Dumpvalves :what in english do they do ? whats the best price wise and engine wise for my 1998 2000 turbo awd (242bhp)

The dumpvalve is fitted between the turbo (compressor side of it) and the throtle body (inlet).
When your on boost, the turbo is producing a positive presure, ie phsyicly forcing air into the engine, when you change gear the throtle snaps shut, blocking off this presure being produced by the turbo which is still spinning at high speed and hence this presure has nowhere to go.
This sends a shock wave back to the turbo causing it to stall and can cause damage to the commpressor wheel of the turbo.
To eleviate this problem, a dump valve is fitted which vents this presure either to atmospere (vent to atmos dump valve) or back into the inlet tract (re-circ dump valve). The former is the one that makes the 'tish' noise (irritating;)). The latter will already be fitted to your car as standard.
They also help to keep the turbo spooled up in between gear changes as already mentioned about the shock that would otherwise stall it and you would have more 'lagg', waiting for it to spool up again.
If you wanted an aftermarket valve, id stick with a re-circ.
But its realy down to personal preff!

Price wise, anything between about 60 quid right up to around 320 for an all singing all dancing HKS item, very good if you like noise! Turbo Smart valves are loud to.
If like me you dont like the tish noise, id go for a Forge re-sirc, i think their around 100-150 quid.
Has to be said though, if yours is doing its job ok there are better things to spend on your car, personally speaking ofcourse;)

[Edited by Leviathan - 11/18/2003 8:44:36 PM]