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Old 6th February 2009, 22:41   #76
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i think it means that the qjd and tdi both run out of steam at pretty much the same rpm , but the qjd gets there faster ( and hence accelerates better) than the tdi
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Old 15th October 2009, 14:57   #77
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Thumbs up Understanding the ratings of engine

Hi friends

what can we understand from the engine specifications of car.

for ex:
for swift ddis the figures are
Torque - 190N-m @ 2000 RPM
Power - 75PS @ 4000 RPM

for fabia PD-TDI
Torque - 155N-m @ 1600 RPM
Power - 69PS @ 4000 RPM

I have driven both the cars and felt that RPM of Swift rises much faster that fabia. For fabia till some where around 1700RPM the car does not have any pickup at all, after that its fine. So driving through city in fabia is not so pleasant experience.

Some of my doubts are
1. What can you judge by looking at these ratings of the car. Can you say which one of the car has got superior pickup, better top speed etc.
2. 69PS @ 4000 RPM what about power after 4000 RPM will it be always 69 beyond that ? Fabia i have not driven above 3200 RPM so does that mean that i have never experienced its max power?
3. 155N-m @ 1600 RPM what about torque above 1600 RPM ? for swift the figure is 190 N-m @ 2000 RPM.
4. If 2 cars exactly the same rating does that mean that performance is also same. Obviously its not i think! But why ?

thanks in advance.

Last edited by Jaggu : 15th October 2009 at 15:02. Reason: please search before opening a new thread, thanks
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Old 15th October 2009, 21:22   #78
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At the risk of repeating myself: read the whole thread, your questions will be answered.

however, for your benefit, I will recap: You can work everything out by the formula BHP= TorquexRPM/5250. Thus, to compare pickup rtc., you need the following: a) The Torque curve, b) the weight of the vehicle and c) the gearing ratios. Thus you cannot judge two cars simply by the max torque and max BHP figures.
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Old 15th October 2009, 21:34   #79
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very nice information, got something to learn from it
thanks buddy
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Old 16th October 2009, 01:56   #80
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Quote:
Originally Posted by revhappy View Post
Does that mean that qjd accelarates faster than the tdi but in the end it loses steam? Whereas the Tdi is more linear in acceleration?
It's just the way you read it!

It can also be read as, QJD has less turbo lag, while Tdi has considerably more of the same. Losing steam is a very different thing, the QJD doesn't lose steam at all.
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Old 16th October 2009, 06:36   #81
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Torque all the way
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Old 16th October 2009, 08:16   #82
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Let me put it this way, as per the equation, "More Torque helps in generating more BHP at lower RPMs.Hence a high torque Low RPM engine can generate same BHP as a High RPM low torque engine."
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Old 10th November 2009, 14:11   #83
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Quote:
Originally Posted by Shan2nu View Post
One more thing is that, though i've said that torque and power are related, more torque doesn't always result in more power. The rpm factor is way too imporant to go un-noticed here.

Lets take 2 different engines for example.
Engine 1 : is a 2 ltr engine producing 200nm or 147.49 lbft of torque at 2500 rpm.

Engine 2 : is a 1 ltr engine producing 100 nm or 73.74 lbft of torque at 5000 rpm.

But see what happens when we calculate the power at the rpm where peak torque is produced.

engine 1 : bhp = torque * rpm / 5252 = 147.49 * 2500 / 5252 = 70.20 bhp.

engine 2 : bhp = torque * rpm / 5252 = 73.74 * 5000 / 5252 = 70.20 bhp.

As you can see, engine 2 which is just 1 ltr produces the same amount of power as that of the bigger 2 ltr'd engine, inspite of having half the torque.

This is pretty much the concept used in F1 engines. When engine capacity is limited, the only way a "Naturally Aspirated" engine can produce more power is if the torque is got at a much higher rpm.

Such engines are not the best when it comes to day-to-day use but can be fun to drive if you can manage to keep them within the "meaty part" of the power band.

Regards...
Shan2nu

What I get from Formula 1 example is that F1 cars due to limitations like samll engine size, shorter stroke can't generate enough torque, so trick to produce Max. BHP is to maximise the RPM's.
But how are Formula cars able to achieve such high RPM's as compared to normal vehicles, from engine design point of view ?
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Old 10th November 2009, 16:48   #84
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Theres however still one info that I havent been able to get either through net or on this community. My doubt is agreed BHP can be increased either by increasing the torque or increaing the RPM. But to increase the RPM you need to increase the torque (to accelerate and hence increase the RPM). so increasing the RPM and increasing the torque is one and the same isnt it ? How else can you increase the RPM without increasing the torque ?
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Old 10th November 2009, 18:03   #85
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Quote:
Originally Posted by high_octane View Post
But to increase the RPM you need to increase the torque
Things are getting really complicated in here, Let me put it in a simple way..

Basically

Torque == Pulling power

Horsepower == Pushing power

Now if you need to pull your vehicle (I mean to accelerate) torque is required and if want to push your vehicle (Top Speed) the you need horsepower..

Then rpm,

Quote:
Originally Posted by high_octane View Post
How else can you increase the RPM without increasing the torque ?
RPM == Revolutions per min
If you see a dyno chart you can get the whole picture of torque and horsepower at any engine speed of a car, to increase the rpm you need to add more fuel in those cylinders, the faster the engine spins the more horsepower it makes and the harder it spins the more torque it will make..
Now faster here means faster and
harder here means against transmission resistance.

As the transmission is connected to both crankshaft and tires, resistance from one is resistance to all and please note that
"When a car is moving transmission is also spinning, ie., less resistance on engine and to spin it faster you need more horsepower, and if a car is standing the engine needs to pull the whole mass here torque is required"

any further doubts pls let me know..

Keep revving..
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Old 10th November 2009, 18:26   #86
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One more doubt here.
Most of the cars have peak torque in a limited range eg. 2000-3000 rpm. But why can't peak torque be extended till redline RPM or over a much broder RPM range. Are there any design limitations here or is it deliberately done ?
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Old 10th November 2009, 18:42   #87
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Its due to the physical limitations of every engine and it mostly depends on the ecu mapping of every individual model and the purpose of use of the car..
Mostly city driven cars need torque in low rpm (You don't want to waste fuel, do you? and sports car have it in top range, since they slow down less frequently..
Did you get it?

Keep revving..
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Old 10th November 2009, 18:49   #88
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Quote:
Originally Posted by r@CYR@y View Post
Its due to the physical limitations of every engine and it mostly depends on the ecu mapping of every individual model and the purpose of use of the car..
Mostly city driven cars need torque in low rpm (You don't want to waste fuel, do you? and sports car have it in top range, since they slow down less frequently..
Did you get it?

Keep revving..
OK so that's way Torque curve dips after peak torque is attained, Guess I am right.
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Old 10th November 2009, 18:59   #89
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Quote:
Theres however still one info that I havent been able to get either through net or on this community. My doubt is agreed BHP can be increased either by increasing the torque or increaing the RPM. But to increase the RPM you need to increase the torque (to accelerate and hence increase the RPM). so increasing the RPM and increasing the torque is one and the same isnt it ? How else can you increase the RPM without increasing the torque ?
Nope, engine rpm doesn't depend on torque. It depends more on the design of the engine, the weight of the internal parts (especially rotational mass), frictional resistance etc.

Which is why a 1.5 Verna VGT engine produces 235nm@1900rpm while an OHC VTEC does only 132nm@4700rpm. Even if i was to bolt on a TC on the Honda engine and boost its torque to 235nm, it would still not be able to produce max torque at 1900rpm since engine isn't designed for it.

Shan2nu
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Old 10th November 2009, 20:11   #90
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Sorry I am going to crank it a little bit

Quote:
Originally Posted by r@CYR@y View Post
"......... to spin it faster you need more horsepower....."
I thought its the other way ??? I mean rpm should control the horsepower and not the other way around isnt it ?

shan2nu and r@CYR@y, I kind of get what you guys are trying to say. you are saying, the increase in the rpm can be controlled by the design of the engine only. That means, the rpm depends on how smoothly the pistons move and hence affect how faster the transmission rotates. This is like a big doubt solved !
So the increase in the rpm levels in order to increase the power of the car (at a fixed speed,remember) cannot be accomplished if the design of the engine is not altered. One way,however, could be by making the same car light (using lighter materials like lighter wheels etc ). On the contrary ,I think the torque of the car (ie the pulling power) can be increased easily isnt it ?

To sum up ...
Torque : related more to how MUCH energy can be produced in the engine(which could be increased by using a high octane fuel and so on)
RPM : related more to how smoothly the pistons rotate overcoming friction due to weight,cylinder interior surface (along which the piston moves) which is dependent on the engine design

Please correct if I am wrong anywhere !
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