well tTm, if you think more deeply about how the turbo works, then the presence of a dump valve will not make any difference to performance!
with a dump valve fitted, be it atmosph or recirc, when you release the throttle, the throttle plate shuts. as the turbo is still spinning, pressure builds up before the throttle plate, and as the engine is still turning, a vacuum forms after the throttle plate. this vacuum is tapped with a feed to open the DV, which is in the presurised part of the system between the turbo and throttle flap.
This vacuum feed opens the DV, and excess pressure is vented, releasing the build up between the throttle plate and turbo.
The reason this needs to be done, is because when the plate closes, a wave of pressure goes back through the turbo, causing the turbine wheel to stall. This is of course compressor stall, and can be very bad for the bearings, thrust bearing in particular. Compressor stall is the 'cool' sound so many turbo cars make when backing off. On larger turbo's with a more manly shaft, it's not a big issue, but on ko3/4 applications, it can cause damage.
Of course, how much damage depends entirely on how much boost you are running.
There is also the issue of spool up time. With a DV in place, compressor stall does not occur, and as such in normal driving, spool up time is reduced between gear changes.
However, many people believe, that when going full throttle, there is a slight advantage to not having a DV, as when reapplying the throttle there is already pressure in the system. It's a strange arguement, which I wouldn't like to get into!
So, that explains the need for a DV. on a standard AGU at 6psi, you can run with no DV all you like, and the chances of getting a problem are VERY minimal.
Recirc; the recirc system simply takes the idea 1 step further, so instead of venting to atmosphere, the DV vents back into the air intake, after the air flow metre.
This allows the car to more accurately control fuelling, as it knows exactly how much air is in the system at any one time, and can fuel accordingly.
With an atmospheric DV, the MAF based audi engines assume the air is still in the system, and fuel for it, hence the over fuelling issue when fitting an atmospheric DV to a MAF based closed loop system.
Thats also not really an issue, apart from passing unburnt fuel into the CAT can kill it over time.
That said, I've been running with a CAT since I've had my car, and I've run it with a recirc, no DV, and an atmospheric, and never had any cat problems in 48000 miles. My current ko3S setup with a miltek 200 cel cat over fuels a fair bit even with the forge 007P, and it's not hurt the CAT yet.
You guys must remember that the AGU engine is TOTALLY different in so many ways to your S3 engines, It's much more simple, has NO boost sensor, and only 1 lambda.
you can feck around with it a LOT more and it's not so fussy.....
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