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Old 5th February 2010, 15:55   #16
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Originally Posted by Shan2nu View Post


Tell me something. Based on theory if you were to race a 560bhp/ton car vs a 520bhp/ton car, which should ideally be quicker?

Shan2nu
560bhp/ton will win easily and if other things including torque remain the same.

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Old 5th February 2010, 16:01   #17
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Quote:
560bhp/ton car vs a 520bhp/ton car ??
What is the difference between those two?
Engine sizes or Induction systems?
Thats exaclty what i was trying to say.

You're already asking me about "mode of induction" and "engine size". The 2 easiest ways of increasing engine torque.

So its impossible to cal the performance of a car based on bhp and weight alone, or else a simple power to weight ratio figure should have been enough to do the math. But its not.

Quote:
560bhp/ton will win easily and if other things including torque remain the same.
Again, you had to mention "torque" bcoz it cannot be neglected. This is the very reason why i said a 7ltr 686bhp car weighing 1600kgs will be quicker than 2.4ltr 686bhp car weighing 1600kgs (even though both have identical power and power to weight ratio).

Take the Verna Diesel itself. It only does 92 or 93bhp/ton but accelerates like a 105-106bhp/ton na petrol car. Its not the power but the 235nm of torque thats making this happen. Had the verna been a 110bhp, 145nm, doing 93bhp/ton, it would have been nowhere in terms of performance.

Shan2nu

Last edited by Shan2nu : 5th February 2010 at 16:15.
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Old 5th February 2010, 16:38   #18
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The OP has assumed his engine to have a constant power at all rpm. So why bring torque or engine size into it? Constant and known power implies that the torque is also constant and known. If he's talking about a real engine, then we can talk about torque. But he's using a model engine with constant power.

I think the OP was surprised at the fact that a ten second car needs so much power. But that is the truth. The Fast and Furious movies make ten second cars appear a bit too commonplace...

Last edited by McLaren Rulez : 5th February 2010 at 16:43.
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Old 5th February 2010, 16:54   #19
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Originally Posted by McLaren Rulez View Post
The Fast and Furious movies make ten second cars appear a bit too commonplace...
I think the black car with a supercharger was of 900+ HP

Considering its size and weight that might not be possible, after all its a movie
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Old 5th February 2010, 16:56   #20
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Originally Posted by r@CYR@y View Post
I think the black car with a supercharger was of 900+ HP
Was it? Wow, I take that back then. Totally realistic
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Old 5th February 2010, 17:05   #21
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Originally Posted by McLaren Rulez View Post
Was it? Wow, I take that back then. Totally realistic
Dose'nt the rear wheels lose traction for that much power at disposal, Tire spin at the starting can be a major problem.

These days even 300 hp cars get traction control for effective utilization of the horse power.
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Old 5th February 2010, 17:13   #22
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The OP has assumed his engine to have a constant power at all rpm. So why bring torque or engine size into it? Constant and known power implies that the torque is also constant and known. If he's talking about a real engine, then we can talk about torque. But he's using a model engine with constant power.
He has menitoned "average car". Which average car or even supercar has an engine that makes 686bhp from idle to redline?

And even if such an engine existed, you would not be able to put down 686bhp from the word go. It would need a CVT transmission that alters power reaching the wheels, which would mean that most of the power lower down the revs would not be of any use.

If you were to dyno test such a car, it would still give you a power and torque curve.

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Old 5th February 2010, 17:33   #23
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Originally Posted by Shan2nu View Post
Which average car or even supercar has an engine that makes 686bhp from idle to redline?
Shan2nu


But a 10 sec car's power output is being discussed here, isn't?
Let's not drift away from it..

Considering the ideal car point, there is no such term in the OP, its just an idealized situation without any external losses such as transmission losses, drag losses etc..
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Old 5th February 2010, 17:35   #24
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Pardon my ignorance, I get it that the OP wants to find out a car's power that would enable it to do 1/4 mile in under 10 seconds.
But By definition a 10 second car means a car that does 0-60 mph or 0-100kmph under 11seconds.
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Old 5th February 2010, 17:38   #25
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Originally Posted by Bumble Beast View Post
Pardon my ignorance, I get it that the OP wants to find out a car's power that would enable it to do 1/4 mile in under 10 seconds.
But By definition a 10 second car means a car that does 0-60 mph or 0-100kmph under 11seconds.
10 second car means a car which can complete a standing quarter mile (402.4 mts approx) under 10.999 secs.
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Old 5th February 2010, 17:43   #26
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Quote:
But a 10 sec car's power output is being discussed here, isn't?
Let's not drift away from it..
What have i been telling you guys. Power output alone is not enough to calculate rate of accelelration.

An Atom does 10.8 with 450bhp
Mclaren F1 does 11.1 with 627bhp
Veyron does 9.95 with 987bhp
Formula 1 car does 8-8.5 secs with 700bhp

So how can you put a fixed bhp figure on a 10 sec car without knowing the other specifications of the engine/car?

Shan2nu

Last edited by Shan2nu : 5th February 2010 at 17:45.
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Old 5th February 2010, 18:16   #27
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I see this thread going off on a complete tangent.

The OP was talking about something completely different from what shade this discussion is talking. I saw the OP's profile and it is really heartening to see a youngster have such inquisitiveness and smarts to use classical physics to pose such a threatening problem.

In any case, my two cents about the issue are as follows.

Whenever we talk about powering houses or of energy reserves... we talk about Energy and not Power. 100 KW of power is useless if it is not sustained for a period of time.
And from there we move to the concept of kilo-watt hours.

Classical physics as applied by the OP, however far from the real values be, still is enough to pose the question.

But considering the short span of time we consider the point to be in motion (10 secs here)... we get a pretty small amount of energy... 1.4 KWH.

In case we use the same to power a generator or something similar... we would end up with a lower usable figure thanks to efficiency constraints.

I hope I was able to clear some of the doubts of the OP.

Best Regards.
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Old 5th February 2010, 18:36   #28
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Quote:
Originally Posted by Shan2nu View Post
What have i been telling you guys. Power output alone is not enough to calculate rate of accelelration.

An Atom does 10.8 with 450bhp
Mclaren F1 does 11.1 with 627bhp
Veyron does 9.95 with 987bhp
Formula 1 car does 8-8.5 secs with 700bhp

So how can you put a fixed bhp figure on a 10 sec car without knowing the other specifications of the engine/car?

Shan2nu
First thing : Atom is not 1600kgs but an average car.

2. The discussion here is on modifying an average car whereas McLaren, Bugatti and F1 are not average cars.

3. The power output should be considered as Horsepower and Torque combined term

4. Regarding the specifications, considering all the conditions being same only power is calculated (Ideal stuff)
For example a Honda accord weighing around 1600kgs (including a bigger engine and all other stiffeners) power required will be at-least 700 bhp
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Old 5th February 2010, 21:48   #29
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Originally Posted by im_srini View Post
The calculations assume a fixed acceleration of 8 m/sec^2 (~0.82 Gs) all the way, usually it's much higher at the start & falls off as aero-drag picks up. For example, at a constant acceleration of 8 m/sec^2, your terminal velocity at the end of 10 seconds is something like 288 Kmph, but the Veyron's speed at the end of the 1/4 mile is "only" 230 Kmph.
That was exactly my point. I had considered an idealised situation, I mean, no drag force or anything. Even then the required power was hovering around 690 HP. In the real world, there are so many -ve factors like friction, drag, power loss etc. which will increase the engine power required.

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Originally Posted by McLaren Rulez View Post
The OP has assumed his engine to have a constant power at all rpm. So why bring torque or engine size into it? Constant and known power implies that the torque is also constant and known. If he's talking about a real engine, then we can talk about torque. But he's using a model engine with constant power.

I think the OP was surprised at the fact that a ten second car needs so much power. But that is the truth. The Fast and Furious movies make ten second cars appear a bit too commonplace...

You're right. I was talking about a model engine or whatever. And you were again right. I'm heavily inspired by the Fast and Furious series

Quote:
Originally Posted by Shan2nu View Post
He has menitoned "average car". Which average car or even supercar has an engine that makes 686bhp from idle to redline?
Shan2nu
Er...again..I was talking about a model engine. We're currently being taught to assume a lot of things while talking about a model engine. Like the Carnot engine stuff. Just let me know if I'm getting anything wrong.

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Originally Posted by EssYouWe View Post
Whenever we talk about powering houses or of energy reserves... we talk about Energy and not Power. 100 KW of power is useless if it is not sustained for a period of time.
And from there we move to the concept of kilo-watt hours.

But considering the short span of time we consider the point to be in motion (10 secs here)... we get a pretty small amount of energy... 1.4 KWH.

In case we use the same to power a generator or something similar... we would end up with a lower usable figure thanks to efficiency constraints.
Thanks for that. The energy required is less, alright. But what I was wondering about was the power. I mean, an average air conditioner takes an hour to use that much of energy. But a 10-sec car does the same in 10 secs...so that got me confused.

So what are the things that make a 10-sec car possible ?? I mean, if a stock Veyron barely manages to do it and there are cars that do 400m in 4 seconds, what is the difference in them ?? Is it just drastic reducing of mass ? I mean, power cannot be transmitted more efficiently than an ideal model in which I was assuming constant acceleration, right ? So all the things that I have not included need an additional power to compensate. So what factor compensates all of these and much more that it pushes a car to 400m/4 sec ??
Thanks a lot for all those replies.
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Old 6th February 2010, 00:30   #30
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Quote:
Originally Posted by Shan2nu View Post
Accelelration doesn't always depend on power or power to weight ratio. Torque also plays a big role in it.
power is simply a product of torque and RPM, isn't it? At low RPM if you are not producing enough torque, you are not producing enough power.

in other words, what you are saying is two engine "rated" at same power produce different power at low RPMS, and the one that produce more power wins.

that way the discussion will be simplified.


That's why I said power-rpm curve needs to be accounted for.
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