Horsepower or Torque???

gIzzE

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Really interesting thread on one of the BMW forums, it is an American based forum and some are getting excited about the arrival of a proper diesel at last, they are getting the 335d.

Just wondering what people know about hp and torque??


The question is what is more important??
 

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Really interesting thread on one of the BMW forums, it is an American based forum and some are getting excited about the arrival of a proper diesel at last, they are getting the 335d.

Just wondering what people know about hp and torque??


The question is what is more important??

Torque + hp:D
 
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gIzzE

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Is that meant to be a + or an = ???? ;)
 

Dryce

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Just wondering what people know about hp and torque??

The question is what is more important??


Torque is an expression of how much pulling capability an engine has (force * distance).

HP is an expression of power which is the rate at which it delivers energy.

The relationship is between the two is the rate at which the engine works - ie revs.

So an engine with high torque but low revs generates the same power as an engine with low torque but high revs. If you can push the revs very high you get a lot of power from not much torque.

Diesels traditionally don't rev very much but produce a lot of torque at low revs. So they generate more power at low revs.

Petrol engines traditionally rev higher and produce less torque. So lower power at low revs but higher power at high revs.

Modern diesels blur this simple distinction - with some technical refinements and a turbocharger they produce an awful lot of torque and rev a little bit more. Upshot is a whole lot of torque and a whole lot of power.

So you don't actually need to ask your original question - you just submit to the inevitable and buy a modern diesel :)
 
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gIzzE

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I found it quite interesting and had never really thought about it before.

HP = torque X revs


Take for example the 330i with 272bhp and only 250lbs of tourque takes 6.1 seconds to get to 60mph, then the 335d with 286bhp and 400lbs of torque takes 6.0 seconds to get to 60mph.

I guess I thought that the one with more torque would always be faster, but that just isn't the case. I now get why people say the torque figure is irrelevant unless you know where it is in the rev range and where it revs to.

I guess it can show you whether a car will perform well round town or on a motorway or whether it will be a good back road blaster or track car.



The other thing was why does the diesel with the same HP feel quicker, even when you compare the 335d with 280bhp and 400lb of torque with a remapped 335i with 380bhp on only 300lb of torque the diesel feels quicker, even though it is over half a second slower??

I think it is because the torque curve in big diesels keep climbing fast, and our bodies can feel change in acceleration rates more than acceleration itself?
 

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The other thing was why does the diesel with the same HP feel quicker, even when you compare the 335d with 280bhp and 400lb of torque with a remapped 335i with 380bhp on only 300lb of torque the diesel feels quicker, even though it is over half a second slower??

The advantage you generally have with a modern diesel is that you're likely to be cruising within a rev range where the engine will deliver more power if you put your foot down. With a petrol engine you'll probably have to pile on the revs - which may mean a change down in gear to get the engine to a position where it will deliver a lot of power. Basically you have to work the petrol engine more - keeping the revs up - to get performance.
 

Mark300SL

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Diesels are generally limited by the speed that diesel fuel can physically burn at - which equates to an approximate rev limit of 5500 rpm
They cant rev very high but by the very nature of diesel combustion they do need heavy duty components like pistons/rods to overcome the high compression figures.
Now we have an effectively heavy set of internal components but a lower rev limit so the stresses begin to even out. To make the large amounts of torque the diesels often tend to have a longer stroke which can be achieved because the engines are not being revved as high. (longer stroke engines increase mean piston speeds, capping rpm limits reduces the ultimate piston speed)

Turbo boost pressures in diesels are generally much higher than petrol equivalents. I race a pretty sucessfull heavily modified Honda that has never seen more than 17psi of boost and thats considered a lot! the average Rover25/45/200/400 diesel sees up to 22lb of boost in everyday use and was set up by the factory at that level.

Finally I try to achieve peak cylinder pressures in a petrol engine at about 14 degrees ATDC, This is achieved by firing a spark into a precharged cylinder before TDC but the burn process actually completing just after TDC
IF we could produce PCP after 14ATDC we would make more torque as there would be a greater leverage applied to the crank but this would involve redesigning 4 stroke petrol engines because of the speed petrol burns at :)
With a diesel however the fuel is injected into a precharged cylinder of superheated compressed air and fuel delivery can and does continue past 14ATDC because it burns slower - effectively applying more torque to an already long stroke engine.

As a die hard petrol head - diesel will be the race fuel of tomorrow as already proven by Audi at Le Mans
 

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As a die hard petrol head - diesel will be the race fuel of tomorrow as already proven by Audi at Le Mans

That presumably depends on whether turbos are allowed!

Also Le Mans and other endurance races involve a different approach to driving than shorter distance races such as F1. Engines have to last longer and are typically detuned (ie. lower revs) compared with those used on F1.

That said the SEAT diesels seem to have done noticeably well in the BTCC.
 

maddog

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Torque feels faster gearing plays a big part on how the car actually functions with that torque and these monster figures from diesels are pretty misleading as the BMW example shows

At the end of the day i prefer a slower car that feels fast than one that IS fast but doesnt feel it
 

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>>effectively applying more torque to an already long stroke engine.

I'm sure Mark's aware of this, but, for a given cc, a given cylinder pressure, and a given crank angle, the torque on the crank does not depend on bore to stroke ratio.

The extra lever arm length of a long stroke crank is exactly cancelled by the reduction in piston area, and hence piston force.

Or, phrased another way, the mechanical argument for saying long stroke gives more torque and short stroke gives more power is somewhat bogus - the more defensible argument is that you can be more adventurous with the valving provision on a short stroke engine, as there's lots of piston area, hence, lots of potential valve area.
 

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Gizze has fallen for the old 0-60 as a test of performance:

the diesel feels quicker, even though it is over half a second slower.

It comes down to gearing, a diesel engined car usually has a v.short 1st gear, unlike a petrol car, this will effect initial traction in that the wheels will spin more readily so its slower off the line. It might also mean the diesel car needs to go thru 2 gear changes, whereas the petrol car needs just 1 gear change.

0-60 is all about gearing, (1st-2nd being ideal or if the car can hit 60 in 1st your laughing for a good 0-60 time) A diesel probably needs 3rd gear to hit 60mph so that accounts alone of the loss of time.

I find mine will spin its wheels as the torque arrives very suddenly in 1st gear breaking traction (its good fun but not ideal for 0-60 sprints). A petrol engine of the same HP wouldn't do this.

One a diesel is in gear although for a given MPH the engine will be turning over at a slower RPM there is much more torque on tap, meaning it feels faster.
 
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gIzzE

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I get the gearing, one of the reasons I hated my manual S4, the most absurd gearing ever for a V8 and done to make it feel fast.

Even comparing say a 335d and 335i, the 335d with its massive torque feels qicker, but it is not, even on a strip up to 110mph.

I used to think that torque was really important, but really HP is all that matters when it comes to speed.

Toque may make it an easier daily driver, and therefore a better car for the situation, but for the first time reading that thread on e90post I realised that HP is everything.
 

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>>This is helpful.

I agree - it's one of the very few discussions of the topic that actually gets it right. So many attempts to explain this are so much BS.

It includes two seemingly contradictory points, which are both true, and once you've "got" them, it makes the rest of the theory easier to grasp.

1) On a manual gearbox car, if you stay in a particular gear, the maximum acceleration is at the engine speed for peak torque. (although you will generally get better acceleration in a lower gear with revs nearer peak power)

2) If you had a CVT, for maximum acceleration, you would always run the engine at the speed for maximum power.


This hints at why, in the real world, peolpe tend to get better performance out of a diesel - it's technical failing in having it's peak power and redline speed so low actually mean that people are driving it closer to the optimum without the engine screaming.

Ford did some research with a petrol engined Ford Granada. The car was standard, but for some instrumentation which recorded the gear change rpm used by members of the public who were given the car for a period of time. Even though the car was not owned by the people, and they could have thrashed it without any fear, most people changed up way before the engine was making anywhere near peak power.

This is, IMO, why the real gains of multi-valve engines weren't taken up by most drivers - they only make technical sense if revved. Although I don't think that such a pairing of near identical production engines existed, but, if the only change made was going from 8 to 16 valves, the power and torque curves in the lower part of the rev range would overlap.

>>I realised that HP is everything.

Yes...., and no!

Yes in that technically it's true that a car with higher HP will post quicker 0 - 60 times than a car with the same torque but lower HP.

However, if you have two cars with the same power, but one has higher torque, the car with more torque will perform better during normal driving, especially when driven "conventionally", without going anywhere near the redline.
 
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Dieselman

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Toque may make it an easier daily driver, and therefore a better car for the situation, but for the first time reading that thread on e90post I realised that HP is everything.

But HP is only a derivative of torque * revs.

Forget 0-60 and -0100 times as has been correctly pointed out by *** they are all about gearing and traction.
A much better check of acceleration is a figure such as 20-50 or 20-70. In such circumstances a turbodiesel will always be quicker due to the abundance of torque supplying greater force to the crank and ultimately the wheels.

A petrol engine can supply as much wheel torque by using lower gearing, thus needs to rev higher and use more fuel.

Peak figures are almost worthless, you need the full graph of torque to see which engine produces more power over a greater spread.

I'm really confused by this as the diesel IS quicker -060, proving that the torque is providing quicker acceleration in spite of longer gearing.
Take for example the 330i with 272bhp and only 250lbs of tourque takes 6.1 seconds to get to 60mph, then the 335d with 286bhp and 400lbs of torque takes 6.0 seconds to get to 60mph.

I guess I thought that the one with more torque would always be faster, but that just isn't the case. I now get why people say the torque figure is irrelevant unless you know where it is in the rev range and where it revs to.
 

maddog

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Not sure how a turbo diesel will be quicker 20-70 and not 0-60 that just doesnt make sense .

I dont find traction an issue with my car at all so the first 10mph should be no slower than a petrol in fact quicker than one of a similar weight due to the engine producing more torque and hp at lower rpm
 

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>>Peak figures are almost worthless

True, but they do give you much more than most people use.

From the peak torque figure and engine speed, you can calculate both the power at that engine speed, and the slope of the power curve (because you know the gradient of the torque curve is zero)

Likewise, at the engine speed for peak power, you can calculate the torque, and the gradient of the torque curve.

These hidden bits of info do allow you to sketch in approximate curves. These approximate curves will miss any sudden changes resulting from effects like ram tuning, but can give good approximations, especially for more basic engines.
 

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Not sure how a turbo diesel will be quicker 20-70 and not 0-60 that just doesnt make sense .

This does not only apply to turbo diesels, it applies to turbo charged petrol cars too. Turbo charging produces a lot of torque and acceleration in mid range is vastly improved.

Many years ago I had a Colt Lancer 2000 Turbo, 170hp, 0-60mph listed in autocar etc as 8.5 seconds & 125mph top speed.

Plenty of cars had faster 0-60 times & appeared on paper to be faster cars, but could not keep up with it on normal dual carriageways, in particular as this is a MB forum, the two occasion I had some fun with a 450SL & 560SEL and totally embarassed them.

Russ
 

bigstuff

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a ford cosworth 20ltr turbo has hp but in 5th gear at 70mph plant your right foot nothing its like a old taxi a v8 sl has torque at 70mph plant your right foot you get pushed back into your seat having said that drop the cozy in to third all hell is let loose so you can do it the hard way or the easy way with the v8
 

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