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  1. #1
    Randomjim's Avatar
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    External wastegate

    I've heard lots of different things about wastegate chatter. If i had a external wastegate would i get chatter or is it a myth? Discuss
    Built in Germany, Perfected in England

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  3. #2
    macca's Avatar
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    external wastegates vent down a seperate exhaust pipe, they sound like you have no silencers when it vents, we have one on a race car i help look after occaisonally and it sounds mad, can't wait until anti lag is set up.

    i think wastgate chatter is something you get from internal wastegates, someone else maybe able to say more.
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    Mods- Forge Dump Valve, The Engine Shop Power Plus based Engine (Low Comp. Forged pistons, Arrow H-Section Rods, Newman fast road cams, polished & ported head), 19" wheels, B&M Short Shifter, Greddy Turbo Timer, RS4 Grill, Weitec GTN coilovers, HKS modified air filter, Autobahn88 FMIC, ChippedUK custom map. Some Turbo mods ongoing.

  4. #3
    Randomjim's Avatar
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    What sort of race car is it?
    Built in Germany, Perfected in England

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    macca's Avatar
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    lancia kappa (never sold in the uk) fitted with a 2litre fiat coupe turbo 20v engine, producing 400-500+bhp, it insane, races in the LMA euro saloon car series.

    check it out here: http://www.nyssaracing.com/index.php
    1998 A4 1.8T Sport - For Saturdays
    2003 VW Golf MKIV R32 - For Sundays
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    Mods- Forge Dump Valve, The Engine Shop Power Plus based Engine (Low Comp. Forged pistons, Arrow H-Section Rods, Newman fast road cams, polished & ported head), 19" wheels, B&M Short Shifter, Greddy Turbo Timer, RS4 Grill, Weitec GTN coilovers, HKS modified air filter, Autobahn88 FMIC, ChippedUK custom map. Some Turbo mods ongoing.

  6. #5
    Randomjim's Avatar
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    Jeeesus sounds crazy, those fiat 20v engines sound good too
    Built in Germany, Perfected in England

  7. #6
    jcb
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    sometimes the noise people hear on rally cars is misatken for the wastegate but is actually turbo stall.

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    Quote Originally Posted by jcb
    sometimes the noise people hear on rally cars is misatken for the wastegate but is actually turbo stall.
    I wonder how turbo stall makes a noise though?

    I'm just guessing here and i'd gladly be informed of the facts...

    On, say a rally car, with no BOV/DV, when the throttle is closed the excess/built up boost pressure is not vented which causes the turbo to stall as the flow of air through it is abruptly stopped. Would the wastegate be open or shut at this point?

    Is the case that you would get 'chatter' from the Wastegate as it should be shut and is being forced open by the excess boost pressure trying to escape.

    Or is the Wastegate wide open?

    As for replicating this on a road car, whether it's turbo stall, or wastegate chatter, neither are conducive with long turbo life. So as cool as it sounds, not something i'd want happening

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    Caesium's Avatar
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    Right I can shed some light here....

    The wastegate is an actuator connected to the compressor housing by a small pressure hose, when the pressure in the compressor reaches a level that is too high (determined by the spring pressure in the wastegate actuator) the actuator moves a rod which opens a small flap inside the turbine housing. This allows the exhaust gases to bypass the turbine vanes and exit down the exhaust, slowing the shaft which gives boost.

    The dump valve works in a similar way but on the intake side as opposed to the exhaust side, when the throttle flap is closed suddenly (when changing gear) the turbo compressor vanes are spinning like mad and producing boost with nowhere to go, the pressure builds up and this pressure acts upon an actuator similar to the wastegate one and opens a vent from the intake hose to either the atmosphere (with a whoosh) or back to the airbox (no whoosh). This stops the compressor stalling, on high boost vehicles this stalling could actually smash the compressor vanes off or snap the shaft.

    An external wastegate does chatter a dump valve does generally not.

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    Caesium's Avatar
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    Also, a rally car would absolutely not be lacking a dump valve unless it didn't have a forced induction system.

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    Quote Originally Posted by A4Quattro
    Also, a rally car would absolutely not be lacking a dump valve unless it didn't have a forced induction system.
    So... What is the noise often referred to on Rally Cars as 'Wastegate Chatter'?

    Does it occur on WOT or gear changes?

    Why would any car with a wastegate and a BOV/DV suffer compressor stall?

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    Caesium's Avatar
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    Rally cars are mostly all turbo's so that noise is wastegate chatter.
    It occurs at gear change, when the engine revs are up but the throttle is closed.
    A car with a BOV should not suffer compressor stall unless the BOV is faulty or incorrectly set up.

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    I've read several times that 'Wastegate Chatter' is actually compressor stall, then again i'd also read that Rally cars aren't typically fitted with BOVs/DVs. I guess you learn not to take everything posted on Internet Forums as gospel

    Anyhoo, i'm struggling with this concept of wastegate chatter... If it happens on throttle shut then the BOV/DV should be wide open and boost will be vented as this is the path of least resistance, how can boost also force the wastegate open off throttle if the spring is strong enough to keep it shut on WOT?

    On the VAG 1.8T the wastegate is controlled by the N75 valve. It should be closed on throttle shut and partially (but never fully) open on WOT. On throttle shut, boost is always (i think) fully vented by the DV (back to the intake).

    So is it the case that on a Rally Car (to continue the analogy) that the boost pressure is simply so great that the BOV/DV cannot vent all the pressure and forces the wastegate open? Once through the compressor would this excess air have to force it's way back through the compressor veins to exit the wastegate, thus causing compressor stall?

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    macca's Avatar
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    shall we move on to anti lag yet???
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    2000 Peugeot 406 HDi - For when it's raining
    1996 Kawasaki ZX7R - For when I'm late for work!

    Mods- Forge Dump Valve, The Engine Shop Power Plus based Engine (Low Comp. Forged pistons, Arrow H-Section Rods, Newman fast road cams, polished & ported head), 19" wheels, B&M Short Shifter, Greddy Turbo Timer, RS4 Grill, Weitec GTN coilovers, HKS modified air filter, Autobahn88 FMIC, ChippedUK custom map. Some Turbo mods ongoing.

  15. #14
    Caesium's Avatar
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    Compressor air cannot exit throught the wastegate until it has been combusted! the wastegate is on the exhaust side and the compressor is on the intake.
    On my twin turbo Supra, the BOV used to open for a second then you could hear the wastegate chattering to relieve the rest of the boost. The BOV releases the pressure and the wastegate stops it from occuring in the first place so to speak.

    As for Anti-Lag, there are several systems you can get to do this, but one of them is to dump raw fuel, through a special injector straight into the turbine housing, this causes the fuel to explode driving the turbo up to speed. Also causes a lot of bang-b-b-b-bang-bang through the exhaust, and flames from the tailpipe....

  16. #15
    jcb
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    not all wastegates chatter. I have two video clips of evo's at a recent rally both on the same stretch, one has no chatter at all the other sounds like a dolphin convention!

    they don't just open and then close after the exhaust gases have all gone.
    they are popped open when the pressure reaches a certain point and as it opens the exhaust gas pressure drops enough for it to close again, pressure rises and it opens again, repeat steps 1 -4 ten times a second. = chatter.

    Tends to be external dumpvalves that do this, as no self respecting rally car will have an internal w/g......totally screws the exhaust gas flow leaving the exhaust housing

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    Quote Originally Posted by jcb
    they don't just open and then close after the exhaust gases have all gone.
    they are popped open when the pressure reaches a certain point and as it opens the exhaust gas pressure drops enough for it to close again, pressure rises and it opens again, repeat steps 1 -4 ten times a second. = chatter.
    I thought you just said wastegate chatter was compressor stall?

    i know i'm stupid but is anyone else not getting a clear picture from this thread?

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    jcb
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    No I said
    Quote Originally Posted by jcb
    SOMETIMES the noise people hear on rally cars is misatken for the wastegate but is actually turbo stall.
    With a relatively unsilenced exhaust system, a blow off valve, straight cut gearbox, antilag system, wastegate and the "compressor stall"......you have a hard job hearing the individual sound components of a rally car lifting off from 100mph at a corner.
    Some people maintain that the "chatter" noise is not the wastegate at all, but just the turbo.
    I am not being that arrogant in telling you what you are or are not hearing, you can hear a wastegate fluttering as it opens and closes (as I described above),
    quick look on google finds this:
    http://en.wikipedia.org/wiki/Wastegate

    which maintains wastegate chatter is a myth and that WRC cars have no BOV, can't find evidence to prove that either way. even on the FIA site. It would surprise me as the prospect of 1.5bar of boost with nowhere to go, wouldn't just stall your turbo compressor it would make the b@stard spin in reverse!

    But.......the noise a turbo makes when boost pressure comes slamming back into the compressor vanes and stops them spinning at a throttle closed position......is most often the noise people associate with WRC/rally cars.

    does that make sense?

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    Quote Originally Posted by jcb
    does that make sense?
    Some of the points you make there are what i'd eluded to in a previous post, so yes they make sense to me.

    Not that something making sense actually makes it correct

    I think i'm slowly getting my head around this... I can understand how the compressor can stall, but still have no idea how their is sufficient sustained pressure in a throttle shut situation to make the wastgate chatter. Or indeed why a stalling compressor would make any noise at all.

    and i really don't know why an external wastegate would 'chatter' anymore than an internal one.

  20. #19
    Caesium's Avatar
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    If an engine was at high revs and the turbo was producing high boost (1.5bar +) and the throttle was shut without a BOV, the compressor would indeed stall, the only problem is the hot exhaust gas still hitting the turbine vanes would try and spin it, the result would be a broken common shaft, and that you would hear as well as see when a huge cloud of smoke wafted out of the exhaust.

    My supra had an internal wastegate amd a piston type atmospheric BOV and that chattered when changing down a gear at high speed.

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    Quote Originally Posted by A4Quattro
    If an engine was at high revs and the turbo was producing high boost (1.5bar +) and the throttle was shut without a BOV, the compressor would indeed stall, the only problem is the hot exhaust gas still hitting the turbine vanes would try and spin it, the result would be a broken common shaft, and that you would hear as well as see when a huge cloud of smoke wafted out of the exhaust.
    In that situation wouldn't the pressure force open the wastegate rather than snap the turbo shaft?

  22. #21
    Caesium's Avatar
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    you'd think....

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    jcb
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    Quote Originally Posted by Tallpaul

    and i really don't know why an external wastegate would 'chatter' anymore than an internal one.
    internal one isn't as obvious when it is shrouded by cast iron exhaust housing and works with a flap. they tend to be set to creep open rather than snap open. (i.e. if it set at 15psi, it actually starts leaking exhaust gas at around 12psi) So I am told or to be precise, so I read in "maximum boost" by corky bell! ; 0.

    external ones are used because they are much more precise and they can be vented to the atmosphere. much more noise

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    Quote Originally Posted by jcb
    so I read in "maximum boost" by corky bell
    ah, i've been recommended that book before. me thinks it's time i actually read it.

  25. #24
    jcb
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    great read. has a few opinions that not everyone follows, but the basics of turbo system design and the reasons behind it are invaluable to anyone thinking about even running a turbo car let alone tinkering with it

  26. #25
    Secret Squirrel's Avatar
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    BS. Wastegate chatter is a myth.
    The sound you hear is indeed compressor stall or compressor surge. Compressed air forcing its way back past the compressor wheel.
    It's most obvious on cars without DVs or BOVs. One of the most well known being the Audi S1.
    However, it can still be heard on cars that have a DV or BOV that is too small. I believe it may also occur if a turbo is making a large amount of boost and the throttle is quickly shut, as the DV or BOV may not open fast enough.

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    lol, so this thread goes full circle.

    i think i'll read the book.

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    A good discussion and some very interesting theories being exuded here.

    Compressor "stall" is actually generally called surge ( the operating point on the left of a compressor map) , it occurs when the compressor outlet pressure gets to a point ( such as throttle closure) where there is insufficent in the system to sustain the pressure, ( unless it is releases dy a DV or BOV) at this point the pressure flows backwards through the compressor causing a "whoosh" noise, it is very bad for turbochargers as it loads the thrust bearings beyond their design limits and generally results in failed turbos.

    Wastegate chatter is a slightly subjective opinion, genarally the wastegate will only open when the ECU has requested it, so it should never chatter, however if the turbine ilet pressure is above the actuator limit it can cause the wastegate to creep open slightly, but this will only happen in extreme circumstances. I think that what is generally refered to as wastegate chatter is the DV operating. The wasegate is an internal sealed unit behind 5 mm of ni-resist steel, you cannot hear it ooperate and you certainly cant hear it chatter!

    Have a look at www.turbobygarrett.com for some further info re comp surge and turbo matching.

    As indicated perviously, the rally car noise is anti lag, where fuel is injected and every 5th firing order is seriously retarted, this causes the greater part of the combustion process to occur as the exhaust valave opens, and as such gives a lot of energy to the turbine and hence very little lag. Very very bad for turbos ( very high turbine temps) and not suitable for std road turbos.

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    Quote Originally Posted by A4Quattro
    If an engine was at high revs and the turbo was producing high boost (1.5bar +) and the throttle was shut without a BOV, the compressor would indeed stall, the only problem is the hot exhaust gas still hitting the turbine vanes would try and spin it, the result would be a broken common shaft, and that you would hear as well as see when a huge cloud of smoke wafted out of the exhaust.

    My supra had an internal wastegate amd a piston type atmospheric BOV and that chattered when changing down a gear at high speed.
    This would not cause the shaft to snap but will seriously damage the bearings in the centre housing.

  30. #29
    Caesium's Avatar
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    how would it damage the bearings exactly.

    The loading on the shaft components under said conditions would be a torsional load, the only load that could conceivably damage the bearings in the short term would be a repeated lateral load.


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    Actually under the coditions you indicate there is of course some torsional loading, but nowhere near enough to shear a welded shaft.

    The main loading is axial down the length of the shaft. At the point of surge the loading on the shaft goes from very high positive ( towards the turbine) to very high negative ( away from the turbine). This loading causes the thrust bearings sufaces to come together and score, this then causes the bearing surface fail as the oil no longer generates a film between the surfaces of the bearing. At this pint the turbine wheel will move excessivlly and will contact the inside of the turbine housing and rub. When the turbine wheel rubs on the housing it goes out of balance and starts to rotate eccentrically, this imbalance will snap the head of the turbine wheel off the shaft.

    The more boost a turbo runs the higher the thrust loading on the bearings, a stsndard "hydrodynamic" ( oil based ) beraing can only hold so much. For high boost and racing applications it is ALWAYS recommended to run using ball bearing systems. These have about 10 times the thrust capacity of a hydrodynamic bearing and a lot less rotational inertia, so spool up quicker and cantake considerably more punishment.

    Sorry to ramble but I hope that makes a little sense..

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    also, you say "in the short term" dont forget this system is rotating at up to 150000 rpm, so it takes a very very short time for catastrophic damage to occur.

 

 

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