In the motoring section of a well known broadsheet yesterday, an equally well known writer states that one of the best ways to 'kill' a DMF on a diesel is to raise the clutch at idle, such as when making progress in slow moving traffic. Apparently, the 'correct' way to drive is to raise the revs above idle, before letting the clutch out.
Our diesel has ample torque to move off at idle in first gear and this is my normal way of setting off, with the throttle only being pressed when the clutch is fully realeased, unless I need to make a swift getaway or I'm on a hill.
My understaning was that getting the DMF to cancel out 'shocks' (jerky gerachanges etc), deal with excessive vibration (30 mph in 6th gear) or holding the car at the biting point, rather than using the handbrake were the real DMF killers.
At idle, the engine on our motor is pretty smooth and surely at c. 1,000 revs when I gently release the clutch, there are less vibrations than if I increased the revs to say 1,500 or 2,000 revs...
I don't let the engine 'bog down' to the point where it is virtually stalling...
Any thoughts on whether the HJ statement is 'accurate'?
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My thoughts are that DMFs wear out over time/mileage - even when the engine is running at normal running speed the DMF is working to smooth out the torque output of the cylinders.
Most TD Mondeos seem to wreck their DMFs between 70 and 100 thousand miles, so there's perhaps a degree of inevitability about it (of course there's always going to be a few that far outlast others).
Pulling away at very low revs perhaps leads to relatively higher energy going through the DMF so it's maybe a bit like clutch wear other than clutches don't really wear through steady speed running - DMFs do.
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The other thing said to kill DMFs is heat, so excess revs and clutch slip should also be avoided. In my Golf, I would consider 2000 RPM excessive unless getting away in a serious hurry. The engine is making 310NM of torque by then, and will set the traction control all of a fluster. Like your car, it will happily move off at idle.
I think its a case of getting a feel for the individual car.and balancing clutch slip against smoothness. 1300 - 1500 works for me in this car. But others are different.
I would say the individual engine delivery characteristis determine the correct approach.
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Minimising revs will maximise torque so I suppose there is some logic there. The car will add more fuel to maintain 800rpm or whatever, increasing the torque proportionate to the power required.
You'd expect the effect on the DMF to be proportionate to the frequency and severity of shock loads. I don't imagine the torque is very high at tickover - racing starts sound like more of a problem. And at tickover you can't let the clutch in too sharply or you'd stall. Using high revs would put more heat/energy into the clutch and heat is also said to be a DMF killer.
I wonder if he's guessing or if there are any actual data from manufacturers to support the theory.
I have a habit of pretty well always setting off at tickover before picking the revs up. 94,000m so far.
Last edited by: Manatee on Sun 3 Apr 11 at 08:59
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I read HJ's comment yesterday, I don't agree, I think the key is to always have the clutch depressed when starting or stopping the engine, my Mondeo was at 140k on the original DMF when I sold it.
Perhaps if in traffic and putting the car into neutral there might be a minor benefit in raise the engine speed slightly as the clutch is released to smooth out vibes though electronically controlled engines (all CR diesels) are doing that for themselves anyway to an extent.
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>> I read HJ's comment yesterday, I don't agree, I think the key is to always
>> have the clutch depressed when starting or stopping the engine
2 Italian engineers published a whitepaper on DMFs, I'm sure it was linked to from here or hj. That was their conclusion, you could do more or less anything to a dmf provided the clutch was disengaged for starting and stopping and it would last.
It was something to do with low frequency irregular shocks that occur strongly during starting and stopping.
Who knows if they're right or just bam pots though :-)
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I did not have any problems with the DMF on my focus, sold at 75,000 miles. The Ceed does not have a DMF, I assume the electronics sort it out as Cheddar says. It certainly drives smoothly.
EDIT - Just seen Skoda's post. I always start and stop an engine with my foot on the clutch, habit. :-)
Last edited by: Old Navy on Sun 3 Apr 11 at 09:49
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Car Mechanics Mag did an article on DMF's and one of the Technical Support Managers from Euro Car Parts had this to say,
" There are many reasons why a DMF will fail, the most common of which can be avoided observing a different driving style. Driver abuse accounts for between 60-70% of all failures. Gunning away from lights creates excess torque through the transmission, as does pulling off in any other gear than first. Travelling in high gear when speed is low and pulling or carrying excess loads are also obvious manners to be avoided. Taxis and many light commercial vehicles are also vulnerable to premature failures due to the way these vehicles are driven.
Some design issues with certain DMF systems can also result in premature failure, notably both the Ford Transit and Ford Mondeo, making these some of the most popular units replaced."
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Pulling away at tickover or a bit more than tickover cannot make any difference to the wear of what is a fairly robust component.
Some of HJ's stuff seems increasingly off-beam these days.
I wonder if spending a lot of time in Thailand has caused him to lose touch, or maybe the column is now written by somebody else.
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>> Pulling away at tickover or a bit more than tickover cannot make any difference to
>> the wear of what is a fairly robust component.
Firstly its not that robust or we wouldn't be talking about it, and yes pulling away at tickover could quite easily wear it out, low revs at high load is a DMF killer.
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If driver error contributes so much to DMF failure, then automatics should have lower failure rate. Is it the case?
One should not have to change driving style drastically to suit a particular car. If that's the case for DMF, then it is a bad design.
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>> If driver error contributes so much to DMF failure, then automatics should have lower failure
>> rate. Is it the case?
Good point. Loads of VAG DMFs need replacing all the time on the forums, but they're always manual. I can only remember one post on briskoda about a DSG auto requiring the DMF changed. Not many people even know the DSG auto boxes in VAGs are driven by a DMF.
Anecdotal and hardly accurate but seems to go along with the idea.
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>>I can only remember one post on briskoda about a DSG auto requiring the DMF changed. Not many people even know the DSG auto boxes in VAGs are driven by a DMF.
The DSG cars have vays of managing torque to protect the gearbox. I still wouldn't trust one of those out of warranty.
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...Firstly its not that robust or we wouldn't be talking about it...
I've done about 100,000 miles in two DMF Ford diesels, neither of which have broken.
Given how common Fords are, if DMFs broke as often as people say, the roads would be littered with conked out Mondeos and Focuses.
And you can't seriously believe pulling away at tickover, or a little above, can make any difference to the wear of the flywheel.
Wheelspin starts, labouring and general car abuse might.
HJ will be telling us next that opening the car's door too quickly wears out the hinges.
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Well to be honest, much as i don't rate him, I think he knows more about the subject than you do.
And again, you cant argue the fact, that if DMFs never went wrong, we wouldn't be talking about them, would we.
Last edited by: Zero on Sun 3 Apr 11 at 11:04
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HJ is enjoying a good bandwagon like any journo would, esp. as the issue cuts across all major manufacturers rather than focussing on just one or two that may be favourites!
The millions of DMF equipped car drivers without problems are surely right aren't they? The few who suffer, and their friends in the media, rightly make loud noises but that doesn't mean they are right ;)
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>> The millions of DMF equipped car drivers without problems are surely right aren't they? The
>> few who suffer, and their friends in the media, rightly make loud noises but that
>> doesn't mean they are right ;)
>>
You could also say that about common rail diesels.
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... I think he knows more about the subject than you do...
He advised someone to pay a £100 scam ticket the other week - guy clearly doesn't understand the difference between the two types.
www.telegraph.co.uk/motoring/caradvice/honestjohn/8395236/No-escaping-parking-fine.html
I used to think his stuff was pretty much spot on.
No longer.
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>> ... I think he knows more about the subject than you do...
>>
>> He advised someone to pay a £100 scam ticket the other week - guy clearly
>> doesn't understand the difference between the two types.
>>
>> www.telegraph.co.uk/motoring/caradvice/honestjohn/8395236/No-escaping-parking-fine.html
Without getting deeply in to legalities, I think he had a point with that one.
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...Without getting deeply in to legalities, I think he had a point with that one...
It's the same as all the others, a legally unenforceable threat from a private scam parking company.
I suspect HJ mistook it for a local authority ticket, or he doesn't know the difference.
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not quite. When you pay for parking you are entering into a contract.
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>> not quite. When you pay for parking you are entering into a contract.
>>
Quite. The only issue is whether a court would uphold that - you are essentially arguing that there is no contract, or that it is unreasonable.
The reasons I thought HJ had a point are -
1. The party has admitted breaching conditions
2. A court's sympathy might be strained by the fact that it was a parent and child space being used improperly.
Not the typical overstaying 'penalty'.
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...2. A court's sympathy might be strained by the fact that it was a parent and child space being used improperly...
It's a civil matter and the civil court is concerned only with damages and costs, it has absolutely no role or powers to punish.
So once again, it comes down to how much the landowner has lost.
A few pounds for the time parked, but even that is arguable.
If the car park was not full, the overstayer has not prevented anyone else paying the landowner to park.
Whichever way you cut it, the damages are at most just a few pounds.
The scam company knows this, which is why so few cases are brought.
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>> So once again, it comes down to how much the landowner has lost.
>>
>> A few pounds for the time parked, but even that is arguable.
I this case its not. Its about contract law. When you pay to park, you enter a contract with the operator and you agree to abide by the T&Cs of that contract. That contract can ( and most commercial ones do) contain penalties for failures to observe the T&Cs, (in this case it will be along the lines of "failing to park as directed"). The parking operator is merely invoking the contract that you freely entered into, the one that is referred to in your ticket. They can take you to court, claim the breach of contract penalty and win. Your only defence in this case is proving the contract clause, or the penalty is "unfair". If the penalty is set at a similar rate to council parking fines, that wont work.
Its not the same at all as the scam parking companies.
Last edited by: Zero on Mon 4 Apr 11 at 08:14
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...Its not the same at all as the scam parking companies...
Even the parking companies don't really believe that.
There's stuff in contract law - particularly with consumers - about unfair terms, which these scam invoices fall foul of.
Even assuming a contract has been formed in the first place, which is debatable.
If there is a contract, it is limited to: "I agree to pay £6 to park for two hours."
Effectively, it would be: "Extra time by negotiation."
But this has never really been tested because cases are almost never brought to court.
We know why they are not brought - the scam company knows it will probably lose, or at best get a few pounds 'pro rata' for the extra time parked.
County courts just do not award punitive damages.
HJ's advice to that guy was garbage.
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>>
>> Its not the same at all as the scam parking companies.
>>
Z you are right, the issue with that case though was whether it is reasonable to be able to park in the parent and child spaces when picking up kids, i.e. when you dont have kids in the car when arriving in the car park.
IMO it should be.
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...the issue with that case though was whether it is reasonable to be able to park in the parent and child spaces when picking up kids...
That has absolutely nothing to do with the unenforceable scam ticket.
It matters not where he parked in the car park, the ticket has no force in law.
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>>
>> That has absolutely nothing to do with the unenforceable scam ticket.
>>
That's right, the unenforceable scam ticket is a different issue to the parking in the parent and child spaces matter.
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>> >> Pulling away at tickover or a bit more than tickover cannot make any difference
>> to
>> >> the wear of what is a fairly robust component.
>>
>> Firstly its not that robust or we wouldn't be talking about it, and yes pulling
>> away at tickover could quite easily wear it out, low revs at high load is
>> a DMF killer.
Load is still minimal at idle power. There are likely to be specific RPM levels that lead to destructive vibration in the DMF; interesting that the study quoted above suggested these occur at start-up and shut-down... well below idle revs.
My old Passat is on 140k on its original clutch and DMF. Have always had my foot on the clutch for starting and stopping the engine - perhaps this is why?
Last edited by: VxFan on Sun 3 Apr 11 at 18:04
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Could the higher compression of a diesel mean it initially fires, and stops more violently?
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>> Could the higher compression of a diesel mean it initially fires, and stops more violently?
It's certainly true that diesels start and stop with a violent shudder which is co-axial with the crankshaft (although doubtless there are more complex forces as well). Modern diesels seem less like this than they used to, perhaps because of the DMF. By declutching the driver disconnects the gearbox, with the considerable inertia of its weighty mainshaft, from the crankshaft/flywheel, thus reducing the axial stresses that the DMF is supposed to damp. I must say I am surprised really by HJ's contention.
NC might well put it better than I have if he could resist the temptation to chuck equations at us...
My own car which runs on petrol won't start without the clutch disengaged, but it has never occurred to me to declutch when turning it off. I also engage the clutch at an idle quite often - indeed to use traction control from rest you have to do that. But to tell the truth I don't know if it's even got a DMF. Must look it up.
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>>2 Italian engineers published a whitepaper on DMFs
Skoda, please could you find the link? (The result sounds rather dubious to me)
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>> it has never occurred to me to declutch when turning it off
Likewise. I always depress the clutch when starting the engine, and I generally reverse the car up the driveway gently enough for it to roll to a halt by itself - to me it feels better to park it with no strains or stresses in the suspension or drivetrain at rest. I shall have to check whether or not I still have the clutch pressed when I switch off.
I'm finding the Mondeo quite easy to stall. I suppose the clutch is a pretty heavy-duty item as it has a lot of torque to handle, and I have an inbuilt dislike of mismatching clutch-plate speeds by more than 1,000rpm, which is often not enough to pull away with. When it stalls it does so quite violently too, which can't be good for the transmission.
Last edited by: Dave_TDCi {P} on Sun 3 Apr 11 at 16:35
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...I'm finding the Mondeo quite easy to stall...
The 2.0TDCi is quite 'petrol-like' in an old fashioned four cylinder pushrod sort of way.
It needs more revs than a traditional diesel to pull away, although this is often disguised by the anti-stall software which does the revving job for you.
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>> The 2.0TDCi is quite 'petrol-like' in an old fashioned four cylinder pushrod sort of way.
>>
>> It needs more revs than a traditional diesel to pull away, although this is often
>> disguised by the anti-stall software which does the revving job for you.
I thought that this was one of the beauties of diesels, the ability to bring the clutch up at idle and the thing will just surge forward with no chance of stalling. Certainly the Seat Ibiza Tdi I test drove recently did just this and was very confidence inspiring, but then it was a PD diesel, noisy but superb grunt coupled with a light body. Anti-stall software for a diesel? God alive, who would have thought that necessary a few years ago.
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...I thought that this was one of the beauties of diesels...
I think Ford made the 2.0TDCi more petrol-like to appeal to a wider range of car buyers.
A first-time diesel driver would not feel it was such a big change.
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>> I think Ford made the 2.0TDCi more petrol-like to appeal to a wider range of
>> car buyers.
>>
>> A first-time diesel driver would not feel it was such a big change.
>>
Agreed.
I used to stall my 2.0 tdci for a while after moving from a VAG TDI. No problems moving from the 2.0 tdci to the Ceed 1.6 crdi.
Last edited by: Old Navy on Sun 3 Apr 11 at 19:38
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Not just fords, there's a tendency since the introduction of common rail for them all to go this way. Not sure I understand why a wider powerband = less impressive bottom end. The hardware doesn't change throughout the rev range yet the difference in torque is pretty big.
Shouldn't be the turbo causing it - they're all variable geometry so should behave like a small turbo at low revs and blow a reasonable volume of air quickly.
Is it the high pressure fuel pump? Seems unlikely.
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>> Not just fords, there's a tendency since the introduction of common rail for them all
>> to go this way. Not sure I understand why a wider powerband = less impressive
>> bottom end. The hardware doesn't change throughout the rev range yet the difference in torque
>> is pretty big.
>>
>> Shouldn't be the turbo causing it - they're all variable geometry so should behave like
>> a small turbo at low revs and blow a reasonable volume of air quickly.
>>
>> Is it the high pressure fuel pump? Seems unlikely.
>>
Its the electronics, they are given petrol characteristics.
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>> Its the electronics, they are given petrol characteristics.
But why don't the electronics give decent grunt at idle to avoid stalling, and a wide power band? Or can't both things be done? Or does it help to protect the DMF from high torque loads when moving off, and prolong it's life?
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>> >> Its the electronics, they are given petrol characteristics.
>>
>> But why don't the electronics give decent grunt at idle to avoid stalling, and a
>> wide power band? Or can't both things be done? Or does it help to protect
>> the DMF from high torque loads when moving off, and prolong it's life?
>>
>>
Decent grunt at idle is not a petrol trait, thats why it is reduced in some diesels so that drivers used to petrol cars feel at home.
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Its electronically killed in diesels because drivers claim "my car wouldn't stop" when they take their foot off.
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>> Its electronically killed in diesels because
Does that mean a tuning chip could be programmed to remove the effect? :)
Like ON, I never had a problem with a VAG diesel, but I've stalled this Ford one a dozen times or so in a month. The wallop when it stops is quite marked, if it's not harming the DMF it won't be doing the engine mountings or exhaust joints much good :( I'm learning my way around the quirk, but it just seems wrong to drive a diesel like a petrol.
Last edited by: Dave_TDCi {P} on Sun 3 Apr 11 at 20:37
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I've found many modern Diesels easy to stall, unlike previous generations, and getting the balance right between enough revs to keep it going and not stall without overdoing it and causing too much clutch slip can be tricky, especially if you drive different vehicles.
I think a lot of the problem is the too high first and reverse gears that many cars and indeed some 4x4's now have, where instead of them being get you moving gears they seem to be good for 30 or 40 mph, and when mated to a turboDiesel which dies below approx 1500rpm makes exacerbates the problem.
Thats where proper torque converter autos score so well in pulling away, the hydraulics allow the engine to gain it's full power without problems.
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I haven't tried to do 30mph+ in first gear in this one yet GB lol, I do agree the overall gearing seems very long-legged though. I guess at 1500rpm it's making 200Nm or more of torque, which would explain the thump when it stalls. A small price to pay for the economy and the grin-inducing mid-range performance on the road.
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>>I think a lot of the problem is the too high first and reverse gears
Use a lower ratio, and you need to strengthen the design of the powertrain downstream of the gearbox accordingly. You would need to size everything to cope with maximum engine torque multiplied up by the large reduction ratio.
As the engines today generally have a higher maximum torque figure, you can specify a higher gear ratio without sacrificing the ability to climb gradients - the vehicle will still probably be traction limited rather than tractive effort limited.
Phrased another way, high first and reverse gear ratios are a side effect of the excessive perfomance figures boasted by many modern engines, another symptom of how grossly oversized the engines are.
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>> another symptom of how grossly oversized the
>> engines are.
Boo hiss :-)
There was an interesting question on another forum about why are final drives in cars normally around the 4:1 mark, and it's because if they were 1:1 the gearbox would be physically bigger than the engine to accommodate the larger gears required to reduce the input speed and handle the extra tooth loadings. Also the inertia in the gears would eat the synchros in short order. In some rally cars theres a step up in rotational speed so step down in torque, before the gearbox to allow a lighter weight gearbox to be used. Well, it was interesting to me at least :-)
Still don't get why we can't have best of both worlds at low revs and high revs and let the driver decide how to use it.
Can't just be to make them more petrol like although I can just about see how that logic would get approved for manufacture.
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>> a side effect of the excessive perfomance figures...
>> another symptom of how grossly oversized the engines are
Spot on, I was going to describe my car's torque levels as being good enough for a lorry 20 years ago but I though that metaphor was stretching it a bit. It seems not!
Last edited by: Dave_TDCi {P} on Sun 3 Apr 11 at 21:47
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>> Spot on, I was going to describe my car's torque levels as being good enough
>> for a lorry 20 years ago
The problem with many is that there's such a sharp drop off once the revs go below torque that you end up with too much useable power and then nothing 200rpm below it, instead of a smooth linear rise from tickover, hence why clutches get slipped.
LR's Disco 3 with the manual box does exactly this, with resultant premature clutch wear.
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>> The problem with many is that there's such a sharp drop off once the revs
>> go below torque that you end up with too much useable power and then nothing
>> 200rpm below it, instead of a smooth linear rise from tickover, hence why clutches get
>> slipped.
>>
>> LR's Disco 3 with the manual box does exactly this, with resultant premature clutch wear.
So, according to NC, first gear is deliberately made higher in order to reduce the loading through the rest of the drivetrain - but with the result that the engines now have a tendency to stall. Certainly seems true with my next door neighbour and his Jag X type 2.0d. I always hear it revving fairly high to move off, as if he doesn't want it to stall.
First gear in my old Beemer was very low - you could creep along in a traffic jam at idle, and accelerate extremely rapidly, but it ran out of revs quickly, and unless you got into second gear smartish, the car behind would catch up rapidly. I assumed that the low first gear was easy on the clutch though. And the drivetrain on that car was pretty strong. I had the propshaft reconditioned (fruitlessly) in an attempt to cure a vibration - the guy at Recoprop told me that BMW use a different kind of joint that are stronger than the usual UJ's - typical germans.
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Phrased another way, high first and reverse gear ratios are a side effect of the excessive perfomance figures boasted by many modern engines, another symptom of how grossly oversized the engines are.
What size of reduction would like to see, what's pushing this oversizing, people wanting faster cars?
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>> high first and reverse gear ratios are a side effect of the excessive perfomance figures boasted by many modern engines, another symptom of how grossly oversized the engines are.
Reading between the lines N_C, could it be said that you are in favour of the sensible cars about which we all feel so ambivalent? So am I, with a large practical part of myself, although a nutter still lurks somewhere in the undergrowth and springs out occasionally.
For example I am greatly attracted by the idea of the Fiat 500 twin-air. Something like that somewhat scaled up would appeal even more. It's true that most people don't need fast cars and don't really know what to do with them, so the run-of-the-mill 140mph repmobile is just costing everyone a lot and doing no one much good.
I was hoping you would have some words of wisdom on what might be making some DMFs fail. No one really seems to know apart from gross clutch abuse which must be fairly rare these days despite the efforts of driving instructors to teach it.
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I have done my share of too-fast driving on public roads. But one of the nicest drives of all the cars I've had was an 895cc Mk1 Polo with about 45bhp. Wonderfully flexible. Make that about 70bhp now to handle the extra weight, and you should be able to make a decent - if not fast - small car.
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>> I have done my share of too-fast driving on public road
Me too. But it's just as much fun rowing a slow car with interesting handling along at a brisk lick. I always liked 2-cylinder Citroens and spent a lot of time haring economically about in Skodas.
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>>Reading between the lines N_C, could it be said that you are in favour of the sensible cars
Very much so, yes. I admire cars where the engineers had made do with very little and had designed something truly fit for purpose. To my mind, there's much more cleverness in the engineering design of, say, a 2CV than there is in many "supercars"
Mike Dunn, at whose feet I was lucky enough to sit when learning about engineering design, would say "An engineer is someone who can make for a shilling what any damn fool can make for a pound"
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A TD5 Landrover DefenderI had was stall city. Worked out why in the end, I was being nobbled by a useless ECU.
The thing was so slow to react to throttle that with it idling, I could stab the throttle to full and lift off before the engine did anything.
Add to that the fact that if it was in rundown mode, the fact you might have 1400 revs on board was irrelevant, pushing the throttle did not result in instant fuel. So if you aborted a take off, lifting off the throttle was liable to cause a stall.
Good engine - silly ECU.
The successor, a TDCi Landrover was fine, and there was no DMF to kill either.
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>> Good engine - silly ECU.
Haha :-) the MED 17.5 or whatever it is I forget the numbering, ecu in the vag 2.0 TSi detects you're going for full throttle and has the throttle plate full open 20ms before your foot hits the floor!
More useless information. It's a service honestly it is :-) sleep well tonight!
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>> Thats where proper torque converter autos score so well in pulling away, the hydraulics allow
>> the engine to gain it's full power without problems.
>>
As the driver of a diesel automatic, I am a somewhat detached observer, but I have found this thread a very interesting read.
Take off from rest is very smooth easy in my car (which has the old-fashioned type torque converter+planetary gearbox), albeit with a very slight initial hesitation. According to a technical bulletin which the dealer showed me, this is down to the fact that the gearbox always starts in second gear in "normal" mode. It only uses first gear in when in "sport" mode.
Maybe this is one of the reasons why the car was surprisingly capable in light snow conditions this past winter.
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>>There was an interesting question on another forum about why are final drives in cars normally around the 4:1 mark,
In the days of a conventional rear wheel drive layout, there was an efficiency advantage to be had by making top a direct drive - straight through. Then, setting the rear axle ratio was simply a matter of setting the maximum power engine speed to, more or less, tally with the vehicle's theoretical top speed (when drag power and power at the wheels are equal)
With a more modern powertrain, in a passenger car, the logic goes something like this
a) size the lowest overall ratio to allow the unlden car to climb a 1 in 2 hill at maximum torque engine speed
or
a i) size the lowest overall gear ratio to allow the unladen car to climb its traction limited maximum gradient at maximum torque engine speed
whichever gives the lowest reduction.
b) size next to top overall gear ratio to give something like optimum gearing, matching peak power engine speed with maximum theoretical vehicle speed
c) size top gear to give quiet, economical cruising at appropriate speeds for the vehicle and market
a) and c) together give an idea of the required ratio span of the gearbox. For a medium sized petrol engined car, a) gives something like 15:1, and c), 3:1
Once you have the span of gears required, and the difference between max torque and maximum engine speeds, you have enough to define the number of ratios required. The ratios of adjacent gears should not differ by more than the max engine [n.b. not maximum power engine speed!] / max torque speed ratio
As you move away from 1:1, gears become less efficient, and it's unusual to have a gear ratio of more than 5:1 in one reduction step.
This means that you do try to make next to top and top (where the car will spend most of its life) near 1:1, and this brings the final drive ratio down towards 3 or 4:1. If you did bring the final drive down to 2:1, and had top gear about 2:1, you would need 7.5:1, or so, for first - the tooth geometry of the pinion would be beginning to be weakened, and the efficiency of the gear pair would be suffering.
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If the point of a DMF is to iron out vibration from pulse impulses, then it ought to be built to do the job. If you have to treat it carefully why have one?
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>> It's certainly true that diesels start and stop with a violent shudder which is co-axial
>> with the crankshaft (although doubtless there are more complex forces as well). Modern diesels seem
>> less like this than they used to, perhaps because of the DMF.
My PD diesel has something called an "anti-shudder valve" which is a flap over the inlet manifold which closes for a second or two at key off. The idea is, by cutting off the air supply to the cylinders, there is nothing for the pistons to compress as the engine comes to a stop. This allows the engine to slow (relatively) more gently to a stop, instead of stopping hard against the very high compression (18.5:1) in a given cylinder.
It certainly makes a difference from a comfort perspective. My brother in law has a Golf with the same engine, and his anti shudder valve has been removed as it was playing up. His engine stops with much more of a "jolt" than mine does.
I got into the startup declutching habit in my youth driving old bangers that needed all the help they could get to fire up in the mornings. Decoupling a stone cold transmission of course, gives the starter and battery an easier time. I've stuck with it ever since.
My Golf's clutch (can be made to slip in top) and DMF (recent noticeable increases in both low rev vibes and general engine coarseness) are both tired at 119,000 miles. I'm getting them changed next month.
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>> My Golf's clutch (can be made to slip in top) and DMF (recent noticeable increases
>> in both low rev vibes and general engine coarseness) are both tired at 119,000 miles.
>> I'm getting them changed next month.
Where will you get the parts DP, and how much do you estimate it will cost?
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Somewhere around the £400 mark for a decent (Sachs) clutch and DMF kit, plus a new concentric slave cylinder/release bearing.
I will be entrusting the work to my VW tech mate who does serious mates rates, and reckons these are pretty painless as clutch changes go, so I reckon it will be somewhere around £600 all in. He also has a trade account with GSF, so can typically save a bit off the retail prices. This tends to be where he sources all the parts when he works on our cars.
Of course, this depends on finding no unpleasant surprises when it comes apart (crank or transmission oil seals, for example).
The car has cost me buttons in the 2 yrs I've had it, and the mates rates will save me a good few quid, so annoying as £600 is, it could be a lot worse. The clutch change on our Scenic cost a grand, and that was without the DMF needing attention!
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I had the same thoughts . .
Regularly done on current Octavia, no problems yet in 3 years.
It has only done 80,000 miles and is still on the original clutch, i don't expect top need to change it.
The prat who wrote in eeds to learn how to drive!
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Well I have had DMF's for years and managed 100,000 miles plus including towing caravans around Europe and I have never ever experinced a DMF failure with any Diesel I have owned, including the infamous Mondeo TDCi's and jaguar Xtype 2.0 & 2.2d
I dont do jack rabbit starts or snatch gear changes and I dont drive in too higher gear at too lower speed, this seems to work fine so common sense really !
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How to kill a DMF - HJ comments... - Lygonos
