The purpose of the dump valve is to prevent the excess air that is in the inlet pipework passing back down the inlet pipework and spinning the turbo in reverse when the throttle is closed. It also stops the air passing backwards through the airflow meter which may cause problems with the meter.
The problem with the turbo spinning in reverse means that there is a bit more lag when accelerating again but the standard mitsi turbo only has a 270 degree thrust bearing (I think??) and so this may also cause problems to the turbo. With a 360 degree thrust bearing its not as much of a problem which is why some of the highly tuned cossies can run without dump valves.
Hi, I'm talking about the dump valve. You see a lot of modified engines keeping the original recirc dump valve. I've also read that these leak pressure on boost to the inlet. Just trying to get it clear what it is trying to do.
I know it stops the turbo stalling on lift off, acting like a recycle valve so the supply is never cut-off.
That's exactly what the BOV does, stops the turbo stalling on lift off but it's not to do with cutting off the supply of air to the turbo (because the turbo has stopped spinning due to the reduction in exhaust gas flow with a closed throttle) it is to stop a reverse flow of air going back to the turbo and possibly causing damage to the blades and bearings and to stop a reverse flow of air through the airflow meter.
Does the BOV leak at a certain boost pressure. ie tight shut at 1barg, but 1.2barg causes the BOV to pass boost pressure (1 and 1.2 nominal figures used!!) The question was does it and a what pressure? Why do modded engine seem to retain the OEM unit. Do after markets only make the wooosh? Any other benifits to an after market??
The BOV will leak when generating boost on a part throttle when there is a bit less pressure on the diaphragm side of the valve than on the inlet side and it may bleed out a small amount of air at this point. It may also leak inwards a bit on idle when there is a very big vacuum in the plenum chamber. Fitting a stronger spring will help eliminate the leaks but will make the valve slower in acting and may IMHO cause potential problems with reverse air flow to the turbo (see the thread on reversing the dump valve for other peoples opinions on this)
Using an aftermarket vent-to-atmosphere (VTA) valve will mean that when the air is leaking in at idle it will be bypassing the airflow meter and the meter will therefore be reading the incorrect amount of air entering the engine (due to it not being able to record how much has entered) and the engine may idle very roughly and sometimes even stall.
This can be overcome with a VTA containing a stronger spring or a dual acting valve (like the HKS super sequential) that eliminates these problems. Note that by using a recirc valve, the idle problems do not happen because the air that is leaking into the valve has already passed the airflow meter.
Apart from that all a VTA gives is a nice whoosh noise which is good enough for some people......
.....Hopes he's got the right end of the stick now