Is torque really relevant?
Discussion
Equus said:
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.
Last time i actually tried to work it out (admittedly using a lot of gusses in the data), there was very little in it. As best I could work out, the diesel had roughly 50% more effective rotational inertia (strictly speaking the moment of inertia) in the engine than the petrol (due to a combination of greater actual rotation mass and greater linear linear kinetic energy in the longer stroke of heavier pistons); higher torque tend to use gearboxes use considerably heavier internals which obviously have more rotational inertia as well (although exactly how that falls out will depend on which gearboxes). Obviously an engine reving 50% higher but with a third less effective mass will have more internal kinetic energy, but only 50% more (I think with the numbers I used before it came out closer to 40% but there's obviously a fair margin for error there without accurate figures).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.
Once you add in the extra total mass of the diesel vehicle (in the case I worked out it was about 5% I think, but it'd be less with big cars like the OP is considering), the overall energy requirements could have fallen either way.
Edited by kambites on Monday 6th July 08:13
Baldchap said:
Torque is rotary force - essentially how hard you can turn the wheels (massive simplification).
Flywheel torque is how hard you can turn the flywheel, not the wheels. A gearbox is a torque multiplier; a car generating X NM at Y RPM will generate the same wheel torque as a car generating X/2 NM and 2Y RPM. That's why power is convenient, torque can be multiplied up and down by gearing so flywheel torque doesn't really tell you anything at all about what's going on at the wheels, whereas power at the flywheel is the same as power at the wheels (baring frictional losses, obviously).1
Edited by kambites on Monday 6th July 08:04
John Locke said:
Baldchap said:
This tends to result in better throttle response and,
One quality which a diesel, indeed any turbocharged engine doesn’t have is good throttle response. For that a naturally aspirated or supercharged petrol engine is needed, or better, an electric motor.John Locke said:
One quality which a diesel, indeed any turbocharged engine doesn’t have is good throttle response. For that a naturally aspirated or supercharged petrol engine is needed, or better, an electric motor.
Generally speaking the more an engine depends on a turbo for power, the worse the throttle response.For example, a 1.6T with large turbo to attain 270hp has a lot more lag than a 1.6T with small turbo for 170hp.
The benefit of a diesel is that if fits normal driving better, it is more stealth and probably more acceptable.
I haven't missed my E39 M5 since I switched to a later 3 series diesel. They are about the same size inside. If you squint the furnishings look similar.
If they were both parked at a traffic light the diesel would win up to pretty much legal speeds UNLESS the M5 started with high revs and abused the clutch, mainly due to the clutch delay valve.
In other words from tickover with your foot down the 3 series is only beaten by a petrol engine if that engine is three quarters up its rev range when it starts, and to be honest it needs to be a pretty powerful petrol engine to do it.
Now to any nearby police officer the petrol car is an attention magnet at that point. I really didn't appreciate how much of a magnet it was until I switched. Since switching I drive at the same speed probably faster point to point, but unlike the M5 I've only ever been stopped once for the manner of my driving and never been followed because of the type of car. This happened on a fairly regular basis on the M5, and its because of the torque. I can go point to point on a country road on the wave of torque without having the engine screaming and without attracting any attention.
The other benefit of course is that it does it all using nearly half the fuel. So its cheap, fast and stealthy.
I think that high torque low power cars are better for our current roads, but I'd still always pick a petrol for a trackday because I prefer the sound.
I haven't missed my E39 M5 since I switched to a later 3 series diesel. They are about the same size inside. If you squint the furnishings look similar.
If they were both parked at a traffic light the diesel would win up to pretty much legal speeds UNLESS the M5 started with high revs and abused the clutch, mainly due to the clutch delay valve.
In other words from tickover with your foot down the 3 series is only beaten by a petrol engine if that engine is three quarters up its rev range when it starts, and to be honest it needs to be a pretty powerful petrol engine to do it.
Now to any nearby police officer the petrol car is an attention magnet at that point. I really didn't appreciate how much of a magnet it was until I switched. Since switching I drive at the same speed probably faster point to point, but unlike the M5 I've only ever been stopped once for the manner of my driving and never been followed because of the type of car. This happened on a fairly regular basis on the M5, and its because of the torque. I can go point to point on a country road on the wave of torque without having the engine screaming and without attracting any attention.
The other benefit of course is that it does it all using nearly half the fuel. So its cheap, fast and stealthy.
I think that high torque low power cars are better for our current roads, but I'd still always pick a petrol for a trackday because I prefer the sound.
julian64 said:
I think that high torque low power cars are better for our current roads, but I'd still always pick a petrol for a trackday because I prefer the sound.
This is one of the reasons an electric family car appeals so much to me - no gearbox or turbos to harm initial repsonse, pretty much completely silent so it's impossible for anyone else to tell how fast you're driving, around 90% efficient under almost all loads, regenerative braking means braking hard isn't throwing away fuel,... John Locke said:
One quality which a diesel, indeed any turbocharged engine doesn’t have is good throttle response. For that a naturally aspirated or supercharged petrol engine is needed, or better, an electric motor.
Not totally accurate to be honest, I can name diesels which have better throttle response than their petrol equivalents. ddom said:
John Locke said:
One quality which a diesel, indeed any turbocharged engine doesn’t have is good throttle response. For that a naturally aspirated or supercharged petrol engine is needed, or better, an electric motor.
Not totally accurate to be honest, I can name diesels which have better throttle response than their petrol equivalents. 
However, it is true to say that turbos never have brilliant throttle response (unless they have anti-lag, hybrid turbos, or cleverness of that sort). They just can't because a turbo is fundementally a feedback loop. They've got a damned slight better, mind. I think one can reasonably argue that in a two tonne barge, the throttle response of a decent rubo engine is plenty good enough.
Edited by kambites on Monday 6th July 09:00
kambites said:
julian64 said:
I think that high torque low power cars are better for our current roads, but I'd still always pick a petrol for a trackday because I prefer the sound.
This is one of the reasons an electric family car appeals so much to me - no gearbox or turbos to harm initial repsonse, pretty much completely silent so it's impossible for anyone else to tell how fast you're driving, around 90% efficient under almost all loads, regenerative braking means braking hard isn't throwing away fuel,... Also I like a heated wheel and seat with some warm air blowing at me in winter with a decent power window defroster. I'd end up with no range in an electric car
.John Locke said:
Baldchap said:
This tends to result in better throttle response and,
One quality which a diesel, indeed any turbocharged engine doesn’t have is good throttle response. For that a naturally aspirated or supercharged petrol engine is needed, or better, an electric motor.It's like they've figured out that most drivers never pushed the pedal all the way so maximum power demand is in the first half of the pedal travel.
So OP, whatever you try, make sure you actually floor it a few times on your test drive.
Love the "I cant possibly drive anything with less than 400 lb/ft at 1200 rpm"
Performance figures and power/torque figures are a small part of the picture, I do like diesels and the way they drive but they have got us addicted to huge amounts of torque.
I dont think the old ways of having a big capacity NA six would cut it for a lot of ex diesel drivers but a lot of stuff is petrol turbo anyway and when a 2 litre turbo has 270 lb/ft or thereabouts at 1800 rpm, its not exactly lacking low down torque, event a humble 3 cylinder 1.5 litre Fiesta ST has more torque than a 3.5 Rover V8, at lower RPM, and the Rover used to be the torqueiest thing mortals got their hands on.
I think that sometimes after something like that, initially anything with less may feel lacking but part of it is acclimatization, I drove a Clio 182 after my 2001 Saab 9-3 and decided I didnt want one as it felt flat as a fart after the turbocharged delivery of the Saab. Trouble is the Clio 182 is widely regarded as a classic, where the Saab 9-3, though decent is largely forgotten and had crap handling. I kind of wish I had took the plunge but it felt limp after the turbocharged Saab, despite supposedly having another 32 bhp. I reckon with a few more miles to dial my brain in to the power delivery I would have enjoyed it.
Torque is sort of like eating stodge/Fast food, hits the spot but try not to miss out on other aspects whilst gorging on the instant gratification it provides.
Performance figures and power/torque figures are a small part of the picture, I do like diesels and the way they drive but they have got us addicted to huge amounts of torque.
I dont think the old ways of having a big capacity NA six would cut it for a lot of ex diesel drivers but a lot of stuff is petrol turbo anyway and when a 2 litre turbo has 270 lb/ft or thereabouts at 1800 rpm, its not exactly lacking low down torque, event a humble 3 cylinder 1.5 litre Fiesta ST has more torque than a 3.5 Rover V8, at lower RPM, and the Rover used to be the torqueiest thing mortals got their hands on.
I think that sometimes after something like that, initially anything with less may feel lacking but part of it is acclimatization, I drove a Clio 182 after my 2001 Saab 9-3 and decided I didnt want one as it felt flat as a fart after the turbocharged delivery of the Saab. Trouble is the Clio 182 is widely regarded as a classic, where the Saab 9-3, though decent is largely forgotten and had crap handling. I kind of wish I had took the plunge but it felt limp after the turbocharged Saab, despite supposedly having another 32 bhp. I reckon with a few more miles to dial my brain in to the power delivery I would have enjoyed it.
Torque is sort of like eating stodge/Fast food, hits the spot but try not to miss out on other aspects whilst gorging on the instant gratification it provides.
I have a V6 3.0 diesel(GLC) and a 4 cylinder 2.0 turbo petrol(Cooper S), and believe it or not the torque actually comes in earlier in the petrol(around 1200rpm). Modern petrol turbo engines feel like diesels anyway but with a few more revs to play with, plus they have a slightly less annoying sound at low revs/idle for me.
Adequate low rpm wheel torque (torque curve and suitable gear ratios) and sharp throttle response makes for good road driving.
Some people appear to lose sight of the level of of torque/power/performance that is required in a road car. There is no hardship in driving a well designed, responsive small petrol turbo with decent low rpm torque, 150bhp and well chosen gear ratios. Eg. The previous generation VAG 1.4TSI.
Unfortunately, it does appear to me that the latest emissions regs (emissions reduction is necessary) have resulted mostly in engines that are less responsive, petrol or diesel.
10 years ago, emissions were worse, but driveability was often better.
We all know that electric cars are the future.
Some people appear to lose sight of the level of of torque/power/performance that is required in a road car. There is no hardship in driving a well designed, responsive small petrol turbo with decent low rpm torque, 150bhp and well chosen gear ratios. Eg. The previous generation VAG 1.4TSI.
Unfortunately, it does appear to me that the latest emissions regs (emissions reduction is necessary) have resulted mostly in engines that are less responsive, petrol or diesel.
10 years ago, emissions were worse, but driveability was often better.
We all know that electric cars are the future.
I've just gone from a Volvo D4 with 190bhp to a T4 with 190bhp.
The petrol feels like harder work to make progress. Without conducting some kind of study, it feels like it's changing down more often than the diesel would. It obviously has to scream more at higher revs to achieve the same motion as the diesel. On balance, outside of low speed urban areas, the diesel was a more relaxing thing to use. At low engine speeds and urban journeys, the petrol is obviously smoother and a more pleasant thing to use.
If petrols are now as full of emissions gubbins and complexity as their diesel equivalent, and hence prone to similar risk of breakdowns, and all other things being equal, I'd probably prefer the diesel if my driving was mainly out of town and the petrol if it was urban.
The petrol feels like harder work to make progress. Without conducting some kind of study, it feels like it's changing down more often than the diesel would. It obviously has to scream more at higher revs to achieve the same motion as the diesel. On balance, outside of low speed urban areas, the diesel was a more relaxing thing to use. At low engine speeds and urban journeys, the petrol is obviously smoother and a more pleasant thing to use.
If petrols are now as full of emissions gubbins and complexity as their diesel equivalent, and hence prone to similar risk of breakdowns, and all other things being equal, I'd probably prefer the diesel if my driving was mainly out of town and the petrol if it was urban.
Gassing Station | General Gassing | Top of Page | What's New | My Stuff