Exceptionally technical write up

As for compressor blades stalling Saying bollox is very harsh, especially when you yourself state innaccuracies. But hey it's nearly the weekend, so who gives a turkey.

 

Compressor surge doesn't cause the rally style chatter, what the spec c thread was discribing is something totally different which, as they say, is compressor surge. You dont get compressor surge when to shut the throttle body. No dump valve fitted will effect the spinning of the compressor wheel by slowing it down, if there is no where for the air to go and the throttle body is closed, hence the exhaust gases are no longer having a postive effect on the turbine wheel, the compressed air in the i/c & post turbo ducting will be higher than the turbo is now producing slowing the compressor wheel down. How noticable this is while driving the car is debatable but it is a fact that DV do a job and that is to keep the compressor wheel spinning while the throttle is closed by releasing positive pressure back to either pre turbo ducting or atmosphere. What the noise is can be dabated also and somebody will know the answer but I think it is the noise of the unreleasable compressed air (because the DV has been disconnected/blocked off) fighting against the compressor blades and air because it has now where else to go to escape.

The noise you hear from a fmic install is either the DV (not the rally chatter we speak of) piston opening and closing continuously, Bailey DV's are know to do this. An under efficient DV not releasing enough air will cause the rally chatter sound as the fmic stores a higher volume of air to be released, for it to be efficient, a bigger DV will be required.

As to is it safe to remove a DV or blank/disconnect it, I dont suppose manufacturers would fit them if they weren't required but you may get a number of weeks/months/years of use out of a turbo without one. General thoughts from what I've read is TD turbo's are normally ok but VF turbo's are normally not ok.

 

Yes you do, it's the same. That is compressor surge.

If the compressor sections pressure ratio is exceeded, it will surge. Closing the throttle plate (especially if the dump valve has been removed) will instantly increase the P/R.


Jase.

 

From wikipedia - Wastegate - Wikipedia, the free encyclopedia

Wastegate chatter myth
There is a common myth in the automotive world about so called "wastegate chatter" or "turbo flutter", a noise created on lifting off the throttle in a turbocharged car. The sound is commonly described as a chipmunk or a rattlesnake and is often stated incorrectly as being a result of the turbo's wastegate closing.

The noise is in fact the air compressed by the turbo passing back through the compressor wheel of the turbo after the airflow is abruptly halted by the throttle plate closing. However, in some cases, i.e. where it doesn't open fast enough or is set up to only react to high boost, some chatter will remain. Surge can occur on diesels when the turbo is attempting to pressurize the air at a higher pressure ratio than the compressor wheel can flow at a given speed. Diesels have no use for a Blow off Valve as they do not have a throttle.

The chatter noise is very noticeable on WRC Rally cars, where no BOV is allowed.

 

thanks you guy's. not only have you helped answer my question, i now have more understanding about the works of post turbo. i will be running a tdo5 18g when i get it fitted so best thing to do is give it a go and eliminate the dv, as currently running a bailey vta.as long as no short term damage is done. in long term the antilag will take its effect in time..

 

Think I will disconnect my BOV if i can get hold of some bungs.. any idea where i can buy some from (not online) like a plumbing shop oor something. I Have a td04 turbo, second hand ones are cheap as chips and if i kill it I may go and be silly and buy a td05 instead

 

Thats why I'm aint one of them sciencenists folks. Science IS fun though!

Good write up Simon. Bit harsh though. People come here to ask questions, get answers and discuss.

Anyway, back to the fun stuff. What is that info based on? Experience? Lab results? Do we have some hard proof we can refer to? Can you post it up? Not patronising, but it sounds like you know your stuff. There is a lot of gumpf on the net about chatter, so without first hand experience it is very difficult to get a definitive answer.

Andy.

 

Did you read that in a text book?

I carried out a test on this and when the TB is closed with no dump valve the Intercooler system depressurised itself, the only place the pressurised air could escape is via the Turbo, also when I did this with this intake removed you can clearly see that the Compressor wheel does not stall (not so the naked eye could see) but is slowed down, as the force of the exhaust gasses on the hot wheel continue to keep the compressor spinning forward.

 

With the throttle shut the exhaust flow rate is also negligible, so reverse flow will have some effect on the speed of rotation.

Subaru and others spend millions trying to get these details right, then we all muck about with them....

My FMIC problem is if I shut the throttle at about 80 mph, (private test track !), the DV doesn't open, the reverse airflow is then read by the MAF as zero flow andd the fuel cuts violently ( OUCH !)

Do I need a second DV just after the turbo to vent more pressurised air ?
TIA

Dunx

P.S. Softest spring fitted to DV already....

 

Agree'd on that statement, but its not what I'd refer to as compressor surge, as the engine requires no air but the turbo is still spinning making un-required air but, to me, it doesn't seem right to call that compressor surge, its the turbo stalling.

Its like saying the engine is reving when its actually de-celarating

 

No you need to read this
Redirect

 

why do i get a funny oily smell sometimes when running without a dump valve but not with one fitted?

**** it tho i like the turbo turkey noise.lol.

 

What you have just described is compressor surge though I.E. the engine cannot flow the required amount of air for that particular boost pressure.

You seem to be inferring that compressor surge & compressor stall are two different phenomena brought on by different events. They are one and the same. When a compressor surges it can be said that airflow over the compressor blades is stalling (or losing its 'grip' on airflow).

Sorry to be pedandtic

 

No, thats cool mate,

I do understand what you are saying but I was just saying that to try and seperate the two, I call one compressor surge, i.e. on boost and the other compressor stall, i.e. off boost. They are technically the same thing but in different curcumstances.

Plus one sounds cool and one sounds terrible

 

My information comes from a chap I worked/work with, both when we were both at Cosworth technology and now at another consulatncy, most recently he was working on the new Skyline engine at concept stage....as well as someone else I work with who is ex TWR....

Wikipedia (after all written by anyone who wants to submit) is wrong, its not the air going back thorugh the compressor, its just the pressure ratio/flow speed exceeding the surge line....most turbos will go over the surge line on lift off as the flow drops rapidlyso even with no pressure ratio change the turbo goes over the surge line, this will nearly always be worse on a small turbo like mine, than on say a big MD321......

As an example the Skyline has a flow straightener in the turbo inlet pipe, this enabled more boost to be used at lower speeds as by improving the flow 'quality' into the turbo, the blades are less likley to 'stall'.

If you can struggle through the reading its very well described in http://www.itk.ntnu.no/ansatte/Gravd...ressorBook.pdf section 1.4, stick to radial/centrifugal flow descriptions though (turbos) not axial flow (jet engines), you will then understand that the urban legend of reversing back flow etc etc is actually urban myth!

Simon