Twincharger 7DSG - Selects higher gear than it should. Thoughts?

[VeedubTSI]

This thread is a full 1 year old.

*Facepalm*

[prise]

It is true that an engine under load at lower revs will normally feel and sound rougher than one at higher revs and less load but this perceived roughness at lower revs is as much a function of the tuning of the engine mounts as it is the engine itself. I have worked as a vibration engineer in the past, hence my interest in this topic.

Engine mounts are carefully tuned by the manufacturer such that the engine on the mounts has a natural frequency of vibration that is below the idle speed of the engine. As the engine speed increases, so does the frquency of the mechanical vibration of the engine. The higher the frequency of the engine vibration (higher RPM) the smaller % is transmitted through the mounts into the car body so higher revs often feel smoother as less vibration is making its way into the car body. This is the vibration we feel, not the vibration of the engine itself.

Another factor making the the engine feel less smooth at lower revs is something called torque ripple. This is the variation in output torque of the engine that results from the cylinder firings. Engine manufacturers fit special harmonic balancers, but at lower RPM and higher engine loads these become less effective in smoothing out the torque ripple which also increases the amount of transmitted vibration.

My theory is that this is often why people perceive lower revs as somehow being worse for the engine. I have a 118TSI with DSG and do agree that it doesn't sound and feel as smooth when its lugging at 1500 RPM compared to 2000 RPM in a lower gear, however provided the engine isn't pinging (which it's not) I have no concern for its longevity. 80% torque at 1500 RPM is not hard on the engine as thermal loads are low and loads from cylinder pressures are modest compared to loads from reciprocating parts it would experience at higher RPM's. I reckon the fact it rarely needs to get above 2000 RPM driving around town should extend engine life.

[idaho]

To me it's obvious, nobody likes a smartie who can master the #$@%@! search function on this forum.

I only used the New Posts function so I feel safe to continue.

It's all about fuel efficiency, chasing the poofteenth of a litre per 100km savings that consumers seem to love. Remember VW detuned the TSI from the MkV GT (120 something kw?) back down to 118kw. No doubt they've programmed the shift points as efficiently as possible.

I think the "detune" is more of a market positioning move as both engines have the same torque over the same RPM range. The only difference is that the 125kw Mk V GT Sport has the power specified 100 rpm higher (6000rpm) than the 118kw Mk VI (5900rpm).

Very Clever ...but seriously:
There is no Problem to solve in the first place. People are trying to make it one and I would like to clear things up, so a myth does not perpetuate.

So far no one has come up with a good reason not to let this powerplant and transmission operate within the manufacturers design envelope.

As various people say the shifts have obviously been calibrated to maximise economy. But you need to think about how an engine works to decide if you want it to operate that way.

Torque is directly related to cylinder pressure. The higher the cylinder pressure the higher the torque that is produced for that power stroke. The TSI engine uses the supercharger to increase the cylinder pressure at low speeds that a 2.5L naturally aspirated engine would not be able to achieve. But the price of the higher cylinder pressure is more side thrust on the cylinder walls (because the conrod is angled during the power stroke and more pressure is being applied over a smaller bore area) and more blowby of combustion gases into the crankcase.

Also, 4 cylinder 4 stroke engines have big torque peaks and troughs during an engine cycle as there is no overlap of powerstrokes from one cylinder to another. The individual strokes cause more vibration at lower speeds (because the rotating mass has lower moments of inertia at lower speeds) which gets transmitted to the dual mass flywheel (also in the DSG). So, while I generally drive our car a gear higher at low engine speeds than my wife does, I don't accelerate hard (full torque) at those same speeds.

In the end it probably is a moot point as the original owner is probably unlikely to keep the vehicle long enough to experience any change in longevity. So it comes to personal preference.

[idaho]

... 80% torque at 1500 RPM is not hard on the engine as thermal loads are low and loads from cylinder pressures are modest compared to loads from reciprocating parts it would experience at higher RPM's...

But aren't the high rpm loads hard on bearings while the cylinder pressure loads are hard on pistons / bores and essentially independent of rpm?

[prise]

Torque is directly related to cylinder pressure. The higher the cylinder pressure the higher the torque that is produced for that power stroke. The TSI engine uses the supercharger to increase the cylinder pressure at low speeds that a 2.5L naturally aspirated engine would not be able to achieve. But the price of the higher cylinder pressure is more side thrust on the cylinder walls (because the conrod is angled during the power stroke and more pressure is being applied over a smaller bore area) and more blowby of combustion gases into the crankcase.

All this is true but you don/t mention that this is happening over fewer piston strokes. Also the mass of the piston and connecting rod being accelerated up and down creates loads that increase with RPM. Golf diesel engines have a good reputation for longevity and they have been lugging at low revs and high cylinder pressures for years. It comes down to what the engines been designed to do.

[prise]

But aren't the high rpm loads hard on bearings while the cylinder pressure loads are hard on pistons / bores and essentially independent of rpm?

As a general rule - yes. However diesel engines have proven that a properly designed bore, piston and ring arrangement can handle high gas pressures and still deliver long engine life so there is no reason to say why one type of wear would be worse than the other for our engines.

[Corey_R]

I was thinking of replying as this was the first time I had seen the thread, but perhaps not. People are often berated for starting new threads without searching for and adding to existing threads, but it seems that searching and continuing on is also frowned upon. If a new group of posters wish to discuss this subject isn't it reasonable to continue this thread rather than start a new one?

To me it's obvious, nobody likes a smartie who can master the #$@%@! search function on this forum.

Sorry, maybe my post was a bit full on...
My main point was, that VeedubTSI was not coming in to ask a question, but to "answer the thread" when the OP is already very knowledgable on the topic and created it to answer other peoples questions in the first place.

Feel free to continue it though if there is new stuff to add...

[VeedubTSI]

Sorry, maybe my post was a bit full on...
My main point was, that VeedubTSI was not coming in to ask a question, but to "answer the thread" when the OP is already very knowledgable on the topic and created it to answer other peoples questions in the first place.

Feel free to continue it though if there is new stuff to add...

*sigh* I felt I was getting moderator bashed after your initial statement. At least I resurrected some topic of some significance as I've seen in the last few hours. I don't know where the lines blur between resurrecting a topic or starting a new one and getting it merged or bashed. All I know is I'm watching the posting dates more carefully next time.

[Corey_R]

If you are asking a question on the same topic as an existing thread, then you should always use the existing thread (no matter how old), otherwise it'll probably be merged (especially if I remember the existing thread).

I was just not sure if you realised the thread was a year old...
Anyway - as I said last night, my first post was a bit full on, so I apologise.

[VeedubTSI]

All good.

[kaanage]

All this is true but you don/t mention that this is happening over fewer piston strokes. Also the mass of the piston and connecting rod being accelerated up and down creates loads that increase with RPM. Golf diesel engines have a good reputation for longevity and they have been lugging at low revs and high cylinder pressures for years. It comes down to what the engines been designed to do.

Another post to put in the "thread revival" category...

Diesels are fabulous for lugging at low rpm due to the high cylinder pressure peaks, lack of throttling and the low bore/stroke ratio. All are inherent in the diesel cycle and also require the engine components to be far stronger than for a petrol engine of similar power output. This makes diesel engines pretty much immune to damage from side thrust loads on the pistons and bores.

None of this is relevant when discussing the suitability of the TSI engine for lugging at very low rpms with large throttle openings. Idaho's point about side thrust is a very valid one given that the TSI engine is square bored, but without knowing other details of the engine internals (the con rod length, piston skirt length etc), we cannot know if sustained running at 1500 rpm with the throttle wide open will lead to premature failures.

It is true that many small Japanese designed cars do suffer from premature engine wear when driven by young women due to excessive throttle openings at low rpm but the engines in these cases tend to be quite oversquare (and hence don't suffer unduly from high piston speed when driven for sustained periods at high rpm - crankshafts are another matter) as the Japanese like to chase high power outputs for the spec sheets. European cars are less prone to this as they tend to be designed more for real world driving so they are less highly tuned (in the non-performance versions anyway) and designed more for torque at low rpm than power at high rpm.

[prise]

Happy to continue the revival.

we cannot know if sustained running at 1500 rpm with the throttle wide open will lead to premature failures.

I don't think anyone was talking about sustained running at 1500 RPM and WOT (at least I hope not for the next owners sake). The thread was about whether the DSG7 shifted up too early and I know that my DSG7 will shift down a gear well before then, however it will hold low revs under moderate throttle openings. Your points about the influence of con-rod length, etc are all good ones - it comes down to what the engines been designed to do.

Out of interest - can anyone tell us what the conrod length on the 1.4 engines is?

[logger]

..we cannot know if sustained running at 1500 rpm with the throttle wide open will lead to premature failures.

Ha.. That was the only line in the post that got my attention too. Sustained running at 1500 rpm with the throttle wide open is not readily achievable with the DSG7. With WOT it will downshift or the rpm will increase in the same gear. Either way the rpm will increase.

[kaanage]

Please feel free to be disinterested, logger.

Happy to continue the revival.

I don't think anyone was talking about sustained running at 1500 RPM and WOT (at least I hope not for the next owners sake). The thread was about whether the DSG7 shifted up too early and I know that my DSG7 will shift down a gear well before then, however it will hold low revs under moderate throttle openings.

OK, "sustained" was a poor choice of words. "frequent" may have been better to convey what I meant, which is regular high loading of the engine at ~1500rpm.

The whole thread revolves around the noticably greater vibration of the engine when running with large throttle openings and when the DSG decides to shift up early. And while it may not be WOT, the throttle must be fairly open for the engine to load up enough for the engine mounts to be stressed in this manner. It may well be that the engine mounts are the items that are actually the most highly loaded rather than the engine itself.

As others have also posted, I've found manually shifting later in these circumstances not only reduces the vibration but also the fuel consumption and increases acceleration so the DSG programming is not optimal in these situations. This makes the notion that "I do not believe the designers would allow them to so readily operate in a regime where it could damage itself" a bit of a leap of faith, in my book. Designers are people and in this world of mass production and cost benefit analysis, things can get missed - otherwise, there would be no such things as recalls.

I'm not against shifting very early when the situation justifies it. With very light throttle openings (eg when accelerating only enough to keep up with traffic going down slight inclines), I actually find that the DSG won't shift early enough.

[MariusGT]

am fairly sure (without digging around for the article 'cause it is SUNDAY!) i read about the simulated testing done on the 1.4TSI engine.

VW had engines tested to over 300,000 kms without too much drama. in fact i'm sure you'll find many a 1.4TSI in europe with kms close to this. whatever the case i understand the push for engineering to focus on fuel economy for marketing, and VW have achieved this while still throwing down some decent performance figures. congrats to them for doing the R&D and being the pioneers in bringing this engine technology to the masses. (has been used in trucks i believe, and some old school rally car--> name??).

--- Dean Slavnich, editor of Engine Technology International and co-chairman of the International Engine of the Year Awards, said: “The international judging panel is hugely impressed by this VW engine. It’s a masterstroke of downsizing technology and a real engineering showcase. I have no doubt that this engine will become the template for a whole new generation of high efficiency, small capacity engines in the years to come.” ---

lastly. isn't the 1.4TSI made from a cast iron block? ie: strong little bugger!?