Torque curve – an education…

After a conversation yesterday I googled torque curve in relation to my own car, and came across this particular video from Chris Harris – 4:10 onward:

https://youtu.be/zKrXFTL_6pw

My own experience in my 320d is the same – it feels positively quick, smooth & powerful up to speed and during the cruise beyond my previous experience of a 2.0 Diesel engine, this perhaps explains why – at those speeds and ratios my car is sitting right in the optimum torque curve (I think).

I’m assuming then – a discussion for all cars in general – this is to do with the torque curve being in the “right”place, but am interested in layman terms how this works.   I understand what torque is, but how do you “map” to gears, speed etc and when does torque stop and BHP start / how do they work together?

Horsepower is calculated from torque and rpm. They will always be the same at 5252 rpm.

Simple explanation is torque is the power the engine produces in one revolution, while horsepower is its overall potential for power output.

So torque is felt as a more immediate response from the engine, but you can have a low torque high rpm engine or high torque low rpm engine with the same horsepower.

” Horsepower sells cars, Torque wins races!”

Trouble with using horsepower to compare cars is it is a peak figure.

You realty want the area under the curve rather than that peak figure to assess outright potential (that’s energy delivered across an entire pull through the revs).

” Horsepower sells cars, Torque wins races!”

So an audi 3.0 TDI (369 lb⋅ft) will win races against a 2005 v10 F1 car (260 lb-ft).

Torque is related to how much force the pistons are exerting against the crankshaft (assuming that the crankthrow is the same, which it is if you are talking about the same engine). Maximum torque is at the point where maximum efficiency is realized. At low engine speeds, the airflow into the cylinder is slow and there isn’t much turbulence, so the air and fuel don’t mix fully and combustion is less efficient. At higher engine speeds, there isn’t enough time to completely fill the cylinder with air, so less fuel is burnt. With engines optimized for low speed, longer, narrower inlet manifolds are used so that it is tuned for a lower frequency, just like a musical instrument. For higher engine speeds, shorter, wider inlets are used. Similar effects operate with camshaft lift and timing. A camshaft that produces maximum torque at low revs will not work well at high revs. A camshaft optimized for high revs will produce poor torque at low revs.

Power is force x speed. Torque is basically the force the pistons are exerting against the crankshaft, so power = torque x revs. If you double the engine speed, but the torque drops by 30%, you will have 40% more power because 2 x 70% = 140%. Maximum power will always be at a higher engine speed than maximum torque, but at some point the combustion efficiency drops so much that higher revs result in less power. This is the point of maximum power. It will always be at higher revs than maximum torque.

My wife has one of the Mercedes SLK’s with the twin-turbo diesel and it does feel quicker than you’d expect for a car with just 200bhp, which I’m assuming is down to the large quantities of torque it produces low down the rev range. If anything it seems to produce a bit too much torque at low revs as the traction control really seems to struggle to keep up in anything approaching slippery conditions.

Basically it all gets more complicated when you put a squishy human brain into the car, because we’re not great at really feeling torque or power- what we can feel best is acceleration, and contrast. So a really peaky engine that makes good power right at the top can still feel faster or more exciting than a similar engine that makes the same power up top but also has better power through the range, purely because it’s that kick where it achieves peak power that feels “powerful”- you’re going from, say, 50 to 100bhp quickly with a really clear contrast. While the stronger engine doesn’t have that kick if it rises in power consistently.

That’s not well explained, so… Example. I took my bike in for a dyno run once, and there was a feller there with IIRC a GSXR750 SRAD, doing his nut because the tuners had “ruined” it and it was weak and slow. They got out the dyno charts and actually, it’d been running horribly with a big flatspot in the middle because he’d fitted an exhaust and filter but not tuned it- and so when it climbed out of that flat spot, it gained power fast and felt exciting. They’d tuned that out, and even though it now had more power up top and was going to be faster absolutely everywhere, it no longer had that exciting quick rise. He went away with a better bike that he didn’t like as much.

I like vtwins and big fat engines- diesels and lazy petrols. Girth. But I get why other people like vtec and 2-strokes.

Don’t you people have gearboxes?

I don’t know much about the physics/engineering bit, but I always look at hp / torque figures when looking at cars.

BMW always have a good dollop more of torques than hp, which means the car will drive well.

The 2.0d is 184hp 380nm torque

My 330e is 249hp 420nm torque (2.0 pertrol)

The diesel already has more power than a lot of 2.0 diesels but also has way more torque than a lot of 2.0 diesels. Hence why nippy for a 2.0d but I guess you are paying for it.

The 330e uses the extra power to get 0-60 1 second quicker.

Both lack top end power so when you get 100 mph+ both start to noticeably accelerate more slowly. (not of course that I have ever done over 100 mph)

You need hp to go fast (top speed)

Torque makes are car more driveable.

Don’t you people have gearboxes?

This it the gist of torque Vs power.

You can overcome a lack of torque with a gearbox, for example an average cyclist puts out more torque into the chainset than an F1 engine manages. But the F1 engine has a 18,000rpm redline, the cyclist about 100.

You can’t do that with power, with all the gears in the world my focus wouldn’t do more than the 115mph it claims it can, at that point it’s using all 100hp to push the air out the way, drivetrain losses, rolling resistance and do all it’s other stuff. But you could keep the same torque figure, tweek the valve springs, ballance the engine, upgrade the fuel pump and injectors etc and turn it from a 5500rpm redline to 11000rpm, roughly doubling it’s power. At this point it will probably go much quicker (155mph ish), but you’ll now do that in third gear due to the ratios. If you put it in 5th it would actually slow down as there wouldn’t be enough power.

Alternatively you could stick a turbo on it, set it to 1bar, and double the torque (approximately).  This would accelerate just as quickly, but use all 5 gears rather than three, and hit the redline at 5500rpm still (and hit the peak power earlier too as the injectors wouldn’t cope).

So, torque sells road cars because in reality what you want is a nice feeling at 2500rpm on a motorway sliproad.

Horsepower wins races because the revs never drop more than about 20% below the redline*.

Saying torque wins races is true, but so is saying higher rpm’s win races. And power = torque x rpm so saying pawer wins races is true squared.

*unless you some reason decided to build an engine with peak power a long way from the redline, which would be pointless as all things being equal more revs means more power.

My wife’s SLK has less power (202 v 249bhp) but more torque (369lb/ft v 310lb/ft) compared to the BMW 330e but is slower to 60 (6.7 for the SLK against 6.3 that I’ve seen in reviews for the 330e) but quite a bit faster (151mph v 140mph) – so not sure how that works in that theory that you need torque for quick acceleration but more power to go fast!

It could be the case that the 330e can’t or doesn’t use it’s electric motors at higher speeds (i.e. it’s relying on just the 182bhp petrol engine) which might explain why it’s got a low top speed for a 249bhp car. Although not as slow as my CX-7 which has 260bhp but only a 130mph top speed – although in it’s case I think it’s electronically restricted to 130).

so not sure how that works in that theory that you need torque for quick acceleration but more power to go fast!

Bluntly, you need torque to accelerate from low revs in the real world. No one commutes to work at the peak power of their car, it would be antisocial!

So, torque sells road cars because in reality what you want is a nice feeling at 2500rpm on a motorway sliproad.

Boo!

Drop it to 2nd, nail it to 8,500rpm then slot it into 5th at 70mph. 🙂

I appreciate that a lot of people simply aren’t used to driving cars with nice gearboxes that you can reliably and confidently do that it in.

Its power that matters for outright performance.

A good level of torque across the rev range is very useful for everyday driving as it makes the car feel responsive lower down, but its power that matters for actual performance.

Its also important to remember you are measuring torque at the flywheel, you have to take into account wheel torque after the gearbox.  Every gear in a car will be a reduction gear so effectively increases wheel torque, however petrol cars will have shorter gearing than a similar diesel car so although torque at the flywheel in a diesel will be a lot higher, torque at the wheels can end up being fairly similar.

Two cars with the same weight:

One has 200ft/lbs and 300hp

One has 300ft/lbs and 200hp

The car with more power would wipe the floor with car with more torque.

For me turbo charged petrols are a good compromise, the turbo helps to produce lots of midrange torque (which can be mapped to produce an almost flat torque cover over a wide rev range) giving nice linear power with a good top end

I’m used to motorbikes (my CBR600 revs to 14.5K but has virtually no low down torque) which is probably why I don’t really like diesel cars – although my wife’s SLK is beginning to grow on me, partly because the 7-speed gearbox works well enough to hide the fact the car only really revs to about 4K.

 for example an average cyclist puts out more torque into the chainset than an F1 engine manages.

Huh? For reals?

The engine in my CX-7 (a 2.3 turbo petrol) has a fair bit more power (256 v 202bhp) but a lot less torque (280 v 369 lb ft) that my wife’s SLK (a 2.2 twin turbo diesel). The SLK is a bit quicker to 60mph and about 20 mph faster although that’s of course down to the weight of the CX-7 as well as the electronically limited top speed.

There isn’t as much in it between the Mazda 6 MPS (which has the same drivetrain as the CX-7) and the SLK for weight though and those two cars have pretty much identical 0-60 and top speed figures – despite the differences in max torque and max power.

Huh? For reals?

0.175 x 100kg (fatty stwer on normal cranks stood up but not pulling on the back pedal or handlebars so actually probably a low estimate).

9810N x 0.175 = 1717Nm of torque.

F1 engines are ~275Nm, about 6 lower (but spinning 180x faster)

^^^ you are out by a factor of ten

Huh? For reals?

Well probably not a modern turbo hybrid engine F1 car but the early 2000’s cars that revved to 20k rpm  – probably.

If it produced 900hp at 19,000 rpm then peak torque was probably no more than 250 ft/lbs.

Your legs can (very easily) produce the same force as your weight (other wise you wouldn’t be able to stand!  You can probably easily double this force.  So for example 400lbs of force onto a crank arm measuring 175mm gives 229 ft/lbs

0.175 x 100kg (fatty stwer on normal cranks stood up but not pulling on the back pedal or handlebars so actually probably a low estimate).

9810N x 0.175 = 1717Nm of torque.

It must be that Friday feeling 🙂

Or is this why I feel so fat lately. Just blame the extra gravity….!

I think there is an extra zero in there; it’s 981N, not 9810.

171.7Nm of torque.

People quoting acceleration & top speeds – you need to take into account things like gearing & how aerodynamic the car is (for top speed), so it gets a LOT more complicated than X engine has more power/torque than Y engine.

My mate used to have a Corolla T-Sport, putting out something like 190bhp. My Ibiza TDi (130bhp) could easily keep up with it on cross-country roads, beacuse he was having to constantly change up & down the gears to keep in ‘in the power’ while I just left the diesel in 3rd and stuck my foot down a bit more.

A quick look at the specs on Parkers – the Corolla is putting out 189bhp, but only 192Nm of torque. The Ibiza was 130bhp and 310Nm of torque, so they were pretty evenly matched; the Corolla have roughly 1/3 more power, while the Ibiza had about 1/3 more torque…..

Drop it to 2nd, nail it to 8,500rpm then slot it into 5th at 70mph.I appreciate that a lot of people simply aren’t used to driving cars with nice gearboxes that you can reliably and confidently do that it in.

my 900cc Peugot 107 will do this

Yes, but a human can’t produce peak torque through an entire revolution of the pedals, and neither can an internal combustion engine. What you are doing is comparing the peak torque of a human cyclist with the average of an internal combustion engine. The average torque of a human cyclist will be much smaller than an F1 engine.

A good cyclist can produce a sustained power output of 500W at about 100 rpm. An F1 car over 1000 times that at  about 20 000 rpm. If a human could sustain their torque output at 20 000 rpm, they would be producing about 100 kW, which is less than 20% of what F1 engines were making. Therefore, an F1 engine produces about 5 times the average torque of a very strong human cyclist.

A V8 F1 engine could smooth the variations across four cylinders per revolution (because each cylinder only fires once per two revolutions), but a human can only fire one leg at a time. A human leg cannot produce much torque at the top or bottom of the downward stroke. The peak force on a piston and conrod is massively higher than a human leg can exert.  Also, the combustion characteristics and conrod length of an internal combustion engine can be varied to alter how the torque is distributed across the firing stroke.

my 900cc Peugeot 107 will do this

Either you’ve swapped the engine (The 107 only came with two engines, a 1.0l 3cyl petrol and a 1.4 diesel), or both your dials are broken. 🙂

The 107 only came with two engines

That’s one more than my car.

When people talk about low-down torque they could just as easily be saying “low down power”. At low revs you will have low power unless the torque is high.

This is the torque curve from my wife’s car – no wonder the traction control struggles at times!

That’s one more than my car.

What are you, gay or something?

Saying torque wins races is true, but so is saying higher rpm’s win races. And power = torque x rpm so saying pawer wins races is true squared.

It also depends on the type of race, the high-revving petrol engines are ideal for circuit racing where there are lots of bends mixed with high speed straights and curves, compared with long distance endurance races like Le Mans 24 Hour, where diesels have proven very effective for maintaining steady speeds through the course of the race, coupled with better average fuel consumption. I’d be interested to know if anyone has tried using a diesel for circuit racing in mixed saloon classes where big V8 engines are used alongside smaller four-pot engines.

So an audi 3.0 TDI (369 lb⋅ft) will win races against a 2005 v10 F1 car (260 lb-ft).

Because 369 x 3000 isn’t as much as 260 x 15000?

Weight ratio and gearing aside the F1 engine produces less torque more often, so does more work.

A 150bhp diesel car will do 0-60 in about the same time as a 150bhp petrol car, if both sets of engineers have done their calculations correctly and optimised gearing for the intended use. (This assumes the same class of car)

JERK.

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“jerk” is the term for what us humans are most responsive too, and it is actually the rate of change of acceleration.  So cars that suddenly start accelerating feel faster to us, compared to cars which accelerate just as hard, but do so for longer, or that accleration ramps in and out more smoothly!

JERK.

VTEC y0!!!

<span style=”font-size: 0.8rem;”>It also depends on the type of race, the high-revving petrol engines are ideal for circuit racing where there are lots of bends mixed with high speed straights and curves, compared with long distance endurance races like Le Mans 24 Hour, where diesels have proven very effective for maintaining steady speeds through the course of the race, coupled with better average fuel consumption. I’d be interested to know if anyone has tried using a diesel for circuit racing in mixed saloon classes where big V8 engines are used alongside smaller four-pot engines.</span>

I think that’s only partially true. Physics says there’s a speed they can go round a corner, if the petrols could accelerate quicker out of the corners they’d win. I suspect those diesels don’t feel much like a pensioners dacia to drive!

I think the rules were written to favour them quite a bit too? A bit like in motorbikes, an inline 4 will always be quicker (insane redline and lots of horsepower), unless your ducatti and threaten to take your ball home unless you’re allowed a 20% capacity advantage, then v-twins win. Torque is correlated to capacity (and volumetric efficiency), so if your v-twin can’t match the i4’s revs, it needs a helping hand in the rules to get more capacity.

Dammit about the f1 vs cyclist. I’m sure it was true at one point (probably when they hit 20000rpm) but the point still stands that a cyclist has about the same magnitude of torque.

Power is how fast you hit the wall, torque is how far you move the wall when you hit it.

VTEC y0!!!

That’s no good on the race track. They make VTEC killer cams to keep the engine in high lift. As TINAS said not enough torque to pull out of the corners when the revs drop.

So cars that suddenly start accelerating feel faster to us, compared to cars which accelerate just as hard, but do so for longer, or that accleration ramps in and out more smoothly!

My dad had a 1.4 petrol with the same max bhp as my 1.9 diesel, but my car felt way faster in normal driving because it was one of those PD engines with a huge slug of torque low down then nothing.

And remember, quoted bhp figures are just peak. They don’t say how long you get that power for…

Yep, feeling fast and actually being fast are different.  Proven very well with my very small engined turbo.  If pulls really well from 2,000 and gives a feeling of acceleration but it runs out at 5,500.  In isolation it actually feels fast but as soon as another car is involved it soon becomes obvious that it is in fact slow…

For those of you with a BMW, switch on your ‘sports’ display that gives you a power and torque display.

Tootling along you can use next to no power ie 40-50hp but maybe 200/250 torques @ anything from 1500-2000 revs. The car is very easy to drive and quiet as a mouse, and pulls relatively well.

However change down, 3000 + revs you build very quickly to max power and torque decreases, it’s all good fun but you can’t drive like that everywhere. There must be some correlation to mpg too? ie if your progressing using torque you are not firing the cyclinder as often so surely using less fuel?

Bloody forum!

Is that why diesel has been popular at Le Mans because you can go longer on the same amount = less fuel stops?

Torquey diesels are great for normal driving where you want to have instant acceleration, but as mentioned indirectly above they run out of revs too quickly.

This means you are using more or less all the torque to accelerate and there is nothing left if you really want to push on.

My 184hp, 400Nm diesel feels quick when driving but it can’t hold a candle to a similar weight petrol car with 200hp and significantly less torque when both are pressing on and using gears correctly.

For those of you with a BMW, switch on your ‘sports’ display that gives you a power and torque display.

Sunday’s 200m round trips for jnr’s events afforded just the opportunity, for which Mrs K has named the “teenage gauges”.   I guess – bearing in mind twonks comments – the 190hp of the BMW engine combined with 8 speeds from the Gearbox allows them to keep the car in the torque range a fair bit.  Makes for a comfortable experience whilst making progress.