I was just knocking the tight restrictions that Melbourne has.. not all are illegal.. they have some weird rules like only 1 intake mod etc..
BOV - Blow Off Valve, aka, by pass valve, diverter valve, dump valve and probably many other things from different manufacturers is to divert air from the system when you jump off the accelerator.
It sits somewhere on the pipe work on the intake side of the throttle body.
You accelerate, turbo builds boost, engine chows down on boost, you smile, tacho reaches redline, you jump on clutch, off the loud pedal, slam the next gear, off clutch, on the loud pedal and continue smiling.
In that split second where you change gears, the "x" psi of air that was rushing in to the engine to get exploded, is now jammed against a metal wall.
It's got nowhere to go.
During that split second to shift gears, the pressure on the other side of the throttle where things are exploding goes from boost to vacuum.
This vacuum runs from that side of the throttle along a small hose, to the back of the factory blow off valve, which sucks open the little piston in it to allow the pressure on the intake side to be vented.
The factory system directs that air to the intake pipe between the air box and the front of the turbo.
Factory cars want things to be quite..... so does the EPA for that matter.
so there will be a bunch of things along the intake to make it all quiet, like air box, silencing chambers, sometimes even padding in the air box.
If you didn't have this factory bov, the air would get jammed against the throttle butterfly, and cause the intake air cavitation which people who own VL commodores have named "dose"... that fluttering tutututu noise.
clixx-io - what you were referring to sounds like the 1980's "pop off valves" where they used a pressure release valve which kind of looks like a blow off valve to release boost when it reaches what they have set it to..
That task has been taken over by a "wastegate" a long time ago.
The external venting blow off valves do the same thing as factory blow off valves but instead of recirculating the air, it lets it out to atmosphere and this has the psshhtt noise or many other types of noises they can be made to do depending on the outlet.
I'm not sure if the renaults have an Air Flow Meter (MAF) but if they do, atmo venting bovs cause slight changes to the mixtures and so you sometimes get issues with puff of black smoke on shifts and some backfires, and sometimes cars stalling.
Police/EPA love finding them on cars.
Atmo venting offers no benefit over recirculating systems in terms of performance... None.