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Old 18th May 2004, 10:49   #1
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Default Torque vs BHP?

ok major point of confusion: what is the diff between torque and bhp and which one will have a greater effect on top speed and acceleration figures?
have heard that diesel engines r more torquier by nature. why??

say i have 2 cars one which has 10 x of torque and 20 bhp and the other has 20x of torque and 10 bhp.
which car will be quicker??

Note from mod : threads merged.

Last edited by tsk1979 : 2nd October 2006 at 12:04.
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Old 18th May 2004, 12:12   #2
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whoaaa its more complicated.. very simple explanation will be good torque will help u with faster of the mark then u need the bhp curve to carry it to the top speeds.. idealy u need a combination.. torque means more lugging power vs bhp higher top speeds.. but believe me its very complicated and has loads of other factors involved..
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Old 20th May 2004, 14:51   #3
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Torque and power are very much related. The amount of power an engine produces at a certain rpm depends on the amount of torque produced at that rpm.

For EG: the Vtec produces 132nm at 4700rpm. Now to calculate the power we will have to convert nm to lb ft which comes to 97.34 lb ft.

Now using the formula (bhp = torque * rpm/5252) we get 97.34 * 4700 /5252 = 87.10 bhp.

So the Vtec produces 87.10 bhp at 4700 rpm.

IMO the ideal torque curve would be if an engine were to produce max torque as low as 2000 rpm and keep it there for as long as possible.

If the Vtec engine was to keep that max torque of 97.34 lb ft beyond 4700 rpm, the engine would have produced 106 bhp at 5719.25 rpm and would have gone all the way to 126.03 bhp at 6800 rpm.

So the power of an engine depends on how much torque is produced at different engine speeds.

Diesel engines produce more torque mainly due to the high compression ratios the engines run on and the fact that they have a smaller bore and a longer stroke.

Actually it isn't that complicated but does take a lot of time before you start understanding it. When i started off with this, i was as confused as you are but when as i kept reading and discussing this with others, i've got a basic idea of what it's all about. *

Regards...
Shan2nu



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Old 20th May 2004, 15:15   #4
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Quote:
Originally Posted by [b
Quote[/b] (souled @ May 18 2004,19:19)]say i have 2 cars one which has 10 x of torque and 20 bhp and the other has 20x of torque and 10 bhp.
which car will be quicker??
Cosidering that both cars weigh the same, both run on petrol, have the same engine capacity and have the same gear ratios, i'd put my money on the car with more power.

But it's not really that easy to say unless you mention the rpm at which these figures are produced.

The fact that car one produces less torque and more power proves that it revvs higher than the other car. So technically the car with more power will have an edge over the other car as far as top speed and acceleration is concerned.

At the same time i wouldn't be surprised if the car with less power beat the other car for the first few meters.

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Old 20th May 2004, 15:17   #5
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neat explanation mate.. tried composing a reply 3 or 4 times to explain but never came close to this..
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Old 20th May 2004, 17:47   #6
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thanks, pal.

Regards...
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Old 21st May 2004, 15:01   #7
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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.

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Old 21st May 2004, 17:08   #8
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Basically power = force X velocity


and force = mass X acceleration

so power = mass X acceleration X velocity



but velocity = distance/time

so substituting again,

power = mass X acceleration X distance / time



but mass X acceleration X distance = work


and since torque is the same as work,


power = torque/time


or in other words torque represents the work done and power
represents the rate at which work is done.

we can rewrite the above as,

power = torque X rpm

rpm basically stands for 1/time since it effectively it stands for rotations (a dimensionless quantity) divided by time.

So since power is the rate of work, it can be increase due to two factors. either by increase in work, or the increase in rate.

basically you can produce high power by doing a small work at a heavy rate, as well as by doing a heavy work at a small rate.



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Old 21st May 2004, 17:34   #9
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now u know why i didnt do my engineering (bangs my head into a brick wall)
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Old 21st May 2004, 22:46   #10
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thanks a lot..not sayin i understood it completely but gettin there!
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Old 22nd May 2004, 08:07   #11
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Quote:
Originally Posted by [b
Quote[/b] (Maamukkoya @ May 21 2004,15:38)]power = torque X rpm
power = (torque X rpm)/5252 which is a constant


Nice explaination Maamukkoya

cya
R
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Old 22nd May 2004, 14:21   #12
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Quote:
Originally Posted by [b
Quote[/b] (Rehaan @ May 22 2004,16:37)]
Quote:
Originally Posted by [b
Quote[/b] (Maamukkoya @ May 21 2004,15:38)]power = torque X rpm
power = (torque X rpm)/5252 which is a constant
Actually speaking,

Power = Torque x 2 pi x rpm

To turn this into Horsepower we need to divide by 33,000. Our final equation therefore becomes:

Horsepower = Torque x 2 pi x rpm / 33000 which simplifies to:

Horsepower = Torque x rpm / 5252.

Regards...
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Old 24th May 2004, 14:01   #13
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Another interesting part is the relation between topspeed bhp and torque...

Was just wondering abt the shape of the torque curve. the torque curve increases and hits the max-torque point; Then it starts falling off, and in the process, it passes through the max-bhp point.

Now for a given max bhp, to get max topspeed, i would think that the torque curve should fall off as 'gently' or as 'slowly' as possible, after crossing the max-bhp point...Again, this should happen without producing a new max-bhp point!

i mean, we have this point at which the engine no longer produces enough torque to overcome rolling resistance wind resistance etc; and for max speed, this should be as away as possible from the max-bhp point, on the rpm ladder...But if this is too far, then mathematically we would have upped the bhp, because of the increase in rpm.

just thinking out loud...



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Old 26th May 2004, 06:16   #14
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Warning some layman's views ahead :

Power Vs. Torque.

There used to be these 2 distinct camps. The power freaks and the torque freaks. Now people have begun to realise that one does not live in isolation of the other.

Torque first. Torque is the twisting force of the engine. How much force the engine can apply at rotating the rear wheel will define how fast ur vehicle will pick up speed. Torque can be required in different RPM ranges based on what the riding application is.
Dirt Track : Most riding is done at low -mid RPM range (unless u are really a pro and can keep the engine boiling). So Max torque in that range
SBK : Here u will notice that the stretches are nearly level and the engine can be kept boiling much more easily. Hence here people look for torque up high.(there is another reason to setting it this high. explained below)

Now about power. Power = work done in unit time. So it is not enough if the engine develops lots of torque at the rear wheel, it should also spin up the wheel to good RPM so u travel fast. The faster u have to travel, the more power (BHP) u need. In rotary motion terms, Power ~ Torque * RPM.
Here lets go back to the SBKs we discussed. Those bikes need to develop serious power. The maximum power can be squeezed out of an engine (with no regard to reliability ) if u can rev it to dizzying RPMs and also develop torque at those RPMs.

How do these relate to each other and to you?

Torque without Power implies low RPM limits. That obviously gets u nowhere. U cant get to any speed so the whole purpose is lost.

Power without torque?? Lotsa RPM but it will take u forever to reach it. So u can even hit the speed of light, but u should live long enough.

Just rambled off there. Hope i make sense.
- Ajith.

P.S. : The above are my own words but i have posted parts of it before on another forum for bikes. Am reposting it here in a reworded form just so i can make out if there are any inconsistencies in my understanding. If any moderator feels this is unethical please delete it but also please drop me a PM explaining why u felt so



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Old 5th February 2006, 22:14   #15
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Exclamation BHP@RPM Queries !!!

Hi,

I have a simple question regarding Performance of a CAR:

If there are 2 cars like Ford ikon 1.6. Now out of these 2 cars 1 is 100PS@6000RPM and other is 100PS@5000 RPM. Now i want to know which car will out perform?? I mean the one with 6000 RPM will be better in terms of pickup, top speed etc. or the one with 5000 RPM is better ??.

Thanks.
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