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Rajeevraj

Explanation of BHP, Torque

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Hello,

  Can some explain me the concepts of BHP and Torque and the phrase "x bhp at x torque"  in very simple terms.

 

I know the physics definitions and tried to read up on the net on these. And also know that a higher bhp at a lower torque is the best.  But could not get a clear picture. Like if i wnated to explain to someone i cannot. :)

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OK lemme try... basically there is a formula to determine the relation between torque/bhp which am sure you would have come across during yur surf... lets consider an example of two cars car A and car B..say A has peak torque at 2500 RPM and B has peak torque at 5000 rpm...now say if both cars are

travelling at 3750 rpm here the torque is gonna be the same for both... now both cars reach a flyover and while moving up both cars' engine rpm drop to 3000, now Car B needs a gear shift down coz its peak torque is at 5000, however the car A doesnt need a gear shift down and the driver

can still accelerate to keep up the speed / power...

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One can also understand the torque as a measure of 'pulling power'. bhp is torque multiplied by the rpm. These together define the 'power band', which  roughly (no exact definition here, FRG, Zavo correct me if I am wrong) extends from a little below peak torque to the maximum bhp. For best performance one should keep the engine revs between these limits.

A racing engine will have very narrow power band, necessitating frequent gear shifts,and making it unsuitable for road use. A normal engine will have a far wider power band not requiring frequent gear shifts.

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Your right..

If you see any Dyno graph.. the peak power comes close to the end of the rev range.. and then drops off.

So your BHP's at the top of the curve so to speak..just before the drop off point.

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Thanks That makes it more clearer. Another related question So RPM is revolutions per minute right?Question is revolution of what? Is it the piston or whatever inside the engine (sorry if it is a stupid statement. I am pretty much ignorant of these things. trying to learn) And torque i know is the tangential force applied. So how does the torque and RPM tie up to an actual engine.

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k, friens..

i have another doubt...

we have seen the example put in by harryccc. that is the example of cars A and B.

A's maximum torque at 2500 and B's at 5000rpm. So we came to know that A will perform well at about 3000rpm. So my doubt is, wheather, the car B will perform well at around 5000rpm range..?

I asked you because, i have a baleno, which generates maximum tourque at aboy 3500rpm. and it performs well at low rpms. and i dont have to shift gears very often. But i feel that, when under spirited driving, the engine lacks the thrust, which i feel below 3500rpm.

but when i test drove the sx4, which is having maximum torque about 4800rpm. it felt very much better under spirited driving.

or is this because of the generation gap between the two engines..?

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yes!!!! there are many other factors like Engine inefficiency, transmission etc.. so practically the output doesnt get doubled if you increase the torque by double due to the factors like mentioned above... hope this answers yur doubt!

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ok, thank harryccc,

but i have the doubt that wheather all the engines making maximum torgue at lower rpm be a less performer at high rpms..since the tordue will start to decrease after the peak (torque vs rpm graph being parabolic.)

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Remember bhp =  torque x rpm. Assume you have an engine which has almost a flat torque from 2000-5000 rpm. Obviously then at 3500 rpm you have more power than at 2000 rpm (why Carispasson finds his Baleno when driven spiritedly sluggish below the 3500 rpm mark).

As for engines with better torque at lower rpm they have a 'less aggressive' crankshaft  profile than their racing type cousins. Engine design is always a compromise between the comfort (read broad power band) and peak output. That is why many performance tuners replace the camshaft. The engine mapping by the EMS also comes into play here.

In general you will find that long stroke engines have a broad power band, and a poor (rpm limit, bhp and roughness) top end, while a short stroke engine  will not have much at the bottom end, but will be  'rev happy'. An extreme case of the short stroke engine in a popular car was the FIAT 128 of the mid 70's. This engine had a rev limit of 8300 rpm!

Take the Esteem and the 1.5 Accent. The latter is a long stroke engine and can (if you leave your lead boots at home) be taken from under 800 rpm all the way in 5th gear. The Esteem will not be as happy doing this. However,  the Esteem is far more happy at high revs.

Old cars (due to the obscure RAC formula of calculating taxable HP) used to have long strokes.

 

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sorry, it was a mistake.

my baleno feels sluggish above 3500rpm.

ok' date=' thank harryccc,

but i have the doubt that wheather all the engines making maximum torgue at lower rpm be a less performer at high rpms..since the tordue will start to decrease after the peak (torque vs rpm graph being parabolic.)[/quote']

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i meant actually the other way. it was a mistake.

my baleno feels sluggish' date=' above 3500rpm...[/quote']

Then either you are not happy with the power profile, which is something you have to live with. If someone has a electronics remapping unit/box then you may gain somewhat.

Another possibility is that you have a gummed up Injection System. This is more likely if you are not using premium fuels. Put a good injector cleaner fluid in the tank. I had a friend who was using normal petrol in his Baleno, and it was very sluggish. Changing to premium transformed the performance within 300 km!

Redundant text removed!

sgiitk2007-11-07 12:02:48

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wow... that sounds great..

feels that, i should have tried it before.

im using only ornirary fuel, since baleno has lower compression ration. now i am going to try it..thanks.

any way, the car is not very sluggish at high rpms. but we wont get the same driving pleasue at high rpms. i felt my baleno to be sluggish, just after test riding sx4... thinks that my soul needs a car replacement soon...he he smiley36.gif

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Compresion has nothing to do with this, it is injector clogging.

To put my experience to paper (figuratively). We are to go to Gwalior from Kanpur (~270km). We are going in a friend's Baleno. I was driving. Just before leaving Kanpur we topped up (~15L0 with Speed. On the short 40km stretch of NH2 which was almost complete, I found the car a brick. So I was cursing all the hype. On eraching Gwalior and before starting back we tanked up with XtraPremium. This time we returned via Jhansi (320km) and I felt the car far better. The proof came when my firend got his driver to drive banck to Lucknow. A few km out of Kanpur he asked the boss, 'Saheb, ky aap ne car mein kuch kaam karwaya hai, ab to gadi ki bhag bilkul badal gayi hai'. (have you had some work done on the car, now the pick up is totally different). Such was the transformation. I guess a couple of tankfuls of premium (Speed/ XtraPremium/Power) is all you need.

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