Please explain torque

[mattanna]

Can someone please explain the relevance of torque to me, and what impact it has on peformance, for instance the bmw 330 diesel has more torque than the m3 yet it is not faster, and huge tractors have loads of torque and horsepower and are slow as hell,is it more to do with how quick the car can get to it's attainable top speed or more to do with pulling power from low revs, has always foxed me this one, and obviously gearing must have an impact?, sorry should be in non supra technical:search:

[Bob]

As far as I understand it, the engine only produces power in the form of torque (turning force). BHP is a function of rpm and torque. I think more torque = better drivability. Tractors and diesels don't rev very high and so the bhp is low - imagine a Supra that only revved to 2.5k!

[Alex]

http://en.wikipedia.org/wiki/Torque

[Alex]

http://vettenet.org/torquehp.html

[garethr]

Can someone please explain the relevance of torque to me......?

 

Here's one way it's relevant....dynamometers measure torque. BHP is calculated from the torque figure.

BHP = Torque (lb/ft) x RPM / 5252

[Daston]

ok in simple terms a car with more bhp will be faster 0-60 and have better topend, however the more touque you have the more mid range acceleration you will have i.e. overtaking and 40-60 etc. Slight hijack is there a way of remapping the engine to produce max tourque early on say around 3k so that the car will pull strong in any gear (people with Griff's will know what I mean)

[black cat]

whats the torque of a 400 bhp supra??

[Alex]

(400*5252)/6850 = 306lb/ft (at 6850rpm)

Obviously I guessed when peak power was made.

 

Someone please correct me if I've made a balls up - maths ain't my strong point.

[Thorin]

(400*5252)/6850 = 306lb/ft (at 6850rpm)

Obviously I guessed when peak power was made.

 

Someone please correct me if I've made a balls up - maths ain't my strong point.

 

You have a strong point?

 

Looks right to me.

[mattanna]

standard 2jz gte torque figure is 430nm torque which is about 320lb/ft so surely 400bhp supe would have more??

[Alex]

standard 2jz gte torque figure is 430nm torque which is about 320lb/ft so surely 400bhp supe would have more??

 

Not at 6850RPM it wouldn't. The torque drops off after it's peak at about 4000rpm.

[Digsy]

OEM engine develpment is all about torque. Sure there will be a headline power figure, but since (as has already been pointed out) power and torque are inextricably linked, the way you get from 0kW at rest to peak power @ whatever RPM is all about the shape of the torque curve. Genarally you will get a specification of "peak power of X kW @ A RPM and peak torque of Z Nm @ B RPM". If you turn the power back into a corresponding torque, then this gives you two points on a graph that you can fit the required torque curve through. More importantly, if you go one step further and define the engine size, then you can define the target BMEP curve, which is really what it is all about, as it is a measure of how hard the combustion process pushes down on the piston. Engines of a certain type tend to have similar BMEP curves (i.e a turbo engine will have a different curve to an NA and an NA with VVTi will have a different curve again). Hence you can take a raw power and torque spec and respond pertty quickly with "well, that'll have to be a 3.0 litre turbo engine running half a bar of boost" or some such.

 

When you dick around with your engine by changing boost pressure, cam duration and timing, etc what you are really changing is the BMEP curve, which changes the torque curve, which changes the power curve. It might not seem the most logical way to look at it, but when you get inside how an engine works, it makes a lot of sense.

 

In SI units, the equations are:

Power (Watts) = torque (Newton.meters) x revs (radians per second)

BMEP (Bar) = 0.12566 x torque (Newton.meters) / engine size (litres)

 

I've seen many a comment on here saying things like "400hp? That's not very much." while completely missing the fact that the car might weight 2 tonnes and yet do 0-60 in 4 seconds or some such. That's torque for you. Put simply, you get power as a by product of the engine RPM because you get more little "shoves" per second, but those little shoves get bigger and smaller through the rev range according to how the engine is tuned, and when they start to tail off it doesnt matter how hard you rev the engine because the product of torque x revs will still be a small number. So its where you place the biggest shoves (peak torque) in the rev range that really counts.

 

Engine tuning is all about shaping the torque curve, yet its hardly ever discussed in detail here because most people just shoot for a headline power figure.

[Wez]

I have been targeting low down torque in my setup, great write up Digsy

[JohnA]

Electric motors are interesting examples.

Lots of torque, unimpressive bhp (as they don't rev that high)

Full torque from zero revs, beats any sequential turbo setup. So they accelerate very fast, and not much need for gears either.

 

Too bad that you have to carry all those batteries, lol...

[Kranz]

John.... just use an APU (Auxiliary Power Unit) gas turbine from a 747 (the one that sits in the tail running while you get on board) or such like to power your electric motor.

 

Would sound great too

[JohnA]

John.... just use an APU (Auxiliary Power Unit) gas turbine from a 747 (the one that sits in the tail running while you get on board) or such like to power your electric motor.

 

Would sound great too

I thought that these APUs need a minimum airspeed to operate.

Not M25-friendly perhaps:innocent:

[Wez]

Going back on thread

 

What are peoples views on what can be done to increase low down torque?

[Alex]

Going back on thread

 

What are peoples views on what can be done to increase low down torque?

 

On a turbo engine...get the turbo up and running faster.

[Thorin]

Or increase CC

[Wez]

OK cool

[Digsy]

Increasing cc will increase torque across the rev range for a given BMEP curve.

 

Longer intake runners or a multimode plenum (which the NA Supra already has) will also raise the curve low down. Buggering around with the cam timing will help too, but with fixed pulleys you will always be trading off low down torque with that hgher up (and hence high up power), If you optimise an engine for low down toraue you would have to rev the nuts off it to get decent power at the top end.

 

The only way to truly open up tuning for both low down torque AND top end power on an NA engine without using sily revs is to use variable systems like VVTi, variable intake manifods, and cam switching.

 

Hybrid electric cars have bags of low down torque because of their electric motors. You could also use a very small, low lag turbo.

 

An serious alternative, although none of the OEMs have really embraced it yet because it is expensive is an electric supercharger like the Visteon VTES (not the eBay PC fan bollocks).

[AndrewOW]

So how come some BPU and BPU+ Supras seem to have a reasonably high BHP, say 400, but the torque value is well over the conversion you've equated to, reaching the mid 300s?

 

My car's flywheel BHP is 376, and my torque is around 290lb/ft, which sounds correct using Alex's conversion table, but if I wanted to increase the low down torque, I could have it remapped, but I would then lose high end torque? I assume this means I couldn't reach top speed, or not have as much torque (acceleration) at higher speed?

 

Dopey questions, but I'm curious!