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Old 28th June 2011, 19:32   #31
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Interesting topic,
I would prefer to understand where the torque and power start and end.

Drags are in significant under 80 kph imo
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Old 28th June 2011, 22:53   #32
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Default Re: Power/Weight ratio : relevance for driving

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Originally Posted by dot View Post
To me,

Everything else remaining same,
1. Car with more power/weight than another translates to a higher max speed.

2. Car with more torque/weight than another means faster 0 to 60/100 numbers.

But the key thing is everything else remaining same like air drag, wheel friction.



That is why if you lightweight your existing car, it will be a better drag racer.
I don't understand at all:

Let us assume we take two car of identical make and model (Chevy beat for example) take the engine out of car A, put a 120bhp engine in the new model (against the roughly 90bhp engine) and then load the car with about 500kg distributed all over. Car B is without changes.

Now let us say you race them on a long straight track side by side. Which one do you think will attain higher max speed? Car A or Car B? My guess is Car A (peak speed for both will be in 150kmph range, drag will be WAY higher than rolling resistance for both cars) - your comments above (point number 1), if true will mean car B.
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Old 29th June 2011, 09:19   #33
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Default Re: Power/Weight ratio : relevance for driving

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Originally Posted by vina View Post
I don't understand at all:

Let us assume we take two car of identical make and model (Chevy beat for example) take the engine out of car A, put a 120bhp engine in the new model (against the roughly 90bhp engine) and then load the car with about 500kg distributed all over. Car B is without changes.

Now let us say you race them on a long straight track side by side. Which one do you think will attain higher max speed? Car A or Car B? My guess is Car A (peak speed for both will be in 150kmph range, drag will be WAY higher than rolling resistance for both cars) - your comments above (point number 1), if true will mean car B.
Interesting point. Why do you think turbocharging a car results in better performance?
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Old 29th June 2011, 10:21   #34
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Default Re: Power/Weight ratio : relevance for driving

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Originally Posted by vina View Post
I don't understand at all:

Let us assume we take two car of identical make and model (Chevy beat for example) take the engine out of car A, put a 120bhp engine in the new model (against the roughly 90bhp engine) and then load the car with about 500kg distributed all over. Car B is without changes.

Now let us say you race them on a long straight track side by side. Which one do you think will attain higher max speed? Car A or Car B? My guess is Car A (peak speed for both will be in 150kmph range, drag will be WAY higher than rolling resistance for both cars) - your comments above (point number 1), if true will mean car B.
have you changed the final drive?
if not, then on a long long unending road, they'll eventually catch up, and be together...endlessly...

(your stock car based on power to weight will still be quicker off the line, and to top speed.)
i think you could ask bhpians who have e- or b-zens.
how does it behave with the weekend shopping and a full load of companions, compared to the single-occupant behaviour during stock days.

Last edited by mayankk : 29th June 2011 at 10:25.
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Old 29th June 2011, 11:27   #35
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Default Re: Power/Weight ratio : relevance for driving

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Originally Posted by mayankk View Post
have you changed the final drive?
if not, then on a long long unending road, they'll eventually catch up, and be together...endlessly...
.

Why would that be? the 90bhp car will match wind resistance + rolling at speed v1, the 120bhp (but heavier) car will meet wind resistance + rolling at v2

If v2>v1 (as it will be, my reasons in previous post) then the 120bhp car will eventually go faster.
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Old 29th June 2011, 16:38   #36
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Default Re: Power/Weight ratio : relevance for driving

Air Drag forces are proportional to the square of speed.
Therefore they are more applicable at the "top speed" levels than the frictional forces (which are dependent on load).

And what is against this - the motive force provided by the engine. The force provided by the engine depends on power (= force x velocity).

Therefore its the power along with the aerodynamic shape that primarily determines the top speed. The vehicle weight may be high or low - but it doesn't matter as much.

At < 60 kmph speed levels - of course its the power and weight which matter. (because air drag has less effect)


Now coming to the acceleration part. Here - weight of the vehicle (or more correctly - the mass) plays very important role, since acceleration = force / mass.
Lesser mass - higher the acceleration.
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Old 29th June 2011, 17:45   #37
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Default Re: Power/Weight ratio : relevance for driving

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Originally Posted by alpha1 View Post

...


Now coming to the acceleration part. Here - weight of the vehicle (or more correctly - the mass) plays very important role, since acceleration = force / mass.
Lesser mass - higher the acceleration.

This was one thing I was missing - overtaking on highways will be easier in lighter cars and higher bhp/weight will definitely help.


In city I till believe the bhp/weight may not be relevant - low end power is usually not well represented by peak power numbers.
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Old 29th June 2011, 18:02   #38
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Default Re: Power/Weight ratio : relevance for driving

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Originally Posted by vina View Post
This was one thing I was missing - overtaking on highways will be easier in lighter cars and higher bhp/weight will definitely help.
Overtaking, or better roll-on figures depends on only one thing: A great torque/weight ratio. And also, flat max torque curve. I don't think bhp has much relevance in overtaking or roll on figures.
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Old 29th June 2011, 18:11   #39
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Default Re: Power/Weight ratio : relevance for driving

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Originally Posted by dhanushs View Post
Overtaking, or better roll-on figures depends on only one thing: A great torque/weight ratio. And also, flat max torque curve. I don't think bhp has much relevance in overtaking or roll on figures.

I think this has been discussed to death elsewhere, but here we go again

For a given rpm, there is no difference between "more torque" and "more power". So if you are trying to accelerate without downshifting you press the pedal and the engine generates more power as well as more torque.
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Old 29th June 2011, 18:37   #40
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Default Re: Power/Weight ratio : relevance for driving

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Originally Posted by dhanushs View Post
Overtaking, or better roll-on figures depends on only one thing: A great torque/weight ratio. And also, flat max torque curve. I don't think bhp has much relevance in overtaking or roll on figures.
As rightly mentioned above by Vina - Power is nothing but a product of torque and rpm.
So it doesn't matter what you take into account.

Where all this plays spoilt-sport is when you change gears. This changes the wheel RPM to engine RPM ratio, as well as torque at wheel to engine torque ratio. (which is again mentioned above by Vina).

So all our discussions have to ensure that you either remain in same gear, or account for the change in torque/rpm due to change in gears.

The good thing about using power in these scenarios is that it doesn't matter which gear. Power = torque * rpm, and thus remains same.
Therefore we prefer using power instead of torque.
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Old 30th June 2011, 15:51   #41
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Default Re: Power/Weight ratio : relevance for driving

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Originally Posted by dustom_99 View Post
I wonder why dont I see any drag reduction system for any indian car for sale. We do not have such products because people dont care whats the drag doing ,they dont connect with it.
A DRS will not have any effect under the IDC where your speeds are under 70 kph legally. Even motorways around "most" of the world have speed restrictions to reduce the complexities associated with it [read DRAG and associated issues with managing it]

You wanna go fast, the track is the place for it

Quote:
Originally Posted by mayankk View Post
i have a feeling you have some sort of technical qualification in automotives.
You should read his other threads to understand what he tries to do - Interesting read though I admit
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Old 1st July 2011, 00:11   #42
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Default Re: Power/Weight ratio : relevance for driving

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Originally Posted by headers View Post
Yeah, agreed - but does drag really affect the drive ability of cars in city speed limits - I guess not.

A CD is relevant above 60 or 70 kms only based on the weight and category of the vehicle!

At slower speeds - one [normal person with normal driving skills] cannot find any difference.

If drag could effect drivability of cars at 60-70kmph, it would be extremely hard on bikers - God in his infinite wisdom (an evolution with its messy patience) didn't decide to make human bodies streamlined for the airflow encountered on a bike - though people do try with their posture.



By the way - I saw this picture File:Energy flows in car.svg - Wikipedia, the free encyclopedia in this article Fuel economy in automobiles - Wikipedia, the free encyclopedia on wikipedia


At highway speeds rolling resistance is about 1/3rd of the total resistance - so a weight difference of 20% between two cars (e.g. same car with driver only and with driver + 2 passengers of avg. weight) will lead to roughly 6% extra rolling resistance on the heavier car - This is hardly anything noticeable without sensitive instruments in highly controlled conditions.
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Old 30th November 2013, 18:48   #43
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Default Re: Power/Weight ratio : relevance for driving

Interesting topic. Is Polo GT TDi the most powerful diesel car under 10 lakhs in terms of power/weight ratio? If so, what is the 2nd most and how far ahead is the Polo?

Last edited by nvssudheer : 30th November 2013 at 18:49.
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