Okay, so I've been reading on here, quite smugly, for years about diesel owners fannying about with changing the dmf to a single, but I've just read a thread where someone said it's not necessarily all deisel cars, some petrol have them too. Is that correct and does my 1.8 bzb engine have one? If so, why is it mainly deisel owners seem to have the issue, is it because of the torque values of their cars or summat?

Yes, the BZB does have a DMF.

 

DMF's are a hotly debated component in the world of cars, some recommend removing them, as they "are only there to make money", others praise them for what they do.

 

Diesel and petrol have (obviously) different combustion characteristics, couple these with the variances in torsional vibration, drivetrain advances, increased injection pressures (Tdi roughly 200 bar - Common rail about 3000bar+) and you alter these vibrations.

 

Now, there are numerous reasons why DMF's fail - most common;

 

Injector imbalance - anything over 10% difference between cylinders will shorten the lifespan

Poor starting - the greatest movement a DMF experiences is during start/stop situations (think how "roughly" the engine moves during cranking)

Compression issues

EGR/Intake systems - air shut off valves, faulty swirl flaps, EGR system malfunctions, intake flaps

Driver abuse - labouring the engine, slipping the clutch, over loading (trailer or vehicle weights)

 

Petrol engines have them because of similar reasons. Higher power outputs (think back to the late 90's - a high powered family hatchback would have about 140bhp max - now we are talking 200bhp + being "normal") due to advances in both fuel systems and turbo/supercharging, downsizing leading to imbalanced engines, and improvements in transmissions requiring better balancing of these torsional vibrations.

 

The DMF has also been proven to improve economy by up to 15% over a non DMF car, so it's not just for NVH reasons that we have them, it's also for reducing emissions and improving fuel economy.

 

Have a read of the brochure - that may help explain things a bit more

 

dmf brochure.pdf

Thanks. The failure/problem rate does seem more pronounced on diesel compared to petrol or am I imagining things??

Thanks. The failure/problem rate does seem more pronounced on diesel compared to petrol or am I imagining things??

 

Diesels do suffer more, due to the combustion characteristics, and the much higher compression/fuel pressures involved. You only have to go back 20 years and diesels with over 110bhp were pretty much unheard of - now they are upwards of 400bhp in some cases

We're going to see a lot more DMFs in the future, on petrol as well as diesel cars.  My guess is that it has a lot to do with cost-effective car production.

 

Increasing power outputs and the demand for fuel efficiency, partly through lighter components, means that ever lighter gear trains are being coupled to ever more powerful motors. Provided it all lasts for 5 years or so, then the manufacturers and fleet buyers are happy.

 

No doubt the manufacturer's accountants have done their sums too, so that DMF+light gearbox costs less to produce than ordinary clutch+robust gearbox.

 

If it makes you feel better about it, think of the DMF as a sort of 'fuse': it's the cheap(er) part of the transmission designed to fail before, and instead, of the gearbox.

We're going to see a lot more DMFs in the future, on petrol as well as diesel cars.  My guess is that it has a lot to do with cost-effective car production.

 

Increasing power outputs and the demand for fuel efficiency, partly through lighter components, means that ever lighter gear trains are being coupled to ever more powerful motors. Provided it all lasts for 5 years or so, then the manufacturers and fleet buyers are happy.

 

No doubt the manufacturer's accountants have done their sums too, so that DMF+light gearbox costs less to produce than ordinary clutch+robust gearbox.

 

If it makes you feel better about it, think of the DMF as a sort of 'fuse': it's the cheap(er) part of the transmission designed to fail before, and instead, of the gearbox.

 

The clutch is the "fuse", that will fail before a DMF (unless there are underlying engine issues of course).

 

Some smaller engines are going back to solid flywheels now too - take the 1.6 diesels CAY# codes - from the end of 2010 ish they went back to solid, mainly due to the extra strain of the start stop systems

Edited 25 November, 20169 yr by octyal

Reversion to solid flywheels on some models is the exception rather than the rule, and is a reliability/cost trade-off.  In a few cases like the low power 1.9 PD it's justified, but the DMF does protect the gearbox.

 

A clutch in good condition is more robust than a DMF to power pulses, though both have springs that can fail if overloaded, or through age. Slip kills clutches, and engine labouring kills DMFs.  The DMF and clutch are often designed as a single unit and will be replaced together, regardless of which bit has failed.  In these combined units, it's more often than not that the DMF fails before the integral clutch.

 

From Wikipedia: 

https://en.wikipedia.org/wiki/Dual_mass_flywheel

 

"Because of expensive unreliability issues with Dual Mass Flywheels^[3] there has been a trend to fit solid flywheels from lower power models of the same engine.^[4] Some mechanics have even welded the dual mass elements together.^[5] These 'fixes' have resulted in instances of gearbox damage and damage to engine crankshafts, as well as refinement and drive-ability issues."

DMF's are not unreliable on the whole (apart from the Valeo one Peugeot used on the early 110 bhp 307 hdi of course) - and will quite easily exceed 200k miles in a car that is well looked after and driven sensibly - see my initial post at the top of the page for DMF killers.

 

It's the manufacturers that are reverting back to SMF in some cases. Cars that are designed to run with a DMF should retain that DMF as it is there for a reason, we have a snapped crank out of an Octavia in the training room to highlight the problems of conversions