Is torque really relevant?

[un1corn]

I'm having a dilemma here.

So, i've got a 2015 530d. I love it, great car. Chucks out 254bhp and 540nm of torque.

I want rid, and i'm looking at the new 530d, which chucks out a stonking 620nm and 261bhp.

However, I don't do the miles to justify a diesel. I've been looking at the 530i and A4 45tfsi, which are both 2.0 litre petrols. Pretty much all these cars hover around 5.6s for a 0-60 time and the 250bhp area, but the torque on the Audi is 370nm and on the 530i is 350nm.

Maybe i'm reading into this or doing it wrong by comparing a diesel and a petrol. I really enjoy my current 530d, i like the way i can floor it on a kickdown and it feels like a train and it makes overtakes on the motorway, country roads or NSL roads a piece of cake (nearly fun?). The new 530d with even more power/torque can't be any worse. Even off the line, my current 530d is great. Takes a few yards to get going but then it's just a constant push and keeps going and going.

However, because i'm looking at petrol, will I be disappointed by taking a 2.0 petrol that throws out the same bhp level but half the torque? I've not driven a petrol for years. The last one I had was a 2007 Vectra VXR which used to just spin the front wheels when you went near the power too much.

In short, will i be left feeling short getting one of these 2.0 turbo petrols over the big 3.0 diesel lump?

I'm only really looking at these cars due to budget. Something like a 540i would solve the problem, but meh. I might just play safe and pay over the odds for the new 530d when I do around 7000 a year. The other issue i have is that these cars are rare, so I cant just pop down the local dealer to drive it. It seems the new car market is saturated with 1.8 150bhp petrols and 170bhp 1.9 diesels.

Any advice you lot could offer would be appreciated. I've spent hours watching various acceleration and noise videos on youtube and it's not made my mind up one bit.

Edited by un1corn on Monday 6th July 01:24

[raspy]

I wouldn't focus too much on stats alone as that won't tell you enough about how well the car drives in the real world.

Yes, torque is still relevant, and in particular "instant torque" from electric motors.

I have a MB C350e, which has 275bhp but 600Nm from a 2 litre turbo petrol with a small electric motor. I love the torque, especially off the line. It makes driving completely effortless. I suggest considering a petrol plug in hybrid if you want to get back to petrol and have a lot of torque.

However, go for an EV (doesn't have to be a fast one on paper) and you'll find the "instant torque" a revelation compared to ICE powered cars, even a humble Nissan Leaf.

If you really want your mind blown regarding "instant torque", test drive a Tesla Model 3 Performance. It's completely bonkers, but in a fun way :-)

[kambites]

Go and drive the cars in question. Flywheel torque figures are tricky when comparing petrols to diesels because petrols rev higher and hence typically have shorter final drive ratios (basically the petrol engine sits at higher revs most of the time). Thus wheel torque in real-world situations doesn't tend to favour the diesel by anything like the flywheel torque figures would suggest. If you're driving an automatic and allowing the box to kick-down anyway, it's even less relevant because the gearbox will do what it needs to do to get you to peak wheel torque (or to put it another way, peak power) for the speed you're travelling at.

Of course this does mean you're using the revs of the petrol engine, which can make you sound a bit of an idiot if there's lots of other cars around.

[Equus]

Unless you have a CVT, yes, torque is relevant.

[Equus]

kambites said:

...the gearbox will do what it needs to do can within the limits of a small number of fixed ratios to get you to peak wheel torque (or to put it another way, peak power) for the speed you're travelling at.

EFA

[kambites]

Equus said:

kambites said:

...the gearbox will do what it needs to do can within the limits of a small number of fixed ratios to get you to peak wheel torque (or to put it another way, peak power) for the speed you're travelling at.

EFA

Yup, but just because a diesel has more peak torque doesn't mean it has more flexibility. It may well be that the gearbox in the petrol can do more to keep the engine in its power band than that in the diesel. In practice modern automatics have so many damned gears and turbocharged engines have such broad power bands that the 'box can usually do a pretty good job of maintaining peak power whatever the fuel.

I've never looked at the figures, but I'd bet if you look at the acceleration figures from, say, 30 to 60 (or pretty much whatever numbers you choose) through the gears including kick-down time, a 530i will be faster than a 530d.

Edited by kambites on Monday 6th July 07:10

[Equus]

kambites said:

Yup, but just because a diesel has more peak torque doesn't mean it has more flexibility

But in the real world, it almost always does.

Proof of the pudding is in the driving. I defy you to drive a diesel against a petrol of similar BHP and say that it makes no difference.

[kambites]

Equus said:

Proof of the pudding is in the driving. I defy you to drive a diesel against a petrol of similar BHP and say that it makes no difference.

Don't need to drive it to understand the numbers, just need to look at the shape of the torque curves and the gear ratios.

I don't deny that diesels often feel faster, they tend to generate greater jerk. They also feel more laid-back because they don't need to be revved to generate the power. However I don't think they generally are faster. So I guess it depends on what the OP is after.

ETA: As a case in point, the F10 520i and 520d had indentical power figures and pretty much identical performance figures all the way up the range. It's only if you artificially lock the cars into particular gears that the diesel is ever faster.

Edited by kambites on Monday 6th July 07:18

[Equus]

kambites said:

Don't need to drive it to understand the numbers, just need to look at the shape of the torque curves and the gear ratios.

Well, if you prefer numbers, I suggest you dig a bit deeper: do the calculations on the change of engine speed required to deliver a given change in road speed, and the rotational inertias involved.

Then you will understand the meaning of your 'greater jerk' (!) and the fact that horsepower-for-horsepower, a car with more torque will have considerably more in-gear acceleration than one with less.

[kambites]

Equus said:

Then you will understand the meaning of your 'greater jerk' (!) and the fact that horsepower-for-horsepower, a car with more torque will have considerably more in-gear acceleration than one with less.

I understand the mechanics thanks, and we'll have to agree to differ here.

[Equus]

kambites said:

I understand the mechanics thanks, and we'll have to agree to differ here.

So do I - like I said, dig a bit deeper in the numbers involved. I understand the mechanics and the numbers, too, and I think you're treating it at too superficial a level to gain a proper understanding.

[John Locke]

Torque (power) really is relevant, but so is bhp (the rate at which the power is produced), even more so the band over which they are delivered. The greater all three, the more responsive the vehicle will be, hence diesels are sluggish until they wake up, petrols ultimately more satisfying to the enthusiastic driver, and electrics a revelation, once one accepts them for what they are.
Make a short list, beg an extended test drive, or better still, rent one of each for a weekend to allow yourself to get the best from them and decide your preference, then spend your hard earned.

[kambites]

Equus said:

kambites said:

I understand the mechanics thanks, and we'll have to agree to differ here.

So do I - like I said, dig a bit deeper in the numbers involved. I understand the mechanics and the numbers, too, and I think you're treating it at too superficial a level to gain a proper understanding.

I know that you think that, and clearly you are going to continue to think that.

Anyway, the answer is still that the OP needs to find a way to drive the cars in question.

Edited by kambites on Monday 6th July 07:27

[alec.e]

Yes it is good to have enough low-down torque for daily driving, however, the quoted figure isn't to get hung up about in my recent experience. My previous daily- a Supercharged V8 Jag had around 600nm and my current Mercedes V8 BiTurbo is 700nm- the Jaguar required less effort to accelerate in daily driving, due to better throttle response.

[Equus]

kambites said:

I know that you think that, and clearly you are going to continue to think that.

Because it's a mathematical truth: it was you who said that you preferred to believe the numbers, so work them out. Properly.

[kambites]

Equus said:

kambites said:

I know that you think that, and clearly you are going to continue to think that.

Because it's a mathematical truth: it was you who said that you preferred to believe the numbers, so work them out. Properly.

I have thank you, at least as accurately as we have the available information for. You clearly believe the same so lets just leave it at that and not polute the thread arguing about who has the better grounding in classical mechanics.

[Equus]

kambites said:

I have thank you, at least as accurately as we have the available information for. You clearly believe the same so lets just leave it at that and not polute the thread arguing about who has the better grounding in classical mechanics.

I know from past experience that you actually like to understand things: I'm not arguing, I'm genuinely encouraging you to run the numbers.

You need to take into account the greater change in revs (and therefore rotational inertia) that a 'low torque' engine has to go through, to achieve the same change in road speed.

That's the reason that when you've finished playing with numbers, the actual, real-life feel and response is quite different.

[nickfrog]

Last time I checked you needed to combine both torque and revs to generate power. So isolating one of the elements of the simple equation as well as ignoring gearing/FD is absurd (particularly when comparing petrol and diesel, which will have dramatically different gearing to accommodate their respective strengths).

[Baldchap]

Torque is rotary force - essentially how hard you can turn the wheels (massive simplification).

Power is torque * rpm / 5252. Basically a human friendly description of how fast we can turn the wheels.

As we can see from the power equation, higher RPM will result in higher power (much moreso over 5252rpm). But, where diesels come in is that they typically generate more torque lower down. This tends to result in better throttle response and, some might say, better driveability at low RPM. Because petrols rev higher, you tend to find they produce more peak power. All things being equal, below 5252 rpm the engine producing more torque is therefore producing more power.

In modern turbocharged petrols, they normally produce peak torque over quite a broad RPM range and also rev well, therefore producing a higher peak power (often also over the rest of the rev range where peak torque has tapered off) but with good driveability lower down. Gearing is typically different to take advantage of these different engine characteristics, so don't judge just on the engine numbers.

[bristolracer]

Are you towing a caravan?

If not, dont overthink it, yes they drive different, but for 99% of normal driving its really not going to ruin your life.

Also your hands wont stink after filling up
Greta will thank you too