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Revs limited to 2500 in park/neutral

Discussion in 'Engine, Drivetrain, Fuel and Exhaust' started by tyler durdz, May 27, 2017.

  1. tyler durdz

    tyler durdz Member

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    Hi guys.
    Do mercedes implement a rev limiter system when the car is in park/neutral?

    I tried to rev my car today (a200 diesel auto 2014) when in park and neutral and it would stop at 2500 each time
    Is this a safety feature? Or is my car in limp mode‽
    It revs normally when driving..

    Thanks guys
     
  2. John Laidlaw

    John Laidlaw Senior Member

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    Revs are limited in neutral/park as indeed a safety feature and engine protection
     
  3. Headhurts

    Headhurts Senior Member

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    Yup my E350 is limited to 3000 rpm stationary.


    Sent from my iPad using Tapatalk
     
  4. OP
    tyler durdz

    tyler durdz Member

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    thanks guys! I started to panic a bit but now I'm calm again :)
     
  5. umblecumbuz

    umblecumbuz Senior Member

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    Open minded about that conclusion. It does not apply to every model.

    Mine revs without restriction when stationary, and the Mot tester (or any garage working to fix a fault) also needs to be able to use higher revs than 2500 when necessary - without putting the car into gear first.

    I'm sure someone will come along with other info!

    Umble.
     
  6. OP
    tyler durdz

    tyler durdz Member

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    hmm. So if it is a "safety" feature, what is unsafe about revving your engine whilst stationary?
     
  7. John Laidlaw

    John Laidlaw Senior Member

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    Mostly engine protection, but I also suppose the left foot braking brigade could make a mess with a bit of imagination and 5000rpm!
    My SL was limited to 3000 in neutral/Park, to be fair I havent tested the 63, I prefer to drive it rather than sit and rev it- might try it quietly in a country place over the next few days and find out. I suspect the neighbours wouldnt appreciate me doing it here!
     
  8. John Laidlaw

    John Laidlaw Senior Member

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    Oh and the workshop can overcome the rev limiter on Star...
     
  9. LostKiwi

    LostKiwi Senior Member

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    It's very bad for an engine to rev under no load. Under load the pistons and rods have a directional force applied which isn't there under no load. The pistons can 'wobble' up and down the bore under no load which can stress skirts and increase ring land wear. You are far more likely to ruin an engine under no load than under load.
    Why would you want to rev it with no load anyway (John don't answer - you have a good reason). :)
     
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  10. OP
    tyler durdz

    tyler durdz Member

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    I was driving home and heard a rattle when i accelerated (i was definitely being paranoid) so thought id see if i could reproduce the sound, and then realised i couldn't rev past 2500
     
  11. Steve@Avantgarde

    Steve@Avantgarde Forum Supporter Authorised Forum Supporter

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    None of that is accurate.

    Whether an engine has torque load or not, no pistons should wobble, in fact, from an engineering perspective, its impossible as the pistons/rings are interference fit. Any loose tolerance would either mean the engine would shake itself to bits, or it would leak vital gases and fluids.

    Engines are set on secure rubber engine mounts to dissipate torque load as well, or the engine would spin at its gearbox.

    You are not likely to ruin an engine on idle, you are far more likely to blow one up putting it under pulling load, like using one to power a car, truck or boat.

    The reason MB's put their limiter there are because 1) gearbox protection for auto gearboxes. You can boil the gearbox oil if revved at high RPM for a period of time 2) Mapped engine electronics, HFM and DDI mapped engine electronics work to their optimum when the vehicle is in motion, its values are based on such things, at idle it adjust fuel quantities, air mass and temperature etc. If the car is at idle it can't run its optimum setting, so selects an idle default (actual values on star will have an idle value, but also value when driving function).

    Essentially, it was this kind of software that allowed Bosch and VW to implement their emissions fooling trigger.

    As an aside engines, loved to be revved, and at times the higher the better, you don't hear an engine sing at 1500 RPM!
     
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  12. John Laidlaw

    John Laidlaw Senior Member

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    Thank you Steve, that's really good information!
     
  13. umblecumbuz

    umblecumbuz Senior Member

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    Now, that one sentence begins to make it all much clearer to a layman like me. The rest falls into place.
    Thank you.

    Umble
     
  14. LostKiwi

    LostKiwi Senior Member

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    Sorry Steve I disagree.
    Pistons are NOT an interference fit. Typical skirt clearances on a piston are 1 to 2 thousandths of an inch per inch of bore.
    Next time you remove a cylinder head try moving the piston laterally across the bore when at TDC. There is movement of the piston in the bore and this is totally normal. When it becomes excessive its called piston slap and is audible when the engine is running.
    http://www.bing.com/videos/search?q...4741E0A15A341BE47C0C4741E0A15A341&FORM=VRDGAR

    If your piston was interference fit in the bore it will seize as soon as it warms up.

    Pistons are also wider at the skirt than at the rings and often taper inwards towards the top ring. This to prevent piton to bore contact during piston rock (this rocking motion is also sometimes referred to as secondary piston motion).
    There is an excellent article about this here:
    http://www.musclecardiy.com/performance/how-to-build-racing-engines-piston-technology-guide/

    Some engines have very short piston skirts and therefore have increased piston rock resulting in high ring and piston wear rates.
    Hillman Imp engines (developed from the Coventry Climax race engines) were well known for this.

    Most race engines that used to blow up would do so as a result of a missed gear allowing a high RPM no load situation which even with rev limiters would cause piston/rod failure due to uncontrolled secondary motion of the piston.

    Engines 'singing' at high rpm is due to cam design, intake design and exhaust deign and has nothing to do with it being 'good for them' to be revved. The only engine I know of which really does benefit from being revved hard has no pistons at all - the rotary (but that's for a very specific reason unique to them).

    Autobox fluid will Alonso be highly unlikely to boil in neutral as it is doing no work. The fastest way to boil fluid is to load the gearbox under conditions such as towing (where high rpm is NOT a factor) or when doing prolonged torque converter stall tests (where RPM don't usually exceed 2000 RPM). In neutral the oil is merely being pumped round the box and coming under little thermal stress.

    Mapped fuel systems run closed loop (I.e. under lambda control ) at normal idle (when the engine is warm) and hence are able to adjust fuel and timing to suit and store variation to base values as trim values.
    They operate in closed loop when cruising as well.
    The only time they do not operate closed loop (and hence hold trim values) is when cold or at wide open throttle. In these situations they operate solely from preprogammed values stored in the map.
     
    Last edited: May 31, 2017
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  15. Steve@Avantgarde

    Steve@Avantgarde Forum Supporter Authorised Forum Supporter

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    When I say interference fit I mean that they are designed to fit snugly into the cylinder bore and are designed not to lose exhaust gases through the bottom, compression anywhere and allow fluids to enter the combustion chamber. It has to be a snug fit from the rings and the piston itself to the bore.

    Piston slap as you have mentioned though is not caused by revving the engine when idle, its usually down to wear and tear of the engine under normal load conditions, such as worn gudgeon pins allowing the lateral movement to take place in the first instance, the beginning of a main or big end failure, a result of overheating, when towing, or engines that are known for poor design or use of poor materials. I think Subaru Impreza's in the mid 00's had this issue and it was a design flaw that highlighted itself around 50k miles.

    Race engines that blow as a result of missing a gear are a result of over revving beyond the engines red line. That isn't high idle over stress, thats asking more of the engine than it can physically move, usually the engine will jump its timing before the bottom end lets go in that situation, having been around race engines most of my life, I know when that happens! I have had to rebuild a few I have blown! But that is why F1 engines have been using pneumatic pop off valve technology since the early 90's, it was a way of moving the valves, quicker and faster to allow the pistons to do the same, thus allowing it to rev (as we used to see in the old V10 and V8 days to nearly 20k rpm) which is 2 stroke territory.

    All engines have an operating rev range in which the engine is safe in which to run at, whether that is 2000 or 6000 rpm, its within its design capability, engines that are freer and give better economy are the ones that have their legs stretched and there is nothing wrong, in the modern engine era that we live in (yes, probably old Austin and British Leyland days wouldn't have coped so well with this, but we are discussing 21st century German engineering here) than using all the revs you are given on the dial.

    And if you haven't seen an auto box boil its oil when running at high rev idle for a period of time, then I would suggest you haven't been around a workshop environment as long as I have. I have seen it a good few times, once where the box then suffered a hydraulic issue, and catapulted itself into the techs toolbox just missing him, it was in neutral at the time.
     
  16. Steed

    Steed Active Senior Members

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    Its not just Mercedes that limit the ability to rev in neutral, my 1994 air cooled Porsche has to be coaxed to rev during the MOT emissions test.
     
  17. LostKiwi

    LostKiwi Senior Member

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    Piston slap has nothing to do with little ends or big end wear. It is purely down to piston skirt/ bore clearance. Forged pistons in brand new engines are well known for piston slap when cold (Google it). This is down to increased clearances required to compensate for thermal expansion of the piston. This is secondary piston motion as mentioned previously where the piston has lateral motion as well as linear motion - in other words the piston can rock as it goes up and down the bore. Gases don't leak past the piston due to the rings having a larger internal diameter than the groove where they sit.

    As for over revs being the cause of blown engines during missed changes even with limiters to protect the engine it still was the prime cause.

    Pneumatic valve operation was implemented to reduce the mass of the valve train. A lighter valve without the mass of the spring can be accelerated more rapidly to open and close it thus allowing steeper cam profiles and/or higher RPM.

    Like you I have been involved in building race engines (including class winning cars at national and at WRC level) and both my father and grandfather had extensive experience of engines which they passed on to me.
     
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