Low Revs and Preventing DMF Failure

Ultrasonic

(I was trying to go through replies in order but just realised I missed a couple, oops!)

Johnmcl7 wrote: »

...the trip computer definitely did favour driving in high gears and low revs but I didn't find it very accurate. Measuring fuel used I found driving that way used more fuel compared to using a lower gear and sitting at slightly higher revs.

That's very interesting, thanks for sharing that. Do you happen to remember what sort of differences you found? What sort of driving style did you find works best? I probably ought to at least give what you suggest a try for one tank, but there are obviously variations between tanks anyway so it will potentially be a question of if the differences are large enough to be certain of. I may have to go to the trouble of making sure I repeatedly refuel at the same pump to reduce variability...

My trip computer is certainly very overoptimistic in its claimed mpg (by ~10 mpg!) by the way, but I've logged both the indicacted and actual (brim to brim) mpg over 26 refuels and there is a fairly linear relation between the two. That is, higher indicated mpg correlates with higher actual mpg. That said I have never forced myself to try higher rev driving for a whole tank to compare. FWIW I've averaged 56 mpg over the last 15 months (including winter), with my best tanks around 61 mpg.

Johnmcl7 wrote:

I don't think there is any solid evidence on DMF failures as carrying out a scientific study on them would be tricky given the miles involved. The belief with low revs is that even if it feels ok (no juddering), the DMF is still having to work harder to smooth out additional vibrations so it's doing more damage than higher revs.

That was exactly what I was wondering if may be the case. It certainly seemed logical to me. I could perhaps do with finding someone online who has had a SMF replacement in the same car, to see what sort of low revs the engine felt OK with without the damping effect of a DMF to hide engine vibration from the driver.

Johnmcl7 wrote:

Gunning the car hard with racing starts is believed to also do more damage.

Think I can restrain myself from doing that .

Ultrasonic

Johnmcl7 wrote: »

As I said above...

Yes, sorry I'd missed your earlier reply. I started writing my reply to you when you posted. I wasn't meaning to ignore your helpful info!

Ultrasonic

forgotmyname wrote: »

I posted about this a while back, Lower revs does not mean better fuel economy.

In my 6 speed diesel mondeo below 60mph in 6th gear gives worse consumption that changing down to 5th.

I repeated the test at 40mph in 4th and 5th gear. 4th gear gave better fuel consumption compared to 5th. I read the throttle position at the same time and 5th gear used 18% throttle travel, Where 4th only required 11% due to the extra torque available. This gave better fuel consumption.

While I'm prepared to believe your 6th gear versus 5th gear trip computer data, as I tried to explain to you in this post I'm afraid your throttle position data really isn't of any use in comparing fuel economy. What you need to know is the rate of fuel consumption at a given speed.

=forgotmyname wrote:

Below 1500rpm is not good for the DMF.

That is exactly the sort of info. I'm after, thanks. Do you have a source for this?

=forgotmyname wrote:

Depending on the car and the power/torque curve. 2000rpm is probably close to the best consumptio.

I really do still struggle to believe this is true, as explained in my reply to Stridor590 above. (Which I stupidly replied to before your post, sorry.)

Ultrasonic

Notmyrealname - I'll reply to the specific points in your other post in a minute, but first I should probably clarify something that I think is causing confusion. My purpose in starting this thread was not to discuss driving at 30 mph in 5th gear, so I didn't take the time to explain when I sometimes do this. I absolutely do not do this every time I am in a 30 mph limited area, most of the time it would not be appropriate, especially in busy city centre traffic. I only do it on flat roads (as I did state) when the traffic is light and I do not anticipate any need to slow down from 30 mph. I mostly do it on a couple of quiet stretches of road on my daily commute that I know well.

For the purposes of my opening post I could just as easily talked about doing 40 mph in 5th gear, or even 50 mph. The point being if the DMF is potentially masking engine distress, ho do you tell when a DMF may be overworked, potentially leading to early failure?

Notmyrealname wrote: »

So you're not in full control of the vehicle.

This was written in response to me saying that I would depress the clutch if I needed to slow down, as well as applying the brake. What I meant was that I would depress the clutch, change down a gear or two, and then release the clutch again. I am no more 'not in control' of my car than any other driver when then change gear. Also, if I'm in busy traffic where my speed is likely to be fuctuating I wouldn't be in 5th gear, it would not make any sense.

=forgotmyname wrote:

5th is only beneficial at 30MPH if you do a constant speed on a flat gradient.

I completely agree - that is the only situation in which I do it.

=forgotmyname wrote:

As soon as you get a rise in gradient or have to vary your speed AS YOU DO IN THE REAL WORLD, it is worse.

I agree it limits it's use, but it does not mean the situation never occurs.

=forgotmyname wrote:

Please look at the URL. It is THE AMERICAN VW website. US roads, especially urban ones, are a lot different to our own. They are a lot flatter, straighter and emptier on the whole.

If you click on the 'choose a country' label you'll see that the same information is provided for the whole world. Here is the Austrian version if your German is up to it; a country with rather different roads to America:

http://thinkblue.volkswagen.com/de_AT/fuel_saving_tips

For what it's worth the sentence that I was most interested in (in the English version) was:

As long as your car runs smoothly and the engine noise is not too loud, the engine speed is not too low.

That is what I'd taken to be the case but it was appreciating that the DMF may be filtering out some signs of engine distress from reaching the driver that made me reconsider if it may be completely true. I should perhaps e-mail VW for their thoughts on my original question.

Notmyrealname

OK, so now we've cleared that up

Forgive me for thinking the worst - doing so has saved my backside several times so I do it as a matter of course.

Back to the original point..

A DMF is basically a two flywheels held together by a flexible compound.

Accelerating in a high gear from a low speed puts additional stresses on that compound. If you think back to when you rode a bike, how much additional effort was needed to set off from a slow speed in the highest gear compared to the lowest one? A lot more. So you were needlessly putting more energy into the drive train of the bike to accelerate. Likewise you're needlessly putting more energy through the compound to accelerate than you would from a lower gear.

Ultrasonic

Notmyrealname wrote: »

Accelerating in a high gear from a low speed puts additional stresses on that compound...

I understand the concerns over accelerating at low revs in high gears. FWIW I've read some websites focusing very much on fuel economy where it is advised to accelerate hard at low revs in high gears to optimise mpg (the logic being that there is usually lower BSFC at high load and low revs than lower load at higher revs). I have ruled out using this approach due to concerns of potentially increasing the risk of a DMF failure, or causing some other problem.

It was the low rev. constant speed situation that I realised was less clear cut, hence this thread.

GolfBravo

Notmyrealname wrote: »

A DMF is basically a two flywheels held together by a flexible compound.

Accelerating in a high gear from a low speed puts additional stresses on that compound. If you think back to when you rode a bike, how much additional effort was needed to set off from a slow speed in the highest gear compared to the lowest one? A lot more. So you were needlessly putting more energy into the drive train of the bike to accelerate. Likewise you're needlessly putting more energy through the compound to accelerate than you would from a lower gear.

Yes, yes, yes, yes and yes.

What DMF is really good at is giving the false impression that your car can move off on tickover or smoothly cruise at 1000rpm. In reality the DMF is working it's nuts off to give you a smooth ride, and driving and accelerating at really low revs put unnecessary stress and heat into the DMF.

Also it is worth mentioning that one of DMF's characteristics is giving the impression of a clutch slip - when DMF gets really hot and you start accelerating from very low revs you can momentarily feel a "clutch slip" - it is in fact the DMF struggling with the high temperature and torque.

Manufacturers fit DMF mainly to protect crappy gearboxes - front wheel drive cars require very compact gearboxes, and unfortunately they don't cope very well with lots of torque. In rear wheel drive cars you have much more space for beefier gearboxes, like the Tremec six speed gearbox designed to cope with 810Nm of torque (with no DMF), for example.

You really shouldn't drive at such low revs - it slowly kills the DMF, clutch and gearbox.

patman99

If you want to eliminate all your worries, why not have the EGR, DMF & DPF removed and the ECU remapped ?.

To have the DPF removed and the ECU remapped costs £400 for an Octavia (unless you know someone with the same engine who has already had this done, you can then use 'EcuFlash' and a laptop to copy the modified firmware onto your ECU). You would get about a 10 - 15% increase in your MPG.

There is a good market for the fitment of SMFs. I have yet to hear of any horror stories about cars suffering massive damage due to replacing a DMF with a SMF, but I have heard of a lot of people who have had £1,000+ bills when the DMF fails.

At the end of the day, it is down to the way the car is driven as to how long a part will last.

Btw, I wanted to know how efficient my car is so invested in an App for my iPad. It allows me to keep track of fuel costs and repairs as well. So far, my mpg is averaging 57.6mpg and is slowly creeping-up.

GolfBravo

patman99 wrote: »

Btw, I wanted to know how efficient my car is so invested in an App for my iPad. It allows me to keep track of fuel costs and repairs as well. So far, my mpg is averaging 57.6mpg and is slowly creeping-up.

Sounds interesting - what app did you get?

Ultrasonic

GolfBravo wrote: »

You really shouldn't drive at such low revs - it slowly kills the DMF, clutch and gearbox.

Thanks very much for your comments. What sort of rev range would be sensible in your view?