dump valve gots rid of the turbo presher when u let of the gass soo wot it does is it lets the turbo spin stiil so if u take it of it will stall the turbo when u let of the gas eg changre gear so the when u put on the gas agen u have got to get the turbo spinging agen by this time the chav in is 1.4 sr is just about to zipp past u hope this helps