Diesel gear change with DMF in mind - OldSkoOL
When do you change gear in your diesel.

A big forum post i found say most shift around 2 - 2.2k.

I take mine up to 3 - 3.5k but sometimes feel the need to do an economy run. Mine has shift lights that do actually come on at 2k. Usually takes me down to 1.5k for the next gear.

This got me thinking, people drive like this all the time, low revs and after reading about DMF failure and someone suggesting lugging from low down puts big stress on the DMF, does it?

Is 1.5k really too low to drive like that all the time? The DMF thing bothers me a little as mine is trying to dampen 400nm so i dont want to lug it under stress if i'm on an economy run.


To confuse matters even further my car just doesn't labour, even at 1k RPM. I've often forgot to change gear whilst slowing down. I've been in sixth gear with the down shift light lit up and 1k on the rev counter and the car doesn't judder or splutter, i get no feedback so i don't know where my labouring point is. What really is too low. I'm no diesel expert :)
Diesel gear change with DMF in mind - DP
Our old 1.9dCi 120 bhp Scenic engine was a much revvier engine than the 1.9 PD 130 in the Golf. I would often extend the Scenic up to around 4k, partly because the power kept building up to (and slightly past) this point, and partly because it just didn't sound or feel unhappy doing so. In fact, it got much more upset being asked to pull from low revs, where it would shudder and almost "throw a strop" if you asked it to pull within 500 RPM of idle speed in anything other than 1st.

The VW PD is much more of a lugger. Will tolerate idle in any gear, starts to wake up by 1500 RPM and that lovely muscular surge of torque seems to fall off sharply past 3500. The punch in the guts this engine gives you with 2500 RPM upchanges is grin inducing and addictive. To do the same thing in the Renault would not show the engine in anything like its best light.

So, Golf - 2k-2.5k
Scenic - 3k-4k

Two very different ways of extracting similar power outputs from near identical capacity diesel engines. One smooth, linear and revvy, the other gruffer and gutsier. Both great engines in their own way.

The worst thing I am aware of for the DMF is clutch slip, as they are not as tolerant of heat as a solid flywheel. I don't know whether low revs stress them unduly. I would imagine many more PD's would have DMF failure if it did, as that's how these engines tend to be driven.


Edited by DP on 07/05/2009 at 00:06

Diesel gear change with DMF in mind - 659FBE
As a PD engine owner I would largely agree with the assessment above. I wish VAG had set the no-fuel low speed safety trip a little lower though; it's a bit too easy to stall it when starting in second gear under circumstances which are just a little marginal.

On the subject of DMFs - which I keep an eye on as I tow a lot with the PD - I have identified one possible cause of failure which is not documented as far as I know. Due to marginal disk handbrake efficiency, many people leave vehicles parked in gear when on a slope.

In a vehicle with a DMF, the locked-in torque caused by this practice can cause deformation of the elastomeric component of the flywheel and possibly cause a permanent "set" in the centering springs, especially over prolonged periods. I'll be watching for this carefully, but I wonder if this is a mode of "use" that Sachs, LuK and others have not considered. The in-service failure rate of DMFs is such that their weaknesses should surely have been identified and addressed during development.

Does anyone have any observations to add?

659.

Edited by 659FBE on 07/05/2009 at 00:23

Diesel gear change with DMF in mind - David Horn
The lugginess of the PD engine is why I love it... suits my lazy driving style because I know it'll go happily from 1000rpm in 4th gear, which makes for incredibly relaxed driving.
Diesel gear change with DMF in mind - ifithelps
Diesel - like petrol - engines have different characteristics which dictate when to change.

My Focus 1.8 TDCi was more of a lugger and needed earlier changes.

My Focus CC3 2.0 TDCi revs more and needs later changes.
Diesel gear change with DMF in mind - Andy P
This is one of the reasons why my next car will most likely be a diesel auto - no DMF to worry about.
Diesel gear change with DMF in mind - moonshine {P}

I have a 2.0 TDCi with the 6 speed box. I would of thought that the rpms at which you change are not as important as how smoothly you change. However, I do find it quite hard to get a perfectly smooth change at higher rpms.

Diesel gear change with DMF in mind - daveyjp
As a comparison the DSG box up changes at about 2,300 when driving sedately, it down changes at about 1,500.

The only exception to this is 1st to 2nd where it will change at tickover if you allow the car to start moving slightly - e.g. creeping in a queue of traffic.
Diesel gear change with DMF in mind - craig-pd130

Interesting point that, 659. My old PD130 was parked for the 5+ years I owned it pointing uphill on a sloping driveway (about a 1 in 6) so I always left it handbrake on and in 3rd gear as a belt-and-braces measure.

No clutch / DMF problems at all. However the handbrake was always applied and the engine off before putting it in gear, so any loading of the DMF would be minimised.

For what it's worth, the experiences I've seen from the VAG owners forums, especially with remapped PD motors mounted transversely (Golf / Ibiza / Octavia / Fabia), is that the standard clutch will give way and start to slip first with the extra torque.

An uprated clutch plate usually addresses this. Not many instances of DMF failure reported on those forums.
Diesel gear change with DMF in mind - OldSkoOL
As no one else has raised any concerns about low revs and DMF failure i'm a bit happier with it now.

As i said, with 400nm of torque mine will pull with no problems from 1.2k and will go well from 1.5k and at 2k its fairly impossible to keep it smooth as the extra torque just jerks the car forward.

So now when i'm doing my economy runs i'm happy to waft along at 1.5k rpm.

Edited by OldSkoOL on 07/05/2009 at 21:01

 

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