of engine a because hp = torque/rpm. They are put in the same car with the same gearing. They will both accelerate off the line the exact same, until engine a has to shift because it got to 2000rpm. Engine b will pull ahead because it can continue to accelerate in 1st until it reaches 4000, and the car with engine a will be behind because it is in 2nd. Kapiche?

So really, since we can change the ratios in the lfs cars, to achieve the highest top speed it's all about hp.

Edit: another thing to think about. You can run a twice as short gear ratio in the car with engine b and reach the same speeds as engine a, ( since b can reach twice the rpms) but since engine b's ratio's are so short it will accelerate almost twice as well because torque to the wheels is twice as much.

[tristancliffe]

The force at the wheels is simple:

F = (Torque * RPM * Transmission Efficiency)/Dynamic radius of driven wheels

As we know Acceleration is Force/Mass. So now we can work out how fast something accelerates. And in each gear it will be accelerating hardest at peak torque.

This doesn't take into account drag (of any sort), tyre slip or a few other things, so is a bit simplified, but will get good enough results for 99% of people.

[Gunn]

Hi :)

the difference is only 20-25 Km/hThat is a big difference actually. The extra horsepower required to add 25kph to a car's top speed would be much more than it may first seem. Adding a lot of extra horsepower would only give a tiny gain in performance for a car at the GTR's level of engineering. So horsepower by itself is no great measure of performance.

It may also be useful to find out out how quickly it takes the GTR to reach the FZ50's top speed. If the GTR car reaches the FZ50's top speed quicker than the FZ50 does, then the performance gap is widened even more than just a top speed rating would suggest.

[Ball Bearing Turbo]

I understand what you're saying but I'm still not convinced that the actual force at the wheels is all that's important, although by the equations that seems to be the case. Forgive my unreasonable questionng, but please help me understand this:

If MORE WORK is done on an object will it not accelerate faster because the total amount force exerted on it OVER GIVEN TIME is greater?

Being able to twist a steel axle with 300ft/lbs of rotational force is a lot less relevant than twisting the same axle with 300ft/lbs over any said amount of time. One could take all day, and one could happen in 1 second which would require more POWER, although the net force is the same. Isn't that correct? Where am I going wrong?

Isn't HP basically the rate at which Torque is applied (550 ft/lbs/sec iirc)? When talking acceleration - wouldn't being able to apply any given torque only be meaningful in the context of how fast it is applied?

Seems reasonable but show me where I am wrong... Yes I know F=MA etc...

[dave_w11]

Sorry but I think you're wrong. Currently delivered torque absolutely dictates your acceleration.

Torque/effective wheel radius = delivered linear Force at contact patch.

Acceleration = Force/Mass

Therefore as weight = mass*g

torque/weight is directly analogous to force/mass which equals acceleration.

Therefore torque to weight is the most accurate way of defining a cars acceleration potential. The reason its never quoted is perhaps because people dont understand it.

That is at the wheels though. An engine could deliver a huge amount of torque at the fly wheel, but once geared to have any reasonable sort of top speed, deliver a relatively small amount of torque at the wheels.


Edit: Small example:

A lot of lotus 7 style kit cars are fitted with bike engines. These may produce a maximum torque of around 70lbft and typically weigh around 450kg, so a torque/weight of 155lbft/tonne.

A popular car engine fitted is a ford zetec, and cars with these fitted will normally weight about 650kg. A google for 'zetec dyno' has found one producing 150lbft maximum torque, so assuming that's a typical value, that is 230lbft/tonne.

The car engined car will still not beat the bike engined one (the bike engined one will batter the car engined one). This being because the bike engine produces useable torque up to 12000rpm, whereas the zetec only to about 7000. The gearing is altered to suite and the bike engine produces more torque/tonne at the wheels.

The bike engine produces ~150 bhp, so ~333 bhp/tonne. The car engine produces ~165 bhp so ~250bhp/tonne. Coincidence? I suppose it could well be and someone could most likely find an example to prove power not to be a good indicator either. For example I suppose if an engine gave a very narrow band of torque at high rpm? So that is maximum torque and maximum power values are both useless :(


Just my thoughts, might be a load of the brown stuff. :shrug:

[Bob Smith]

It may also be useful to find out out how quickly it takes the GTR to reach the FZ50's top speed. If the GTR car reaches the FZ50's top speed quicker than the FZ50 does, then the performance gap is widened even more than just a top speed rating would suggest.
OK. This is using GRC coz I'm too lazy to actually drive and time it, but the results should be reasonably accurate:
FZ50 top speed: 173mph
Time to get there: 90 seconds
Time for FZR to get there: 28 seconds

The difference will always be great though, since the last few mph take ages. For instance it still takes 60 seconds for the FZR to get flat out, so it took 32 seconds to get from 173 to 181mph. Likewise the FZ50 reaches 170mph in just 57 secs. So it took a whopping 33 seconds to gain 3mph.

That is at the wheels though. An engine could deliver a huge amount of torque at the fly wheel, but once geared to have any reasonable sort of top speed, deliver a relatively small amount of torque at the wheels.
You've got that the wrong way around. You gear down, not up. This goes back to skiingman's F1 comment, their torque isn't that impressive at the engine but with so many revs there gearing will be really short, so torque at the wheels will be monstrous. Add to that low weight and you've got something that flies.


A lot of lotus 7 style kit cars are fitted with bike engines. These may produce a maximum torque of around 70lbft and typically weigh around 450kg, so a torque/weight of 155lbft/tonne.

A popular car engine fitted is a ford zetec, and cars with these fitted will normally weight about 650kg. A google for 'zetec dyno' has found one producing 150lbft maximum torque, so assuming that's a typical value, that is 230lbft/tonne.

The car engined car will still not beat the bike engined one (the bike engined one will batter the car engined one). This being because the bike engine produces useable torque up to 12000rpm, whereas the zetec only to about 7000. The gearing is altered to suite and the bike engine produces more torque/tonne at the wheels.

The bike engine produces ~150 bhp, so ~333 bhp/tonne. The car engine produces ~165 bhp so ~250bhp/tonne. Coincidence? I suppose it could well be and someone could most likely find an example to prove power not to be a good indicator either. For example I suppose if an engine gave a very narrow band of torque at high rpm? So that is maximum torque and maximum power values are both useless :(

Hang on, you're not thinking about gearing reduction enough here. The bike engine revs twice as high, so to get the same speeds you need to double your gearing reduction, doubling your torque in the process. So now you've got the same torque (nearly) but less weight. See? So it accelerates quicker.

One big reason you can't give out torque-to-weight figures in reality is that it totally depends on the gearing. And then the difference between a peaky engine and a flat torque curve makes a big difference. So really you need a plot of the torque curve for each gear to analyse things. Just what GRC does. :D

[dave_w11]

You've got that the wrong way around. You gear down, not up. This goes back to skiingman's F1 comment, their torque isn't that impressive at the engine but with so many revs there gearing will be really short, so torque at the wheels will be monstrous. Add to that low weight and you've got something that flies.


I meant a small amount of torque at the wheels compared to what an engine with less torque, but at higher revs could at the wheels after gearing. Not compared to the torque at the flywheel. I didn't put it very well though :sorry:


Hang on, you're not thinking about gearing reduction enough here. The bike engine revs twice as high, so to get the same speeds you need to double your gearing reduction, doubling your torque in the process. So now you've got the same torque (nearly) but less weight. See? So it accelerates quicker.


I would say that amounts to the same thing I was trying to say: That although the torque/tonne is lower at the flywheel, after gearing (thanks to the higher revving bike engine), the torque at the wheels is higher than that of the car engine with higher flywheel torque.


One big reason you can't give out torque-to-weight figures in reality is that it totally depends on the gearing. And then the difference between a peaky engine and a flat torque curve makes a big difference. So really you need a plot of the torque curve for each gear to analyse things. Just what GRC does. :D

That was the main point of my post aswell.


What is a GRC?

[noob4ever]

well i guess thay "souped" up the GTR's just like it was back in highschool..
Rims, bodykit, muffler and an intake, maybe a chip too. :) theres your 20 extra mph :)

[KiDCoDEa]

well i guess thay "souped" up the GTR's just like it was back in highschool..
Rims, bodykit, muffler and an intake, maybe a chip too. :) theres your 20 extra mph :)

hehe :)
BRILLIANT :thumb:

[Tweaker]

well i guess thay "souped" up the GTR's just like it was back in highschool..
Rims, bodykit, muffler and an intake, maybe a chip too. :) theres your 20 extra mph :)
Yeah they aren't much of a difference to their road-car counterpart.

To me they are really just the road cars made into some mid entry level race car... nothing like a GT1 or LeMans class car, but almost there.

[skiingman]

Therefore torque to weight is the most accurate way of defining a cars acceleration potential. The reason its never quoted is perhaps because people dont understand it.
It'd be nice if there were some big conspiracy to that effect....but thats just not the case.

A given acceleration over a given period of time requires a given amount of work to be done. This work cannot be done without power. All the torque in the world is nice, but means absolutely squat if the torque can't be produced far enough to the right of the graph.

I can give a very realistic and very obvious example which proves this point.

" torque/weight is directly analogous to force/mass which equals acceleration."

So I've got this electric motor that produces 2000lb-ft of torque at 1 rpm. It produces 300lb-ft at 1000 rpms. This is not an unrealistic torque curve for an electric motor.

Although it will smoke the tires even in fourth gear from a stop, no amount of gear swapping in the world will make it accelerate faster over any reasonable time period or distance than a 160hp gasoline engine making about 150lb-ft of torque.

By stating this:
Torque/effective wheel radius = delivered linear Force at contact patch.

you are implying that you don't understand the function of a gearbox. Bob did a good job of explaining its torque multiplication duties.

If torque to weight were a better indicator of performance than power to weight (obviously false with a basic look at physics) the izzy Celica would be faster/quicker than the duzzy Celica. Its not. Same goes for ZO6 vettes, which really don't make more torque than stock vettes. They move the torque curve further right, increasing the PRODUCT of torque and RPM......horsepower.

But perhaps the rate at which the torque is being applied gives a better idea of how much energy is being put into the system, therefore HP: Weight should be more important....

Who cares if you've got all the force in the world if it takes a vast amount of time for it to be applied?

Right on, and thats exactly why the figure given is hp/weight, not torque/weight. Given non-silly gearing you can get pretty much the same result of calculation from hp/weight as you can by plotting torque available at a given engine speed/gear combo over time. Its a lot less labor intensive too, unless you have something cool like the tool Bob is using.

[tristancliffe]

If you want to work out the top speed of a car, the rate of acceleration at any point, the force at the wheels, or pretty much any other performance related parameter, you use the torque curve and/or the gearing. You never ever need to use a power curve or power figure, as they don't actually tell you anything. Besides power is just a function of torque, and is so ridiculously easy to work out that if you REALLY want it you can have it...

[Bob Smith]

What is a GRC?
The gearing tool I made for LFS, since you missed the link in my sig I'll put one here too: GRC (http://myweb.tiscali.co.uk/thefloatingwidget/lfs_grc.html)

[colcob]

I think we're just misunderstanding eachother Skiingman. I'm talking about the actual torque delivered at the wheel (ie. post gearing) for a given moment. You seem to think I'm talking about the quoted torque peak figure, regardless of rpm.
Of course, higher power output effectively means being able to carry on producing high torque at higher revs, which means staying in a lower gear for longer, producing higher wheel torque for longer, hence acheving better acceleration.

Basically what I am saying is that the torque/power curve is all you need. Its the main defining characteristic of an engine.

[tristancliffe]

the torque/power curve is all you need. Its the main defining characteristic of an engine.

100% true. Anyone who tries to argue doesn't understand engines or physics enough to argue.

[J.B.]

Hmm, and there was me trying to stay out of all internet torque/power wars. I thought I had seen enough of them but I can't resist. :)

First of all I think skiingman's posts are spot on. I am also quite suprised at some of the other posts I have been reading here. Examples:

Power to weight is irrelevant, I'd say it's the most useless statisitic you can have about a car IMO.

Therefore torque to weight is the most accurate way of defining a cars acceleration potential. The reason its never quoted is perhaps because people dont understand it.

You never ever need to use a power curve or power figure, as they don't actually tell you anything.

Instead of posting long explanations I'll just make up a few questions to get people thinking: (multiple answers possible)

1) When is the best point to shift up?
a) when the next gear puts the engine at an rpm of higher power
b) when the next gear puts the engine at an rpm of higher torque
c) when the next gear will deliver more torque to the wheels

2) To get maximum accelleration out of an engine you can use a continuously variable transmission (CVT) that can keep the engine at a constant rpm. Which rpm should this be?
a) max engine torque rpm
b) max engine power rpm

3) You want to improve the accelleration of your car by getting a new engine. The dealer will only tell you either the max torque or max power value (no rpm value). Which one will give you more useful information?
a) max torque
b) max power
c) both the same

4) You want to improve the accelleration of your car by getting a new engine. The dealer will only tell you either the torque curve or the power curve. Which one will give you more information?
a)torque curve
b)power curve
c)both the same

5) With a given gear ratio you can go through the engine's rpm range by varying the speed of the car. At which rpm will you find the highest acceleration?
a) at max engine torque rpm
b) at max engine power rpm

6) At a given vehicle speed you can go through the engine's rpm range by varying the gear ratio between engine and wheels. At which rpm will you find the highest acceleration?
a) at max engine torque rpm
b) at max engine power rpm

7) Which question, 5) or 6) is irrelevant when you are a girl and are trying to determine the overall drag-strip performance of your car? (bash tristan not me :))
a) 5)
b) 6)

Can you take my challenge? :D

[tristancliffe]

I'll have a crack!

1) When is the best point to shift up?
a) when the next gear puts the engine at an rpm of higher power
b) when the next gear puts the engine at an rpm of higher torque
c) when the next gear will deliver more torque to the wheels
When the next gear will deliver more torque to the wheels than staying in the same gear. Tractive Effort ;)
2) To get maximum accelleration out of an engine you can use a continuously variable transmission (CVT) that can keep the engine at a constant rpm. Which rpm should this be?
a) max engine torque rpm
b) max engine power rpm
The rpm of most torque for most acceleration
3) You want to improve the accelleration of your car by getting a new engine. The dealer will only tell you either the max torque or max power. Which one will give you more useful information?
a) max torque
b) max power
Technically both, as one can be derived from the other, but personally I would go for the torque curve so I didn't have to bother converting it.
4) You want to improve the accelleration of your car by getting a new engine. The dealer will only tell you either the torque curve or the power curve. Which one will give you more information?
a)torque curve
b)power curve
c)both the same
Both the same, as the important one, Torque, can be derived from the power curve
5) With a given gear ratio you can go through the engine's rpm band by varying the speed of the car. At which rpm will you find the highest acceleration?
a) at max engine torque rpm
b) at max engine power rpm
At max torque! Tractive Effort at wheels/Mass of car = Acceleration
6) At a given vehicle speed you can go through the engine's rpm band by varying the gear ratio between engine and wheels. At which rpm will you find the highest acceleration?
a) at max engine torque rpm
b) at max engine power rpm
Max Torque rpm
7) Which question, 5) or 6) is irrelevant when trying to determining the overall drag-strip performance of your car?
a) 5)
b) 6)
5 is irrelevant for drag racing, but drag racing is irrelevant cos only girls drag race. It's for people who can't do corners. Anyway, you can change your gear ratios, but in a flat out sprint you can't choose the best speed for acceleration
Can you take my challenge? :DI just did.

[colcob]

Personally, I think much of the confusion is do with difficulties in explaining/understanding, rather than us actually having it wrong in our heads.

Anyway, I'll have a crack at your test, I havent looked at tristans answers yet, and engines arent exactly my strong point, as you've noted.

1) When is the best point to shift up?

c) when the next gear will deliver more torque to the wheels

2) To get maximum accelleration out of an engine you can use a continuously variable transmission (CVT) that can keep the engine at a constant rpm. Which rpm should this be?

b) max engine power rpm
I cant quite work out why in my head, but it stands to reason that if you are going to run an engine at only one speed, it should be the speed which derives the greatest power from the engine. It think its because running at max power rpm would effectively allow the CVT to be running in a lower gear, thus delivering greater actual wheel torque throughout the speed range.

3) You want to improve the accelleration of your car by getting a new engine. The dealer will only tell you either the max torque or max power. Which one will give you more useful information?

b) Honestly not sure. Max power and max torque are both of questionable use and dont tell the whole story, so I'd probably plump for max power.

4) You want to improve the accelleration of your car by getting a new engine. The dealer will only tell you either the torque curve or the power curve. Which one will give you more information?

c)both the same, one derives directly from the other.

5) With a given gear ratio you can go through the engine's rpm band by varying the speed of the car. At which rpm will you find the highest acceleration?
a) at max engine torque rpm (aerodynamic effects notwithstanding)


6) At a given vehicle speed you can go through the engine's rpm band by varying the gear ratio between engine and wheels. At which rpm will you find the highest acceleration?

b) at max engine power rpm, because it will yield the shorter gearing thus greater delivered torque to the wheels.

7) Which question, 5) or 6) is irrelevant when trying to determining the overall drag-strip performance of your car?
a) 5)


I'd just like to point out that I didnt make it very clear that I was referring to actual wheel torque in most of my previous points, which is a quite different thing to engine torque alone, as the engine power capabilities will determine what gearing is used to turn engine torque into wheel torque.

[J.B.]

Thank you for taking part Sirs. I won't tell my opinions yet, maybe some other people will give it a try. And I clarified questions 3) and 7).

[skiingman]

Basically what I am saying is that the torque/power curve is all you need. Its the main defining characteristic of an engine.

Yes, but you need the WHOLE thing, not just one figure from it. Thats the disadvantage of talking about torque. Uber torque at weaksauce rpms will not get the work done. Sure, if you have uber torque at macho rpms, you will get the work done in a hurry.

The best way to express a torque curve in a single figure is horsepower. This is why you see horsepower figures quoted versus weight, not torque versus weight.

500hp/ton ALWAYS results in a predictable acceleration over time, regardless of the actual torque curve...assuming we have decent gearing. So a 500hp electric motor with 5000lb-ft of peak torque will get from a>b in about the same amount of time as (opposite end of spectrum) a 500hp gas turbine making 80 or 90lb-ft of torque.

If you made the absurd statement that the 5000lb-ft/ton car is going to be faster than the 90lb-ft car, you better not be betting big money. You could very well be wrong.

Bob made these statements:
"Power to weight is irrelevant, I'd say it's the most useless statisitic you can have about a car IMO."

Obviously its not irrelevant. Fairly simple physics can show with the single hp/weight figure results nearly as accurate as his torque/gear model...which requires a LOT more knowledge of the way the system works. It would be very bad to assume the electric motor's torque curve would be similar to a gas motor and plug it into a torque and gear calculator.

"Power to weight tells you nothing really."

In fact, it tells you quite a bit. Knowing weight and something as seemingly unrelated as fuel consumption can give you fairly a fairly accurate representation of of the acceleration available at a given speed. What it doesn't give you a clue about is the gearing you'll need to achieve that. Thats a rather critical part of the setup, and thats why a gear calculator is indispensible.

Outside of the automotive world there are plenty of places where torque is as seemingly irrelevant as horsepower is here. Your typical airbus captain doesn't much care about the torques the turbines are producing, but the simple power to weight calculations are absolutely vital to getting off the ground at the appropriate speed and distance. If you think about it for half a second, the rate of acceleration that massive airliners achieve at a max performance takeoff is mind boggling given their mass.

[tristancliffe]

LOL!!!

I like Q.7 now
And my answer to Q3 would now be Max Power. I would then use a bit of common sense, and look at the engine 'type'. A good, drivable, tractable, but not neccessarily highly tuned engine tends to produce similar hp and lbft figures (it doesn't work with Nm and kW). So a car with 500hp and 500lb-ft of torque will be much nicer to drive on the road that a 900hp 200lb-ft engine (pretty much regardless of gearing).

So if I was looking an engine that I knew produced 900hp, but didn't know the torque I'd ask myself a few questions? How long is the stroke, what capacity, what layout, which manufacturer. From that I'd be able to hazard a guess as to it's vague torque characteristics, and make the correct purchase.

[tristancliffe]

Outside of the automotive world there are plenty of places where torque is as seemingly irrelevant as horsepower is here. Your typical airbus captain doesn't much care about the torques the turbines are producing, but the simple power to weight calculations are absolutely vital to getting off the ground at the appropriate speed and distance. If you think about it for half a second, the rate of acceleration that massive airliners achieve at a max performance takeoff is mind boggling given their mass.

The Airbus captain won't care one iota about torque on the turbines, as turbofan engines produce thrust, not torque (although torque on the driveshaft is obviously a factor in the production of thrust, but the internal workings of the turbofan are not of much interest to the Captain).

[colcob]

Yes, but you need the WHOLE thing, not just one figure from it.

Yeah, thats exactly what I was trying to say. Like I said, I was was talking about wheel torque, which is the car equivalent of thrust. How much of it you get at a given speed depends a lot on the engine power.
And I never said power to weight was irrelevant, it clearly isnt.

Interesting debate this though. Its certainly helped to clarify some of my slightly vaguer notions about torque/power.

[Fonnybone]

As much as i like talking physics and stuff, i think this is a case of
of 'when it seems overly complicated, it MUST be simpler'. Personally
i think it is. There is one major problem in all the comparisions, we
don't REACH top speed in LFS, at least in most cars. There is no track
that is long enough (like when manufacturers test out their top speed...)
and to prove my point i made an experiment a while back. There was some
setup hacks for grip back then and you could literally take turns flat out,
accumulating speed lap after lap. NOT losing speed would be a better
word. There was nothing making the car accelerate faster, it simply didn't
lose much speed in turns turning any track into an endless stretch to
accelerate. Considering all the rest was stock, the XRGT Turbo could reach
260+km/h on Blackwood, it still had guts left for another 5-10kmh i'd say.
Try it now. If the XR GT Turbo can eventually reach 260km/h, i'm SURE the
GTR cars can reach close to 300 if given the room. The S2 oval has just
reinforced this by allowing cars to reach higher speeds than any other tracks.
If the oval hadn't been released, what 'top speed' figures would you use for
LFS ? 'Top Speed' and the fastest you've gone in 'x' car is not the same thing.

Comparing the Ferrari Maranello to a GTR is unfair for a few reasons.
Like i said above, the Maranello's top speed was surely tested on a track
providing it room to do so, a distance VS speed chart would be better for
comparing than a top speed figure imo as we have no actual data on top
speed of LFS cars. The Ferrari is a road car with little drag from aerodynamics
compared to it's equivalent race version, which could very well be compared
to the GTR, and in fact SHOULD be the car to compare it with, and not the
street car. What are the specs of the racing versions of the Ferrari ?

[Bob Smith]

2) To get maximum accelleration out of an engine you can use a continuously variable transmission (CVT) that can keep the engine at a constant rpm. Which rpm should this be?

b) max engine power rpm
I cant quite work out why in my head, but it stands to reason that if you are going to run an engine at only one speed, it should be the speed which derives the greatest power from the engine. It think its because running at max power rpm would effectively allow the CVT to be running in a lower gear, thus delivering greater actual wheel torque throughout the speed range.

You got it right. On CVT you have the most WHEEL torque at power rpm, it doesn't matter if engine torque is lower.

[Shotglass]

You got it right. On CVT you have the most WHEEL torque at power rpm, it doesn't matter if engine torque is lower.

are you sure ? looking at the graphs grc produces id say the most torque at the wheels is still at the torque peak of the engine (the optimum point for the transient behaviour though might be a little higher in the rpm band depending on how flat the torque curve is)

[colcob]

Yeah, but remember that torque at the wheels = engineTorque*gearRatio.

So for a given speed, if you run at a higher engine revs, you run a lower gear ratio. Which, if you are running at max power RPM, will result in greater wheel torque.

[skiingman]

Yeah, but remember that torque at the wheels = engineTorque*gearRatio.

So for a given speed, if you run at a higher engine revs, you run a lower gear ratio. Which, if you are running at max power RPM, will result in greater wheel torque.
Regardless of the technical benefits, I still hate driving CVT cars. The modern ones are getting good at sort of simulating gear-changes to comfort the ear, but I still get the feeling I'm driving a go-kart or a snowmobile. I suppose you likely get used to it. :)

The Airbus captain won't care one iota about torque on the turbines, as turbofan engines produce thrust, not torque (although torque on the driveshaft is obviously a factor in the production of thrust, but the internal workings of the turbofan are not of much interest to the Captain).

An interesting case where the captain cares all about torque and HP is only a figure of academic interest is the PT6 (or probably about any turboshaft/constant speed prop setup) captain pays attention only to torque. Since his prop turns (more or less) at a constant speed, torque is all he needs to know. Its also the only thing that can be easily directly measured, which is probably why they have a torque gauge and not a power gauge.

[Shotglass]

Yeah, but remember that torque at the wheels = engineTorque*gearRatio.

which basically means that you scale the torque curve ... so the highest torque at the wheels for any gear ratio is always when the engine develops the most torque ...
so if your gearing always fits the current wheel speed perfectly youll get the most torque to the wheels at the engines torque peak

[Stregone]

But at a constant rpm more torque = more power. So the torque-meter is really showing the captain total power output.

[Hallen]

An interesting case where the captain cares all about torque and HP is only a figure of academic interest is the PT6 (or probably about any turboshaft/constant speed prop setup) captain pays attention only to torque. Since his prop turns (more or less) at a constant speed, torque is all he needs to know. Its also the only thing that can be easily directly measured, which is probably why they have a torque gauge and not a power gauge.

Absolutely correct. The PT6A-65AR for example uses 3650lbft of torque at max continuous power, 3800lbft for takeoff and will go has high as 4400lbft for emergency power. It will put out more, but every second you are there, you run the risk of throwing a blade off a turbine. This is at a prop RPM of 1700 at takeoff and 1425 for cruise (1300 RPM is the minimum for cruise). The horsepower is somewhat meaningless because we need to factor in the properties and efficiencies of the prop, which produces the thrust.

And Tristian, the Captain does care how and why his engine works the way it does. The way it is measured may obscure the mechanics of it (use % power not actual numbers), but the pilots do know how the power and how much power is being produced.

[StewartFisher]

which basically means that you scale the torque curve ... so the highest torque at the wheels for any gear ratio is always when the engine develops the most torque ...
so if your gearing always fits the current wheel speed perfectly youll get the most torque to the wheels at the engines torque peak
Look at the wheel torque/wheel speed graph in GRC again and load up any reasonable gear setup. Now try to imagine a smooth curve which maximises the torque at all wheel RPM, assuming that the transmission is infintely variable. This line will not go through the peak torque points in any gear. I'd imagine that it would go through the peak power point in each case.

[Bob Smith]

Yep, for any gear you get maximum acceleration at the torque peak. But, for any speed (assuming you can pick any gearing), you get maximum acceleration at the rpm of peak power. Say torque peaks at 5000 rpm and power at 8000rpm. And let's also say torque has dropped by 40%. For any certain speed, to reach this speed at power rpm requires 60% more gearing reduction than to reach this speed at the torque peak. So even after the reduced torque from the engine is taken into account, there is still 20% more torque at the wheels. So, as far as the wheels are concerned, you really get the most torque at peak power (for a multi-gear vehicle).

It sounds wrong but it is right.

[J.B.]

Good, it seems there is quite a lot of agreement in this thread now. That's a good sign that we do understand the subject properly. Well here are my answers to my questions from above.

1) When is the best point to shift up?
a) when the next gear puts the engine at an rpm of higher power
b) when the next gear puts the engine at an rpm of higher torque
c) when the next gear will deliver more torque to the wheels

Obviously c). The interesting thing though is that a) and c) are actually mathematically equivalent.

2) To get maximum accelleration out of an engine you can use a continuously variable transmission (CVT) that can keep the engine at a constant rpm. Which rpm should this be?
a) max engine torque rpm
b) max engine power rpm

b). colcob and others have already described why.

3) You want to improve the accelleration of your car by getting a new engine. The dealer will only tell you either the max torque or max power value (no rpm value). Which one will give you more useful information?
a) max torque
b) max power
c) both the same

b) will give you an idea. a) is useless without an rpm value to go with it or some other information about the engine.

4) You want to improve the accelleration of your car by getting a new engine. The dealer will only tell you either the torque curve or the power curve. Which one will give you more information?
a)torque curve
b)power curve
c)both the same

c) both contain exactly the same information.

5) With a given gear ratio you can go through the engine's rpm range by varying the speed of the car. At which rpm will you find the highest acceleration?
a) at max engine torque rpm
b) at max engine power rpm

a) since the torque multiplyer is constant, the highest torque at the engine will give you the highest torque at the wheels.

6) At a given vehicle speed you can go through the engine's rpm range by varying the gear ratio between engine and wheels. At which rpm will you find the highest acceleration?
a) at max engine torque rpm
b) at max engine power rpm

b) this question is essentially the same as question 2)

7) Which question, 5) or 6) is irrelevant when you are a girl and are trying to determine the overall drag-strip performance of your car? (bash tristan not me )
a) 5)
b) 6)

a) since you can't choose your speed when you are trying to accelerate.

So the prize goes to colcob, who got just about all right. :present: :wow: :chicken: :balloons: :trophy_si

[Shotglass]

Look at the wheel torque/wheel speed graph in GRC again and load up any reasonable gear setup. Now try to imagine a smooth curve which maximises the torque at all wheel RPM, assuming that the transmission is infintely variable. This line will not go through the peak torque points in any gear. I'd imagine that it would go through the peak power point in each case.

your thinking of it in the wrong way imho ... were talking about a gearing that constantly changes while you acellerate
to help you image this look at the graph while you constantly change one gears setting (the scroll wheel of your mouse will work best for this) and look at what happens the peak torque at the wheel constantly moves towars higher speeds while you change the gear ratio (it should follow some more or less hyperbolic curve) when you think about it its ovious that the highest torque at the wheels and therefore the highest accel when using a ctv happens when youre at the peak torque of the engine and not somewhat to the right of the curve where the engine produces less torque

b) this question is essentially the same as question 2)

look at bobs last answer in this thread ... your mistaken ... (and you even contradict your answer to question 5)

[FunkedUp]

:huh:

Perhaps if your gearing is off...

Power limits the acceleration of the car. Not torque. 1500Nm of torque does you little good at 400rpms.

Absolutely correct (MSME here)

[dave_w11]

The gearing tool I made for LFS, since you missed the link in my sig I'll put one here too: GRC (http://myweb.tiscali.co.uk/thefloatingwidget/lfs_grc.html)

Doh, I appear to be blind. Thanks! Great tool!

[spoop]

I didn't read any posts below the quiz, or google anything, but i'd have to say:
1c(this was hard:))
2a
3b
4c
5a
6a
7a

[skiingman]

Absolutely correct (MSME here)
Nice! I'm a year or so into my BSME...quite a ways to go...

[Resound]

It'd be nice if there were some big conspiracy to that effect....but thats just not the case.

A given acceleration over a given period of time requires a given amount of work to be done. This work cannot be done without power. All the torque in the world is nice, but means absolutely squat if the torque can't be produced far enough to the right of the graph.

I can give a very realistic and very obvious example which proves this point.

" torque/weight is directly analogous to force/mass which equals acceleration."

So I've got this electric motor that produces 2000lb-ft of torque at 1 rpm. It produces 300lb-ft at 1000 rpms. This is not an unrealistic torque curve for an electric motor.

Although it will smoke the tires even in fourth gear from a stop, no amount of gear swapping in the world will make it accelerate faster over any reasonable time period or distance than a 160hp gasoline engine making about 150lb-ft of torque.

By stating this:
Torque/effective wheel radius = delivered linear Force at contact patch.

you are implying that you don't understand the function of a gearbox. Bob did a good job of explaining its torque multiplication duties.

If torque to weight were a better indicator of performance than power to weight (obviously false with a basic look at physics) the izzy Celica would be faster/quicker than the duzzy Celica. Its not. Same goes for ZO6 vettes, which really don't make more torque than stock vettes. They move the torque curve further right, increasing the PRODUCT of torque and RPM......horsepower.

Right on, and thats exactly why the figure given is hp/weight, not torque/weight. Given non-silly gearing you can get pretty much the same result of calculation from hp/weight as you can by plotting torque available at a given engine speed/gear combo over time. Its a lot less labor intensive too, unless you have something cool like the tool Bob is using.

Close, but not quite *grin*. Acceleration from a given speed to a given speed is a quotient of horsepower and weight; that is, acceleration over time. This is determined not by peak power of the engine, but the area that you'd get if you graphed the torque curve of the engine (torque x rpm equaling power, of course) over the used rev range. Acceleration as an instantaneous value, that is at any given moment you're accelerating at Xmetres/sec/sec, is a straightforward torque/mass (and drag) equation. In other words (torsional) force divided by mass equals acceleration at that moment.

So acceleration from point to point IS determined by horsepower, but that's NET hp (over distance) not PEAK hp (instantaneous) which is a fairly meaningless value in this application because you're only at peak hp for a moment and using a work over time derivation for an instantaneous value is just silly. At any given moment best acceleration will be at peak torque.

It's only when you hold a single point or very narrow section of the rev range over a period of time significant to the exercise that you're performing (in this case, pushing a vehicle) that peak power becomes the most significant value. Because you're not accelerating hard, torque over TIME becomes more significant that torque over DISTANCE. So at high power/low acceleration (at or close to top speed) horsepower is more significant. So saying that overall torque is more important or power is more important is a bit simplistic, the emphasis will shift dependant on specific factors.

Or as people who think less and do more would say, torque makes you accelerate, power gives you top speed.

[Ball Bearing Turbo]

Right... That's some great articulation, thanks for putting the TIME factor into better words than I could previously...

That being said, an vehicle with a high peak POWER output rating is LIKELY to have more area under it's torque curve, and therefore able to perform more work over time and therefore accelerate faster.

[skiingman]

This is determined not by peak power of the engine, but the area that you'd get if you graphed the torque curve of the engine (torque x rpm equaling power, of course) over the used rev range.

Thats only true if you don't have the perfect gearbox.

Thus all my statements including "with reasonable gearing" as a precaution.

Acceleration as an instantaneous value

Not the way its expressed in automotive publications and the ramblings of enthusiasts. Time to distance or speed is what "acceleration" tends to refer to in these contexts. Not that its right...its obviously not. I've incredibly rarely heard any automotive enthusiast talk about actual accelerations.

Again, all my statements say "acceleration over time" which is what car people talk about.

Time to either speed or distance depends on the power produced. Torque can be used to determine this if you know all of the relevant figures. Since the relevant figures are often difficult to know and consume a good bit of mind-space, relating hp/weight figures is very simple, effective, and accurate.

So at high power/low acceleration (at or close to top speed) horsepower is more significant. So saying that overall torque is more important or power is more important is a bit simplistic, the emphasis will shift dependant on specific factors.

This is all pseudojargon. There is no particular end of the scale where one or the other is more effective. The laws of physics are the laws of physics, given certain data you can obtain other data.

Neither is more important...the laws of physics are rather non discriminatory. However, the entire point of the discussion is that a single hp/weight figure is far more meaningful than a single torque/weight figure for determining time to speed or distance...aka "acceleration."

An 800hp/ton car will always go from a to b in less time than the 400hp/ton car, so long as the gearing is appropriate. Even if the 400hp/ton car exhibits a larger value for the integral of the torque curve this is still true.

The 800hp car could perhaps only make power at 6000rpms...a single speed motor. Area under the curve would be zero.

Or as people who think less and do more would say, torque makes you accelerate, power gives you top speed.
You can't have one without the other. The torque at the top speed must be sufficient to produce the power.

Its really critical to note here that torque at the wheels does the acceleration as colcob explained.

[colcob]

I thats the fundamental point of misunderstanding that started this whole thing off. Me and bob were thinking of instantaneous acceleration values, and you were thinking of acceleration over time, which granted is how most people probably think of acceleration.

I think also its more accurate to say that the overall acceleration over time is the integral of the tractive effort curve for the speed range, not the torque curve for the rev range.

This obviously includes the gearing effects, so your single speed engine would just have a straight, descending line from its CVT gearbox, with a bit fat area under it :)

[Fonnybone]

Ahhh, and now it's comes down to torque vs horsepower...pretty soon we'll
go over aerodynamic drag vs downforce, what's this thread about again ?
Hehe. I like how force(torque), acceleration and work(hp) get all mixed up
in a big bowl of 'power'.

I'd just like to clear up some details. It's hard to get good results when you
start out wrong ;) Skiingman, it is wrong to say :

" torque/weight is directly analogous to force/mass which equals acceleration."

Let me start over at the first chapter of physics 101. From reading your
posts, i supposed you are familiar with F=ma, where F=Force(Newton),
m=mass(kg) and acceleration(ms²). Now, 'weight' is a human concept and
it describes what FORCE you exert on an object. Mass is the actual
molecular mass of all the stuff that make up your body. So, what you're
actually saying there is: Force/Force is analogous to Force/Mass. To get
weight, you use mass*acceleration, where acceleration is earth's gravity,
9.8ms².

It's a common error and the fact that some scales give mass while others
give your weight but that all of them express this the same way just adds
to the confusion. Typically, a household scale is a plate with a spring under
it. By measuring the compression of the spring, the scale can tell you what
FORCE you exert on the earth. Since earth's gravity is relatively constant,
it's easy to 'tune' all scales so you only have to step on it to get the correct
weight. Doctors on the other hand have proper scales that measure mass.
The way they do it is very simple. It uses weights to compare. By moving
weights around until the scale evens out, you get the value of the mass your
body displaces. Again, mass and weight are 2 different things. If you start out
mixing those, the rest can only be wrong.

I read detailed explanations on concept barely known yet no one notices
this obvious problem statement. Tell me when we get back to why GTRs
dont reach higher speeds on the straight.

[colcob]

Well it wasnt skiingman who said that, it was me. And I think you'll agree I (and most of the contributors to this thread) can manage without an idiots guide to physics.

If one value equals another value times a constant, the two values can be described as analogous, ie they have a proportional relationship.

So as force*lever arm (a constant) = Torque, Torque is proportional to force. As Weight = Mass*g(a constant), weight is proportional to mass. Hence the statement :

torque/weight is directly analogous to force/mass which equals acceleration.

is entirely correct.

Or if you like, (torque/weight) * K = Force/Mass

where K= 1/(leverArm*g)

[Fonnybone]

If one value equals another value times a constant, the two values can be described as analogous, ie they have a proportional relationship.

So as force*lever arm (a constant) = Torque, Torque is proportional to force. As Weight = Mass*g(a constant), weight is proportional to mass. Hence the statement :

torque/weight is directly analogous to force/mass which equals acceleration.

is entirely correct.

Or if you like, (torque/weight) * K = Force/Mass

where K= 1/(leverArm*g)

There, now i understood, i didn't realise you were comparing torque and force.
It's not about being an idiot, it's about being clear. If you think all people you
don't understand what you say are idiots then when it's everyone who
doesn't seem to understand, maybe it'll be more important to you.

Ok, so now that you guys have talked this over and concluded that
you were saying the same thing, can we move on ?

I hope skiingman doesn't take this personally ;) That statement would make
any physics teacher react. Of course, they might understand faster ;)

[Shotglass]

Hehe. I like how force(torque), acceleration and work(hp) get all mixed up in a big bowl of 'power'.

uh ... huh ? hp is a dimension of power not of work