taken from another forum :)
The "Pigeon" mod is to remove any dump valve and just let the compressor surge when you close the throttle.
You can run like this but the damage is cumulative and I shall try to expalin why.
When you spec a turbo (and by spec here I don't mean select one froma website I mean have meetings with the turbo manufacturer who is about to commit a production line to your specific turbo application for the OEM you are developing the engine for) you do a number of tests to develop the application.
From basic power curve work to develop the appropriate turbine/compressor spec to altitude and high temperature work. When I talk about testing a turbo, I'm also talking about a fully instrumented unit and that means a minimum of mass flow, pressure and temperature in and out for both the compressor and turbine as well as a slew of other parameters including turbo speed.
All of the above are analysed and among other things all the pressures across the device are analysed. This allows the overall thrust on the turbo shaft to be calculated which will determine the design and life of the turbocharger thrust bearing. This is why the aftermarket does hybrids (big comps on small turbines while the OEMs do not, at least in volume production at any rate) as the imbalance in the sizes of both items causes excessive thrust bearing loads.
So now you're sitting there saying why the hells he going on about thrust bearings???
Well, when a turbo is at full boost and spinning the pressure is as balanced (exhaust side to inlet side) as we can get it at the design stage so that the load on the thrust bearing is kept to the design amount. Now hen you take your foot off, two things happen:
1) the pressure in the inlet rises very sharply indeed.
2) the exhaust pressure rapidly fall to nothing.
You can see where I'm going with this now can't you? SO TRANSIENTLY, we have a very high thrust bearing load spike. Now this won't kill your turbo immediately but IT WILL shorten the life of the turbo.
Your man two posts up whose turbo died very shortly after he did this mod is right: that extra movement in the shaft caused by the additional shock thrust loads will have caused the piston ring seal in the turbine to run on parts of the shaft it hadn't run before and that will have hastened it's demise.
But surely Dump valves prevent comp blades falling off?
Old wives tale!!
Seen any rally cars with a dump valve? No.
F1 engines? No.
The reason most comp wheels fail is foreign object damage or overspeeding.
A recirculation valve (proper name) is there to add durability to the turbo thrust bearing in much the same way that a water jacket aids the life of the bearings by preventing oil coking.
If you stop the water flow by removing the pipework or remove the RV to add noises, the turbo won't die. But You WILL shorten it's life.
On and while I'm on a hobby horse:
Externally venting DVs:
1) They make your car drive like crap
2) You sound like a tw*t
3) You will kill your cat
and 4) I can guarantee you that a car making funny noises does not loosen the knicker elastic of the girl you are trying to impress....... :rolleyes:
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