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Old 17th March 2009, 17:12   #16
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Originally Posted by breezydrive View Post
Where you put Ford fiesta 1.4 TDCI, Bhp per tonne is too low, but the car is not that bad to drive!
I think the 0-60 and 0-100 figures make much more sense than bhp per tonne because all you see the max bhp and not the bhp curve at various rpms, a car might have very high bhp at very high rpm, but at low rpm it might have very low bhp which is not good for highways. Swift petrol falls in this category.

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Old 18th March 2009, 08:26   #17
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Originally Posted by revhappy View Post
I think the 0-60 and 0-100 figures make much more sense than bhp per tonne because all you see the max bhp and not the bhp curve at various rpms, a car might have very high bhp at very high rpm, but at low rpm it might have very low bhp which is not good for highways. Swift petrol falls in this category.

Cheers,
Nitin
I think you mean to say the torque curve. Power curves usually are the same for most of the engines, they go up incrementally up to a certain rpm and then drop down, the torque curves are the ones you need to worry about.

In a good engine, over 80% of the torque will be available in the 1500-4000 rpm range, which is the range most people drive in. And then the peak torque will be somewhere in between.
Of course I'm talking about gasoline engines here, but as always there are exceptions there as well (high revving motors).

Last edited by sujaylahiri : 18th March 2009 at 08:28.
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Old 18th March 2009, 08:47   #18
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Its true that BHP/Ton makes sense.
But the end result i.e. the on road real world performance of the car will depend no several factors. One thing that can play a very big role till lets say 100-110 kmph is the gearing. Till 100-110 kmph the drag co-efficient will not make much of a difference. I am aware that anything above 60kmph and most of the power goes to cut through the air.

Also where the engine makes power is important. Swift is relatively high geared machine, but that G-series motor is strong in top end of power band and makes up for the high gearing. Other examples are Esteem and Baleno.

But at the end of the day BHP/ton makes sense for performance.
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Old 18th March 2009, 12:54   #19
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I believe torque tells you about the acceleration capabilities of the car and BHP/tonnne highest speed achievable. Henceforth, if torque is high or maxes out at low rpm then car has good acceleration at lower speed. Also, the max BHP is generally at high rpms which is usually not acheived in short stretches so it doesnt really convert into high speed in say city driving but may be achieved in 4 way lane highway driving.
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Old 18th March 2009, 16:55   #20
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I believe torque tells you about the acceleration capabilities of the car and BHP/tonnne highest speed achievable.
Then why does a Veyron with just 530bhp/ton have a top speed of 407kmph, while a 1500bhp/ton F1 car with minimal aerodynamc resistance, could only manage 400kmph.

Its always a combination of nm/ton and bhp/ton (along with other factors) that dictate performance, be it acceleration or top speed.

Quote:
Also, the max BHP is generally at high rpms which is usually not acheived in short stretches so it doesnt really convert into high speed in say city driving but may be achieved in 4 way lane highway driving.
That doesn't make sense. Max bhp can be achieved in every gear. In 1st, my car achieves max bhp at 56kmph and i dont need a highway to do that.

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Old 18th March 2009, 17:39   #21
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Originally Posted by Shan2nu View Post
In 1st, my car achieves max bhp at 56kmph
Shan2nu
Wouldnt that kill your car?
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Old 18th March 2009, 17:45   #22
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Wouldnt that kill your car?
Nope, VTEC goes to 58kmph in 1st (7100rpm).

Its whats you call...."revhappy". LOL

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Old 19th March 2009, 05:25   #23
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Originally Posted by WheelWake View Post
I believe torque tells you about the acceleration capabilities of the car and BHP/tonnne highest speed achievable. Henceforth, if torque is high or maxes out at low rpm then car has good acceleration at lower speed. Also, the max BHP is generally at high rpms which is usually not acheived in short stretches so it doesnt really convert into high speed in say city driving but may be achieved in 4 way lane highway driving.
A simple formula: Power = Torque x rpm (for a given rpm) is what we use for sizing motors and gearboxes. There are some other constants involved but these 3 are the only variables.

As a general rule of thumb, the higher the motor revs, the lower peak torque it'll have compared to low revving motors. The torque will obviously depend on the torque curve, which is dependent on many factors.

So a "heavier" vehicle (low bhp/tonne) will not necessarily have a lower top speed, it all depends whether the engine has enough torque left to counter the drag before it runs out of rpm (reached redline).

Think of it this way; torque is the work the motor does (provide traction, counter drag) and power is the net result of all the work the motor has done (resultant motion).

As an example, a Formula 1 motor (2.4L BMW V8) will have to work 19,000 times in a minute to provide maximum acceleration when required, whereas a Hotrod motor just needs to work 5000 times in a minute.
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Old 19th March 2009, 05:48   #24
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while a 1500bhp/ton F1 car with minimal aerodynamc resistance
actually f1 cars arnt the most aerodynamically efficent(slippery) most road cars have a better coef of drag. all those wings and spoilers add huge downforce at the cost of creating drag.

thats why you need to turn a special key in the veron to lower the spoiler to get to its top speed.
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Old 19th March 2009, 07:50   #25
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Originally Posted by Shan2nu View Post
Nope, VTEC goes to 58kmph in 1st (7100rpm).

Its whats you call...."revhappy". LOL
I agree. In most Honda VTECs you really need to be at the upper end of the RPM spectrum to get the true performance. However it is different for a car like say a Golf GTI which has low end grunt.
'Revhappy' is correct! Just love the sound and the feel of VTEC.
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Old 19th March 2009, 09:29   #26
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Originally Posted by equinox22 View Post
I agree. In most Honda VTECs you really need to be at the upper end of the RPM spectrum to get the true performance. However it is different for a car like say a Golf GTI which has low end grunt.
'Revhappy' is correct! Just love the sound and the feel of VTEC.
indeed! VTEC loves to show 5000+ RPM and give you that thrust! It asks for more when you feel that you should shift up a gear!
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Old 19th March 2009, 13:27   #27
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Quote:
Originally Posted by Shan2nu
VTEC goes to 58kmph in 1st (7100rpm).
At what rpm does the revlimiter cut-in in the Vtec ?
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Old 19th March 2009, 16:04   #28
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actually f1 cars arnt the most aerodynamically efficent(slippery) most road cars have a better coef of drag. all those wings and spoilers add huge downforce at the cost of creating drag.
When i said 400kmph, i wasn't talking about your regular F1 car with the big wing n all. I was talking about this - Honda F1 sets land speed records at Bonneville

A normal F1 car would start struggling at around 360-365kmph.

Moreover, we are not comparing 2 cars with equal power to weight ratios here, the F1 car has close to 3 times the ptwr of that of the Veyron.

So you can't just say that a certain car will have a higher top speed, just bcoz it has a higher power to weight ratio. There are many other factors to be considered (including aerodynamic resistance).

Quote:
At what rpm does the revlimiter cut-in in the Vtec ?
7100 rpm is as far as it goes. Max power comes in at 6800rpm.

Quote:
I agree. In most Honda VTECs you really need to be at the upper end of the RPM spectrum to get the true performance. However it is different for a car like say a Golf GTI which has low end grunt.
It doesn't matter what type of engine you have, it'l still manage to achieve max power within city limits.

Shan2nu

Last edited by Shan2nu : 19th March 2009 at 16:06.
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Old 19th March 2009, 16:09   #29
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Its an old thread, but the question still applies as long as we use the word BHP

As per me its not the BHP/TON but Peak BHP/rpm range and also the torque graph that matters.

Its of hardly any use if your vehicle has a great BHP/TON value but if its generated at a high rpm..no point as you are hardly going to use it
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Old 19th March 2009, 16:22   #30
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Quote:
As per me its not the BHP/TON but Peak BHP/rpm range and also the torque graph that matters.

Its of hardly any use if your vehicle has a great BHP/TON value but if its generated at a high rpm..no point as you are hardly going to use it
Thats why we have gear ratios.

If you take a diesel that produces max power at 3500rpm and a petrol that produces max power at 6000rpm, you can still design the gearing in such a way that both reach their max power rpms at the same vehicle speed.

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