although i dont know anything about diesel engines i know a bit about petrol dump valves.
dump valves stop the turbo stalling but without a dump valve the standard valve does same job except it routes the air back to the air box so the maf can adjust fuel amounts (dump valves can cause many problems, oil being thinned down too much by fuel or borewash). with air being dumped to atmosphere the maf does not know the air has been blown out causing overfueling. this is why you can hear many subaru's popping away (back firing)
i have dump valve and it sounds great also love the pops and bangs, letting off the gas pedal near bus stops is very amusing as people think there bus is pulling up

however i don't let girl friend in car anymore as every pshhhhhh and she bursts out laughing
