What is a Dump Valve?
Often referred to as a DV or BOV or similar. It is a device that goes 'tish' when you change gear venting excess turbo pressure to atmosphere...
Are there different kinds of DV?
Yes. The most popular I have come across are atmospheric and recirculating. Atmospheric simply dump excess boost pressure to the atmosphere and you get a loud 'tish'. A recirculating type DV dumps excess boost pressure back into the intake. For each of these 2 types you can have either a piston or diaphragm internals...diaphragms tend not to last as long as the piston type as the diaphragm splits...If you really must have a diaphragm variant then you might consider one that uses Kevlar otherwise you might be forever repairing the damn thing.
Which one, atmospheric or recirculating?
Atmospheric - You get the distinctive 'tish'. It is easier to fit but messes up your fuelling on a 165 and 185 when you change gear as all of a sudden, a rush of air passes through the AFM and consequently the ECU sees "loads of air" and adjusts the fuelling to suit i.e. lots of fuel. The 205 does not suffer from this problem as the fuelling is calculated in a different manner i.e. via a MAP sensor.
Recirculating - a lot quieter than an atmospheric DV. It is much harder to fit as an entry needs to be made back into the air feed to the turbo. However, this is much better for fuelling as excess boost pressure when dumped will help to close the AFM flap so reducing fuel delivery.
Why do I want to spend my lovely money on one of these?
There are a number of reasons why you would want to fit one:
Sound - excess boost pressure is vented to the atmosphere; the escaping air makes a noise.
Turbo life - consider this, if your turbo is spinning away producing, say 15psi, and you change gear. Firstly you physically close the throttle and prevent any air getting into the engine, but the turbo is happily spinning away at ~60,000 rpm making boost, where does all this air go? Nowhere! Closing the throttle will drastically slow or even stop the turbo from spinning, (known as compressor stall). This places stress on the impellor, shaft and bearings. All this added stress will reduce the life of the turbo, or in extreme circumstances damage the turbo.
Driveability - As we have now established the turbo will slow dramatically from the explanation above. Therefore when you change gear the turbo will have to spin up to speed again. If a DV is fitted then on gear change excess air is dumped and the turbo remains spinning as the produced air can flow out through the DV. Now, when you accelerate, the turbo is already spinning and does not require so long to spin up in order to produce pressure.
There's loads of Dump Valves, what are tried and tested ones?
The higher pressure recirc valve was fitted to the 165 and holding 15psi without fault for approx 6 months.
Where can I get one of these from?
You could try: