DUMP VALVES THE MYTH AND RUMOUR...
Ok here goes guys.....
a) There is no such thing as a 'flutter' valve you can only buy a dump valve or blow off valve. Both of which will whoosh/hiss, this is all they are intended to do.
b) The flutter noise you refer to is caused by either compressor stall within your turbo resulting in any boost the dump valve cant deal with coiming out via the air filter.
Example, ive got a grp A Forge on my 21, and ive got a grp A in my toolbox off a Escort Cos from Collins. Put ya thumb in the Forge one and I can freely push the d/v open, do it to the Collins one and I break my thumb. Buy yourself a soft spring, your dumpvalve will go off at at 3mph in slow traffic, buy a tough spring it will just flutter even at high speed, cos YOU WONT BE PRODUCING ENOUGH BOOST TO FORCE THE SPRING OPEN. Therefore the boost cant go anywhere, it causes slight compressor stall within your turbo and the flutter you hear is the wastegate opening and closing really fast emitting via your air filter.
It really dont matter whether you buy a grp A,B or Z.....if the spring is right it'll go off, if it aint it will flutter.
That said I can make my 'LOUD' Forge one flutter by reducing the internal diameter of the take off pipe, if I use a take off pipe of equivalent size to the dump valve stem, it'll produce a loud whoosh, if I use the narrow one supplied by Samco, I get flutter.
I know ive gone on a bit, but thats the truth of the matter, it bugged me when I became a 21 owner, cos a Halfords boy told me to buy a 'Butterfly valve' lol and having never seen one, I thought i'd do my homework.
Me
x
p.s Blow off valves are basically expensive dump valves, although they are adjustable and often incorporate a dump and recirc method, thus avoiding compressor stall completely in ya 50psi roller bearing Skyline. Andy Francis (a 21toc member) recently fitted a ?297 Scooby blow off valve to his 21, it was a little more high pitched than my ?60 Forge one. The Scooby owner agreed it wasnt worth ?297.