I know this is harking back to basics and I really ought to know having driven my diesel for 20k miles now, but I'm not convinced by the often quoted 'in gear acceleration' of turbodiesels.:confused:

Surely, it's only an advantage over a similairly powered petrol if said petrol has a very peaky engine? And the 30-70mph argument . . well, a diesel might be able to cover 30-70 in 3rd, but the petrol probably would keep up in 3rd from 50-70 then carry on to about 85mph, by which time the diesel would be long behind?

Maybe this is a bit daft, but the only advantage performance wise I can see of diesels is that the torque is low down and ready at normal driving revs to help pull the revs up before the power is there, as opposed to a petrol which might need a downshift?

Please no argueing over diesel v petrol, I'm trying to seriously understand this. Reason for asking is I recently did a head to head against a slightly modified Focus 1.8, we had almost identical power to weight ratios and we accelerated just about identically. I would have thought that this 'in gear advantage' and 'outrageous torque' would have made a difference, but it didn't?

Just wondering . . .

Doesn't in gear acceleration only favour the diesel if both petrol and diesel are accelerating in the same gear :confused: So if both are in the right gear for their powerband, with similar power/weight, chances are it'll be pretty even....

Chris

i expect the diesel has got a relitively wider power pand than the petrol though, due to the fact it uses a vnt turbo.

Does this thread help?

http://www.briskoda.net/forums/fabia/bhp-torque-explained/59443/

Chris

that is a very interesting article chris. there's only one factor which has been left out from the allready complicated subject. 'inertia'

Turbodiesels and petrols drive different from turbo petrols, which drive different from hybrid cars

Each one has it's own strengths/weaknesses. A massive turbo would increase lag a LOT but then you'd get a ton of power kicking in.

I like the way the vRS (Fabia) drives for sure, but I'd have no problem enjoying the vRS (Octavia Mk1/Mk2) either - just wouldn't be able to afford to run it at the kind of speeds I tend to cruise at + mileage I do.

Shame really :(

I know this is harking back to basics and I really ought to know having driven my diesel for 20k miles now' date=' but I'm not convinced by the often quoted 'in gear acceleration' of turbodiesels.:confused:

Surely, it's only an advantage over a similairly powered petrol if said petrol has a very peaky engine? And the 30-70mph argument . . well, a diesel might be able to cover 30-70 in 3rd, but the petrol probably would keep up in 3rd from 50-70 then carry on to about 85mph, by which time the diesel would be long behind?

Maybe this is a bit daft, but the only advantage performance wise I can see of diesels is that the torque is low down and ready at normal driving revs to help pull the revs up before the power is there, as opposed to a petrol which might need a downshift?

Please no argueing over diesel v petrol, I'm trying to seriously understand this. Reason for asking is I recently did a head to head against a slightly modified Focus 1.8, we had almost identical power to weight ratios and we accelerated just about identically. I would have thought that this 'in gear advantage' and 'outrageous torque' would have made a difference, but it didn't?

Just wondering . . . [/quote']

If your car had much more power than the focus you would have taken off.

However, you share the same power ratings, the only time your torque figure would apply is if your car and his car floored it at the same time in the same gear at the same speed. If he drops it down a gear though, given you both share the same power, acceleration would be the same, if not quicker, since he has a much wider rev range to play with by the time you would need to change up.

Look at this link mate: http://www.allardmotorcompany.com/pages/atmr/petrol-vs-diesel/petrol-vs-diesel.asp

The petrol has to drop gears or stay in a lower gear longer, but diesel has a wider powerband. IF the two keep the same gear ("in-gear acceleration") (the petrol driver has to be incompetent, lazy, half asleep, etc) Diesel wins hands-down.

If racing, it ain't gonna happen. On the road, the petrol driver has to deliberately down-shift and floor it. You just floor it.

HTH

Bas

In reality, the *turbo*diesel gives a lot of torque at low rpm, (A Non-turbo diesel is another matter altogether, they're absolutely awful to drive!) which gives the average driver the impression that the car is 'fast' although, in reality, the torque drops off quite quickly as revs rise. For relaxed driving, the TD doesn't need a lot of gearshifting when tootling around.

In my opinion, a decently-mapped turbopetrol is superior from a driving point of view as it has high low rpm torque and a wide powerband. Fuel economy can be much better with a diesel though.

In my experience -and unsurprisingly- a TDI 130 Golf accelerates at almost exactly the same rate as a 130bhp 2.0 Focus, but with better fuel economy.

However, you share the same power ratings, the only time your torque figure would apply is if your car and his car floored it at the same time in the same gear at the same speed.

Exactly - the gears are all about torque multiplication as that article pointed out, so just cos you have 300lb/ft of torque coming out the engine and the petrol only has a puny 140lb/ft, doesn't necessarily mean the car with the biggest torque has the biggest torque at the wheels, especially if the petrol driver drops a cog or two for great torque magnification....

Chris

Exactly - the gears are all about torque multiplication

Exactly, I've posted very similar points in the past, although people don't seem to believe it.....

TDs rev slower than petrols, therefore the gearing is taller/longer/higher. The torque at the wheels is therefore reduced...

Why do people go on and on about "in gear acceleration" ? Its not like you can accelarate very well when not in gear is it ?!?!

Why do people go on and on about "in gear acceleration" ?

To mask defiencies in every other aspect of the car's performance.

Rob.

Thanks everyone. Very interesting reading, especially the first linked article. It all makes sense now, I don't think I'd really thought about the different gearing levels in the two types of car!

IMHO it comes down to being able to be more lazy and still get a fairly decent response.

That said, modern petrols are getting more torquey (sp?) all the time, and there is probably more potential for 'off-factory' tuning.

That said I'm still looking forward to that supercharger/turbo/small engine combo that VAG is developing IIRC?

Comparing my old Cav 2l 16v T and my current Octy 1.9 8v PD130, the Octy is OK but the Cav could worry some expensive cars. The Octy has 10% more peak torqe (I think) but only 65% the power. The Octy is c.100kg heavier. Both have 6 speeds, the Cav would pull in top from 30mph and you never had to change down for any hill - unlike the Octy. 0-60 6.4 v. 9.7 (1s advantage from 4wd?), top 145+ v. 127mph, 30-70 6.2 v. will have to test! Wasn't much point revving past 5500rpm in the cav, 4000 in the octy.

All that shows is that they are both very different cars, but in all honesty I think petrol turbo (flatter torque and power curves over a much wider range) + more power wins in a race and on the road in flexibilty. I would be interested to try driving a higher power PD Golf etc. though - just might not be so clear cut.

theres no real replacment for cubic capacity

has anybody on here ever worked on rolls-royce diesel engines??

the ones i worked on were 27litre supercharged monsters fitted into diesel locomotives. 2500bhp. now that's a proper engine

How about this for a diesel!

25480L, 100,000+hp and 5,600,000+lb/ft at 102rpm

http://people.bath.ac.uk/ccsshb/12cyl/

Comparing my old Cav 2l 16v T and my current Octy 1.9 8v PD130' date=' the Octy is OK but the Cav could worry some expensive cars. The Octy has 10% more peak torqe (I think) but only 65% the power. The Octy is c.100kg heavier. Both have 6 speeds, the Cav would pull in top from 30mph and you never had to change down for any hill - unlike the Octy. 0-60 6.4 v. 9.7 (1s advantage from 4wd?), top 145+ v. 127mph, 30-70 6.2 v. will have to test! Wasn't much point revving past 5500rpm in the cav, 4000 in the octy.

All that shows is that they are both very different cars, but in all honesty I think petrol turbo (flatter torque and power curves over a much wider range) + more power wins in a race and on the road in flexibilty. I would be interested to try driving a higher power PD Golf etc. though - just might not be so clear cut.[/quote']

2L 16 V Cav at 9.7 secs to 60? Were you 4 up with bootful of rocks? The 8V 1.8 would go to 60 in mid 8s with decent tyres on. "Lightly" modded, my 1.8 ran around 7.8 to 60 and high 15 / 93mph 1/4s

Chris

How about this for a diesel!

25480L' date=' 100,000+hp and 5,600,000+lb/ft at 102rpm

http://people.bath.ac.uk/ccsshb/12cyl/

Jeez the PD lump is heavy enough!

Chris

yep that is a biggon.

i've not worked on many marine engines but i have played with some of the common ones like the volvo penta's, lister, duetz

2L 16 V Cav at 9.7 secs to 60? Were you 4 up with bootful of rocks? The 8V 1.8 would go to 60 in mid 8s with decent tyres on. "Lightly" modded' date=' my 1.8 ran around 7.8 to 60 and high 15 / 93mph 1/4s

Chris[/quote']

Yes, wot I writ ain't that clear!.

It was a Turbo 4x4 Cavalier and the 6.4s 0-60 refers to that, the 9.7 is for the Octy.

How about this for a diesel!

25480L' date=' 100,000+hp and 5,600,000+lb/ft at 102rpm

http://people.bath.ac.uk/ccsshb/12cyl/

5,000,000 lbs/ft torque? Same as 2.0 furby, non?

Sorry...