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Old 21st September 2008, 12:35   #31
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I'm sorry Opendro, but that formula is only an approximation (as are most formulae). The key error in the formula is that you assume that the car will redline in its highest gear (5th in most Indian cars).

If you drove a number of different cars (especially cars available in India) you will find that almost no Indian car can redline in 5th. In fact, if anybody knows of a stock car that redlines in its highest gear, I would be quite pleasantly surprised to know of it.

Most cars can redline in the second highest gear (i.e. 4th in most cars available in India). For example, my Swift reaches approximately 165 km/h in 4th gear. Now, if I put 500kg of lead in the back seat of my Swift, do you think it would still be able to reach that speed in 4th (or any) gear? How about if I detuned the engine to make only 50bhp instead of 85bhp? Or if I hung my bedsheets to dry from a clothes rack mounted on the roof? Would it still be able to reach 165 km/h?

So therefore, power is usually the most important factor in determining a car's top speed. Since your formula does not include any mention of power, or the two other key determinants of top speed (aerodynamic resistance, rolling resistance) there's no point in using it at all.

May I also propose that there's no point really in being fascinated about top speeds. How often would you reach your car's top speed? If it is often, then you're breaking the law, endangering your own life and the lives of others on the road.

If however, like a number of people here, you are interested in spirited driving, pay heed to more significant performance measures such as 0-60 and 0-100 timings, cornering abilities (measured in g-forces, or max speed around a particular radius corner) and other such measures that have more real world implications.
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Old 21st September 2008, 20:37   #32
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Originally Posted by tsk1979 View Post
Lots of vehicles like that.
Esteem, Indica, Getz P etc.,
Most petrols sold in India will not redline in 5th gear.
Some of them will take a lot of time to redline in 4th gear too.

Nowadays other factors also play the role. For example ECU limits etc., which electronically limit the rpm to a certain value in each gear
Very true. My civic also gave me my top speed ever in 4th Gear. Even at a speed of 192kmph my rpm was around 3500 - 4000. Civic is capable over 7000+ when an electronic limiter stops/cuts off. Also 170kmph is always seen in 5th gear around 3500 rpm. I really cannot think that my Civic can ever touch 7000+rpm in 4th/5th with these observations. Yes, it maxes out in the 1st and 2nd gear, but top speed is nowhere near those figures...I guess there is something seriously wrong in these computations. I also came to know from some Civic forums that the car is even electronically limited to 210kmph.

Speed of a car depends on many parameters as another BHPian said. I guess its not so simple. Further why one must know that - only to get tempted to do the pumping - breaking the law?
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Old 22nd September 2008, 01:58   #33
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Originally Posted by opendro View Post
Jat I didn't say anything about fuel economy.
This was just to put an example for understanding that top speed is related to power required and available (more power, more fuel)
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I did not ignore the torque altogether.
there is no standard formula, except for generalising with inclusion of torque. your formula won't hold good, because the net torque available will change with the gearing. higher the gear, lower the torque on wheels and that is decided by gear ratios which maker want to have. the 5th one is overdrive.

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Another thing. I think many people have assumed that the max RPM as the techometer redline or whatever.
max rpm is limited by design parameters and has no relation to tachometer.

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1. To remove the notion that a powerful engine can do higher top speed (I remember how people were trying to calculate using torque when Verna's CRDI engine was plonked on Getz)
higher powe (or torque) is reduced by gearing to match the speed and fuel economy.

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I could be wrong, but I somehow lost the value I used to hold about team bhp, because the arguments and criticisms are very sub standard and less substantiated or at least I felt that way in this thread.
well it is an open forum and each person has his own independent views on it. but i would say that people here quite experienced and knowledgeable.

and i am sorry if i have hurt you anyway. my intention was not that. but to clarify your approach. i am not saying that you are wrong. but your approach is incomplete.
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Old 22nd September 2008, 19:20   #34
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Personal opinions apart, I think this is a good thread and contains some valuable information. This thread made me think about things I wouldn't have given a thought otherwise.
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Old 24th September 2008, 17:54   #35
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I do understand the increasing resistance at higher speed, reduced torque on the wheel at higher gears, etc. etc. In spite of understanding that, I had still put that formula forward with certain assumptions.

Okay, guys. I will not comment (meaning neither agree not disagree with others arguments) until I get some opportunity to test some of the vehicles. Even 140+ on a santro feels scary even though it feels that it can pull much more. That makes me feel that I'm very unlikely to be able to test on Indian road conditions though. Moreover, I'm a diesel guy and some of my friends will have to agree to my testing when opportunity comes.

However, if you think that the top speeds in the magazines are to be believed, I would still try to apply my formula and see how far it can be generalized to Indian cars at the moment. My next post will have at least some list of cars applied.

OT: On that post of mine that looked upset, I was actually thinking of how people (specially distinguished members) didn't agree when I argued in another post that two engines with similar torque curve and gear ratios will pull the cars of similar weight equally, irrespective of the displacement of the two engines (Linea 1.3L MJD versus Octavia 1.9L TDI in particular). I think I overdid it
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Old 24th September 2008, 18:50   #36
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Quote:
Originally Posted by opendro View Post
OT: On that post of mine that looked upset, I was actually thinking of how people (specially distinguished members) didn't agree when I argued in another post that two engines with similar torque curve and gear ratios will pull the cars of similar weight equally, irrespective of the displacement of the two engines (Linea 1.3L MJD versus Octavia 1.9L TDI in particular). I think I overdid it
The Gypsy king puts out 80bhp.
The Indica Xeta puts out 65 BHp
Both weigh 1 tonne.
Gypsy king is more poweful.
But the Indica Xeta can touch 150, given enough road, but gypsy will struggle beyond 130.
The reason is wind resistance. Gypsy has a breadbox shape and at high speed wind is a very strong limiting factor,
Infact in vaccuum, your formula will hold very well, but with air playing spoilsport, most cars fall very short of the red line in 5th gear limit.
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Old 26th September 2008, 21:32   #37
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Originally Posted by laluks View Post
Very true. My civic also gave me my top speed ever in 4th Gear. Even at a speed of 192kmph my rpm was around 3500 - 4000. Civic is capable over 7000+ when an electronic limiter stops/cuts off. Also 170kmph is always seen in 5th gear around 3500 rpm. I really cannot think that my Civic can ever touch 7000+rpm in 4th/5th with these observations. Yes, it maxes out in the 1st and 2nd gear, but top speed is nowhere near those figures...I guess there is something seriously wrong in these computations. I also came to know from some Civic forums that the car is even electronically limited to 210kmph.
Such less RPMs at such high speeds? If what you are saying is true, then the Civic must return very good mileage even while doing speeds above 150kph, which I'm sure it doesn't!!
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Old 26th September 2008, 23:58   #38
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if civic returns about 8~9 kpl at steady speed of 150 kph and above, i would say, it is an excellent mileage
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Old 27th September 2008, 00:24   #39
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[quote=GTO;983414]I have only seen some diesels hit max rpm in 5th gear. AFAIK, no Indian petrol car does.
T/QUOTE]


An exception is the Palio 1.6 engined vehicles which reach max speed only in the 5th gear. Both 4th and 5th gears are over drive gears in the 1.6 engined gearboxes but yet top whack is in 5th gear.
4th gear runs out of steam by 160 and then we are forced to change to 5th to reach an indicated max Odometer speed of 185kmph.

Last edited by drpullockaran : 27th September 2008 at 00:28.
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Old 27th September 2008, 12:04   #40
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Quote:
Originally Posted by drpullockaran View Post
An exception is the Palio 1.6 engined vehicles which reach max speed only in the 5th gear. Both 4th and 5th gears are over drive gears in the 1.6 engined gearboxes but yet top whack is in 5th gear.
Notice I said max rpm and not max speed.
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Old 28th September 2008, 22:46   #41
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Just as I thought I would not be able to test cars top speed practically in Indian roads, and I would use megazine reports, then I saw Sep-08 ACI, that mentions Indica Vista Quadrajet top speed is 151Kmph. What!! I have crossed 140Kmph in Indica DLS V2, non-turbo, just 53bhp, with four adults on plane road (from Krishnagiri towards Chennai to be specific). Carwale.com has 141Kmph as top speed for Santro Zing. And I have crossed that with two adults on the same road. How can I trust such megazine reports? I will however try to get as much info as possible and post further.
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Old 28th September 2008, 23:20   #42
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Quote:
Originally Posted by opendro View Post
Just as I thought I would not be able to test cars top speed practically in Indian roads, and I would use megazine reports, then I saw Sep-08 ACI, that mentions Indica Vista Quadrajet top speed is 151Kmph. What!! I have crossed 140Kmph in Indica DLS V2, non-turbo, just 53bhp, with four adults on plane road (from Krishnagiri towards Chennai to be specific). Carwale.com has 141Kmph as top speed for Santro Zing. And I have crossed that with two adults on the same road. How can I trust such megazine reports? I will however try to get as much info as possible and post further.
140 only? Well in that road at the slight downward slope I went beyond the speedo end(160kmph+).
On straight highway(no slope, I have done 145kmph on the speedo).
But the speed was not 145, it was 132kmph.
And how did I know the actual speed, the GPS told me, (accuracy of 0.2kmph).
This is how car magazines test speed. using vbox etc., to measure true top speed. What you see on the speedo is usually +5-+10%
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Old 29th September 2008, 02:53   #43
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I once took my cousins indigo diesel to 160kmph, god only knows what the true speed was.

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Old 29th September 2008, 17:22   #44
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I once took my cousins indigo diesel to 160kmph, god only knows what the true speed was.

Shan2nu
You can try calculating the deviation of the speedometer from "supposedly" correct speed calculated using:

Speed at a given rpm in a given gear for a given tyre spec (in Kmph) =
(width * 2 * aspect ratio / 100 + rim diameter * 25.4) * PI * rpm * 60 / gear ratio / final drive / 1000 / 1000
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Old 29th September 2008, 20:56   #45
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Hi OpenDro, I thought we'd do well to put up some concrete (relatively) numbers & see where it goes. The figures are for my car, a 2000 Alto Vx. Let's start with the tires, I'm running 175/60R13 Yokos...
Name:  A_TireProfiles.jpg
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I guess that gives us a tire circumference of around 1.698 meters. The first, second, third, fourth, fifth, & final drive ratios are 3.416, 1.894, 1.280, 0.914, 0.757, & 3.589. At 6000 RPM, the theoretical maximum speed in each gear would be...
Name:  B_TheoreticalTopSpeed.jpg
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As you can see, just using the engine RPM & tire dimensions gives you a top speed that is impossible ! Let's include torque, power, & factor in air & rolling resistance & see where it goes. I got this graph off team-bhp sometime back...
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From which you get this...
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Based on this & the gear ratio data we can get this, which (I think) is the force put down at the contact point in each gear for 1500, 3000, 4500, & 6000 RPM...
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Now that we think we know what's propelling us forward, let's see if we can get at what's pushing us back. The two primary forces are rolling resistance & drag (wind resistance).

For drag, I've assumed a Cd of 0.32, an air density of 1.127 Kgs/cu.meter (40 deg C), & a frontal area of 1.7781 sq. meters. Frontal area's not usually mentioned in the specifications, hence I took this...
How to deduce top speed of a car without breaking speed limit-h_altofrontandsideview.jpg

And made this...
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And counted the number of masked pixels to get at the frontal area (approximately at least). For the purposes of computing rolling resistance, the mass assumed was 750 Kgs & the Crr assumed was 0.0100...
Name:  F_DragAndRollingResistance.jpg
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If you then put everything together...
How to deduce top speed of a car without breaking speed limit-g_opposingforcesgraph.jpg

It's not terribly accurate but you can see that the top speed where the opposing forces negate each other is around 170 Kmph at 4500 RPM in fifth & 180 Kmph at close to 6000 RPM in fourth. Yeah, the Alto doesn't do 170 or 180, but the transmission losses have not been factored in (I think typical values are 10% to 15%), if they are, the top speed numbers would be a bit tamer. And, if I remember correct, the top speed of the Alto Vx, as per specifications, is 156 Kmph.

Gross simplifications have been made here, but the top speeds predicted are definitely more realistic than values like 225 Kmph - there is more to top speed than just RPMs & tire dimensions.

Last edited by im_srini : 29th September 2008 at 21:04.
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