Thread: Torque curve ?

Originally Posted by mark2

From a purely theoretical perspective:

Work is done when a force acts over a distance (FxS)

Power is the ability to do work, in relation to time
P=W/t

In the context of an engine, torque is the relationship between power and RPM. If you know the power (or torque) of an engine at any given rpm, its torque (or power) at the same rpm can be calculated using a simple formula.

Basically, the lower the rpm for a given power output, the higher the torque at the same rpm.

I agree with all the above comments in relation to the practical application of torque.

Blcknight:
I'm not sure I entirely agree that a 2.25P develops its torque from a long stroke, I've always considered them to be a relatively short stroke engine when you compare the stroke length to similar engines. I think the inherant low end torque is more due to cam and induction design. Given that these things will happily cruise on 4000 rpm, they are an amazingly flexible engine.

Or to put it another way - power equals torque times rpm (with correction depending on what units of torque and power you are using).

The Torque peak arriving early in the motor's RPM range is essentially a limit imposed by the head design, not the bottom end (sorry, bklnight). A motor which in all other ways is identical but which can "breathe" better will develop torque at higher rpm - because the combusion isn't restricted. Normally, however, people talk about this as developing more power (which it does,as well) though if a motor had a perfectly flat torque curve, you could gain power by increasing the rpm without affecting torque at all (otherwise known as using higher rpm). A motor which has a longer stroke for identical displacement will generate more torque across the rpm range (all other factors being equal). The longer stroke will exacerbate any limits imposed on induction or exhaust and will probably limit the torque at higher rpm.

EDIT: I just read the corvette example - with identical gearing, he's right(ish). In 4wd land we might chose lower gearing to effectively increase torque, with a sacrifice of top speed and acceleration. The forst 4wd my father owned was a 2-stroke Suzuki LJ50, known locally as the "Liklik Bigpella" ( well, we were in PNG - it means "the little big man"). The motor made next to no torque, high in the rev range compared to contemporaries, but went over mountains due to low gearing.

I suspect that ultimately it's the "area under the curve" of torque that matters - more torque at low AND high revs. And power, as the link says, doesn;t really matter except as a pointer to the torque curve.

Originally Posted by scrambler

The Torque peak arriving early in the motor's RPM range is essentially a limit imposed by the head design, not the bottom end (sorry, bklnight). A motor which in all other ways is identical but which can "breathe" better will develop torque at higher rpm - because the combusion isn't restricted.

care to take a wager on that little fact? remembering that Im talking about what design engine crank is better for what. Changing the head over a given crank will make a difference to that engines preformance but it doesnt matter what you do to the head a short throw crank will never produce the same amount of torqe as a long throw crank. (which is what Im talking about)

a 2.25 is full bickies at lets say 4500Rpm its a long stroke motor.

the wrx motor is a short or square stroke motor ( I cant remember which) and develops peak torque at 4500rpm. (as an example)

care to explain to me how an engine that redlines at the same RPM as another engine thats just getting onto max torque doesnt have its torque curve lower in the RPM range?

and if it was all about combustion chamber design as opposed to crank throw why the hell would there be different cranks that let you have more throw in the same engine if you could get the torque just by bolting on some new heads? ( I know which of those 2 is mechanically easier to install).

Yeah, high torque motors are usually designed with a long throw. The problem with getting power out of these motors is their inability to breath well at high rpm due to the small bore diameter and so valve size/capacity. A long throw isn't good for high revs anyway.

This said, inlet tracts and head design does make a big difference, and generally a 2V head will perform well at low revs (in a car up to about 2500/3000 rpm) which apparently relates to turbulance and atomisation, and afterwards the multivalves seem to have it in the bag where they have the ability to breath deeply.

Oh Oh, everybody is forgetting the flywheel mass

Revs mean nothing if the engine doesnt have enough torque to get it up there into the torque curve.

Imagine dragging a caravan with a 13b rotory (Naturaly asperated for this example) in comparison to a 300TDI engines are similar capacity but the rotory in NA form makes very little torque but has a very smooth HP curve which just keeps getting bigger until it revs it self into oblivian but you wouldnt get it off the line where as the 300TDI would be a little slow off the line until on boost and then away you go.

Torque gets you going, and if you have a heavy duty drivetrain then you could gear up an extremely torquey engine to do massive speeds with very few revs and as long as it was HP matched to the torque curve it would get to those speeds relatively fast but again, size vs weight vs torque and HP.

Originally Posted by Blknight.aus

care to take a wager on that little fact? remembering that Im talking about what design engine crank is better for what. Changing the head over a given crank will make a difference to that engines preformance but it doesnt matter what you do to the head a short throw crank will never produce the same amount of torqe as a long throw crank. (which is what Im talking about)

a 2.25 is full bickies at lets say 4500Rpm its a long stroke motor.

the wrx motor is a short or square stroke motor ( I cant remember which) and develops peak torque at 4500rpm. (as an example)

care to explain to me how an engine that redlines at the same RPM as another engine thats just getting onto max torque doesnt have its torque curve lower in the RPM range?

and if it was all about combustion chamber design as opposed to crank throw why the hell would there be different cranks that let you have more throw in the same engine if you could get the torque just by bolting on some new heads? ( I know which of those 2 is mechanically easier to install).

It might have the "torque curve" lower, but that's due to less torque higher, rather than more torque lower. The Subaru STi puts out 392 Nm of torque to 4000rpm. The 2.25 LR motor puts out 166 Nm at 2500rpm and 134 Nm at 4000. What's the bet that the STi puts out more than 166 Nm at 2500 rpm? It's just got MORE higher up. The displacement is pretty close. Don't have time to find whether the STi is over- or under-square, but the 2.25 is marginally oversquare ("short-stroke") at 90mm bore and 88mm stroke. http://www.csse.monash.edu.au/~lloyd...Rover/Engines/

Are you guys talking about over square/square and under square engines, I cant remember whats what, over square is bore bigger than stroke or something right, It was explained at trade school but that was soooooooo long ago and I was probably sleeping lol.

Then on top of that, air flow increases HP more than torque with heads, It all comes down to volumetric efficiency with head work and you will never get it anywhere near 100% efficiency so it's kinda a mute point bringing head work into it.

Im not sure how to explain how all this marries up but it isnt just 1 thing does this and you change that and it will do something else, you build and engine to do a certain job so I dont see how there can be a this is better than that arguement unless you add what uses they are for, e.g. Motor bike doesnt need much torque but gets speed from revs and can get away with having a nice short stroke to get lots of revs.

Truck wouldnt go far with a motor bike engine as would a motor bike with a truck engine (other than those harley looking things with the 350 chevs in them but they arnt all that practical).

I can end this really really easily.Hook a 18ft boat onto a Jap v6,it doesn't matter what brand their all **** and try to haul it up a boat ramp in high first,then put the same boat onto a Disco v8 and do the same thing.That is HP V's torque. Pat